Space Geodetic Constraints on Plate Rigidity and Global Plate Motions. Giovanni Federico Sella Louisiana State University and Agricultural & Mechanical College

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2001 Space Geodetic Constraints on Plate Rigidity and Global Plate Motions. Giovanni Federico Sella Louisiana State University and Agricultural & Mechanical College

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SPACE GEODETIC CONSTRAINTS ON PLATE RIGIDITY AND GLOBAL PLATE MOTIONS

A Dissertation

Submitted to the Graduate Faculty of Louisiana State University and Agricultural and Mechanical College In partial fulfillment of the requirements for the degree of Doctor of Philosophy

The Department of Geology and Geophysics

by Giovanni Federico Sella B.S., University o f Wisconsin, Madison, 1990 M.S., Louisiana State University, Baton Rouge, 1994 May 2001

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

My advisor Roy Dokka has supported me unconditionally and allowed me to

tackle projects far from his main research area. His selflessness in allowing me to

pursue my interests and his personal generosity is testimony to his abilities as an

advisor. Tim Dixon at University of Miami., my unofficial advisor, without whose

guidance and home (a total of over 8 months of my sleeping life was spent on the

Dixon living room couch) this research would never have started or ended. Tim

offered not only his house, lab and ideas, but above all his enthusiasm and

encouragement at all times. Jackie and Ian Dixon made me laugh as I tried to keep up

with Tim’s 15th idea of the day.

My committee D. Henry, O. Huh, P. Bart and J. Hanor showed support, and most

importantly flexibility, on my constantly evolving research project. Funding for this

research was provided by: NSF, NASA, Louisiana Board of Regents, Shreveport

Geological Society, GCAGS, and the Department of Geology and Geophysics.

Chevron and BP offered me two excellent summer internships.

At RSMAS I had three mentors: Fred Farina first taught me all the rudiments of

GPS processing and script writing, and later had to find a tailor for adjusting my suit

for my wedding. Lin Mao came to Louisiana for the first GULFNET episodic

campaign at the end of which he swore he could not think of anything more boring

and returned to writing all the scripts that allowed me to process my data; Edmundo

Norabuena challenged my interpretations regularly and forced me to better articulate

my ideas.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Without the SOPAC archive and all the organizations and individuals around the

world that freely provide RINEX data to it, and hence to users like me, none of this

project would have been possible. My deepest thanks to the team that keeps the

archive running seamlessly, especially M. Scharber. Additional RINEX data was

generously made available to me by the following people: Dr. S. Miyazaki and Y.

Kikuta, GSI; Dr. T. Kato, University of Tokyo; P. Digney, AUSLIG; Dr. M. Sato,

Hydrographic Department, Japan; J. Prieto-Moris, IGN-Spain; P. Nicolon, IGN-

France; J. Pinto, IPCC; Dr. S. Wodwinski and S. Naman, University of Tel Aviv; Dr.

K. Larson, University of Colorado-Boulder; G. Martin, City of Tucson; P. Colucci at

GeoDAF; Dr. S. Yu and J. Ng, Academia Sinica; M. Caissy; GSD, J. Ozard, NYEC.

Dr. Chris Harrison served not only as a sounding board for various off the wall

ideas, but more importantly unreservedly made his knowledge of statistics and plate

tectonics available to me during each of my stays in Miami. Dr. Ajoy Baksi for

stimulating converstations, my only regret is that we did not talk more often.

UNAVCO has helped me in many ways from my first days that I collected GPS data

to now, and hopefully in the future. In particular, K. Conquest, K. Feaux, C.

Guillemot, F. Boler, L. Esty, E. Manzaneres, and S. Fisher. Critical help and support

was provided by Dr. D. Jefferson and Dr. K. Hurst at JPL, and J. Mader at NGS..

My friends helped me laugh through my days in Baton Rouge: Aline, Myriam,

Maria Giulia, Barry, Jane, Stephanie, Charles, Jeremy, Troy, Emil, Hodge, Katie,

Libo, Kris and Alex. Connie for bailing me out of endless tangles. Tommy Smith for

extracating me from UNIX system black holes. Rick Young for holding me by the

scruff of my neck from a roof top while I stood on the edge of the handrail of a forklift

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40 feet off the ground to install a GPS site. In Miami the first days passed quicker

thanks to the good humor of Alekandra, and the last ones were a hoot with Enrique,

Julien, Pete, Xavier, and Sidonie. Episodic campaigns was collected with the help of

Aline, Rick, Karen and Katie.

My rather drawn out graduate career would not have ended successfully without

the generous help of the department staff, especially, Sara Marchiafava, and also W.

Bush, C. Duncan, L. Nichols, S. Nettels, R. Oubre, J. Payne, S. Purnell, L. Strain, S.

Wilkes. The troika of MGG in Miami: K. Fleites, A. Miller, and T. Villamor, helped

make each of my trips so much easier and obtained new equipment with a speed that

still boggles my mind and makes me want to order more equipment!

The support of my parents has been unflagging in this complicated period and my

gratefulness to them cannot be put in words. Lastly and most importantly, Silvia there

is no way to thank you for everything, and I mean everything, but especially for

putting up with my exceptionally long hours and the heat of Baton Rouge. May I

never ask you to repeat the experience again in our on going adventure.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGEMENTS ...... ii

LIST OF TABLES...... vi

LIST OF FIGURES ...... vii

ABSTRACT ...... x

CHAPTER 1: INTRODUCTION...... 1

CHAPTER 2: RIGIDITY OF NORTH AMERICA...... 5 Introduction ...... 5 Data Collection and Analysis ...... 7 Results and Discussion ...... 14 Conclusions ...... 32

CHAPTER 3: GULFNET AND DEFORMATION OF THE NORTHERN GULF OF MEXICO...... 34 Introduction ...... 34 Previous Studies ...... 35 GULFNET...... 39 Data Collection and Analysis ...... 44 Results and Discussion ...... 45 Conclusions ...... 48

CHAPTER 4: REVEL: A GLOBAL PLATE VELOCITY MODEL ...... 49 Introduction ...... 49 Data Collection and Analysis ...... 52 Results ...... 57 Discussion ...... 60 Comparisons to Geologic and Geodetic Data ...... 66 Conclusions ...... 103

REFERENCES ...... 105

APPENDIX: SITE COORDINATES, VELOCITIES AND RESIDUAL VELOCITIES ...... 122

VITA ...... 128

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

3.1. Episodic site coordinates, monument description, PID number and year established ...... 43

4.1. Xv2, rate residual, and angular velocity for each plate with respect to ITRF-97 ...... 58

4.2. Relative angular velocities for adjacent plate pairs ...... 59

4.3. £v2, rate residual, and angular velocity for the Antarctic plate with respect to ITRF-97 ...... 71

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

2.1. Site locations and antenna types ...... 8

2.2. Example coordinate time series for station GODE ...... 11

2.3. Site velocities relative to ITRF-97 for 81 sites on rigid North America .. 13

2.4. X 2 and mean rate residuals versus total observing time for sites located on rigid North America ...... 17

2.5. Histogram of WRMS values for north, east and vertical positions for all sites on North America ...... 20

2.6. Same as Figure 2.5. but for sites on the rigid North American plate including sites within a radius of 1,800 km of the center of Hudson B a y ...... 21

2.7. WRMS plot for pairs of stations, one with a Dorn Margolin antenna and one with a different antenna, grouped by distance between stations ...... 23

2.8. WRMS plot for stations in the southwestern USA separated into sites with braced monuments and sites attached to a building or pillar ...... 24

2.9. Vertical site velocities with respect to distance from the center of Hudson B a y ...... 27

2.10. Measured vertical velocity versus predicted vertical velocity for sites within a radius of 1,800 km of the center of Hudson B a y ...... 28

2.11. Site velocities relative to rigid North America for sites within a radius of 1,800 km of the center of Hudson B ay ...... 29

2.12. GPS site velocities relative to stable North America excluding sites within 1,800 km of the center of Hudson Bay ...... 31

3.1. Regional tectonic map of the northern Gulf of Mexico from Worrall and Snelson, 1988 ...... 36

3.2. Location of continuous and episodic GPS sites in the northern Gulf of Mexico ...... 40

3.3. Location of continuous and episodic GPS sites in southern Louisiana ....42

3.4. Residual velocities with respect to rigid North America for continuously operating GPS sites in the northern Gulf of Mexico ...... 46

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.1. Histograms of (a) total length of time series at each site and (b) number of data for each plate ...... 54

4.2. Estimate of GPS velocity error as a function of time for the stations used in this study ...... 55

4.3. Histogram showingX 2 for each plate ...... 64

4.4 Mercator projection showing space geodetic site locations for the Amurian plate ...... 68

4.5. Similar to Figure 4.4, for the Antarctic plate ...... 70

4.6. Measured seafloor spreading rates and transform fault azimuths along the Australia-Antarctica plate boundary...... 73

4.7. Similar to Figure 4.6, for Antarctica-Pacific ...... 74

4.8. Similar to Figure 4.4, for the Arabian plate ...... 75

4.9. Similar to Figure 4.6, for Arabia-Nubia and Arabia-Eurasia ...... 77

4.10. Similar to Figure 4.4, for the Australian plate ...... 78

4.11. Similar to Figure 4.4, for the Caribbean plate ...... 80

4.12. Similar to Figure 4.4, for the Eurasian plate ...... 83

4.13. Similar to Figure 4.6, for Eurasia-North America ...... 85

4.14. Similar to Figure 4.4, for the Nazca plate ...... 87

4.15. Similar to Figure 4.6, for Nazca-Pacific ...... 88

4.16. Similar to Figure 4.6, for Nazca-Antarctica ...... 89

4.17. Spreading rates and transform azimuths predicted for Nazca-Pacific (calculated at 12°S, 250°E) and Nazca-Antarctica (calculated at 40°S, 268°E) plate pairs ...... 91

4.18. Similar to Figure 4.4, for the Nubian plate ...... 93

4.19. Similar to Figure 4.4, for the Pacific plate...... 94

4.20. Measured seafloor spreading rates, adjusted for outward displacement, at various times in the southern Gulf of California ...... 96

4.21. Similar to Figure 4.4, for the Philippine plate ...... 98

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.22. Similar to Figue 4.4, for the South American plate ...... 100

4.23. Similar to Figure 4.6, for Nubia-South America ...... 101

4.24. Nubia-South America spreading rates (full rate) versus age for last 25 million years ...... 102

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

The Global Positioning System (GPS) has emerged over the last 10 years as the

premier space geodetic technique for geologic observation with the benefits of both

accuracy and economy. GPS data from more than 260 stations were processed

utilizing GIPSY-OASIS software, and all available RINEX files for these stations

beginning on January 1, 1993. The uniform processing strategy, combined with a

rigorous independent error estimate of site velocities and the broad geographical

distribution of these sites resulted in a global high-precision surface velocity data set.

This data set has been used to: 1) compare the effects of different GPS antenna and

monument types on data noise; 2) examine in detail the rigidity of the North America

plate; 3) constrain the rate of deformation in the northern Gulf of Mexico;. 4) define a

model for Recent (Holocene) time global plate motions.

The weighted root mean square scatter about the best fit line through the daily

position estimates are used to assess the effects of different antenna and monument

types on North America. The data set is sensitive to glacial isostatic adjustment as

indicated by the decrease in Xv2 from 1.3 to 1.0 by excluding those sites on rigid North

America within a radius of 1,800 km of Hudson Bay. The resulting 64 site solution

for the angular velocity of North America give a mean rate residual of 1 mm/yr.

/ GULFNET, a regional GPS network created in 1997 in the northern Gulf Coast to

measure strain across this active “passive margin” is described. Preliminary' results

suggest that deformation is occurring as predicted by the regional paradigm for

deformation, i.e. Gulfward motion along normal faults.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A new model for Recent global plate velocities (REVEL) describes the relative

velocities of 14 plates. For several plate pairs, statistically significant differences are

observed between the geodetically and geologically determined velocities reflecting

changes in plate velocity through time. Some of the rate differences reflect gradual

slowing associated with convergent plate boundaries, and may reflect increased

resistance to subduction or convergence associated with decreased slab pull and/or

crustal thickening.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1: INTRODUCTION

The Global Positioning System (GPS) has emerged over the last 10 years as the

premier space geodetic technique for geologic observation with the benefits of both

accuracy and economy. This has resulted in the establishment of over 1,700

continuous GPS stations around the world whose data are made available to the public,

through the efforts of many individuals, institutions and geodetic agencies. In this

dissertation, GPS data from approximately 260 of these stations were processed

utilizing the GEPSY-OASIS software and all available RINEX files for these stations,

beginning on January 1, 1993. The uniform processing strategy, combined with a

rigorous independent error estimate of site velocities and the broad geographical

distribution of these sites has resulted in a global high-precision surface velocity data

set. This data set has been used to: 1) compare the effects o f different GPS antenna and

monument types on data noise; 2) examine in detail the rigidity of the North America

plate; 3) constrain the rate of deformation in the northern Gulf of Mexico; 4) define a

model for Recent global plate motions.

The first part of this dissertation, Chapter 2, describes how 120 GPS stations on

North America were used to understand the effects on data quality of non-geodetic

continuously operating stations. North American stations were chosen for two

reasons: 1) North America has the largest number of geodetic and non-geodetic

continuously operating GPS sites, except Japan, and the complete RINEX files for the

vast majority of these sites are available on-line; and 2) Over 80 sites have time-series

longer than two years and are located on the rigid North American plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. It is important to understand the data quality at the non-geodetic sites because

incorporating them permits a much broader and representative geographic distribution

of sites for defining the spatial limits of the rigid part of the North America plate. The

results suggest that non-geodetic quality GPS sites can provide valuable information,

although the stations tend to be noisier than geodetic quality sites. Incorporating the

weighted root mean square scatter about the best fit line through the daily position

estimates into the error model for a site is a useful way to represent site-specific errors.

Chapter 2 continues with a detailed analysis of the rigidity of the North American

plate utilizing over 80 continuously operating GPS sites that are both geodetic and

non-geodetic quality. The robustness of the solution for the angular velocity of North

America is assessed by using a standard statistical measure of data misfit, X2 per

degree of freedom (X2), which in this case describes the misfit o f the velocity data to a

given rigid plate model. A number of related issues are examined, including glacial

isostatic adjustment, rigidity of the Colorado Plateau and related extension across the

Rio Grande Rift, and possible differential motion across the New Madrid seismic

zone.

Chapter 3 expands the discussion of areas adjacent to rigid North America by

focusing on the deformation in the northern Gulf of Mexico with respect to rigid North

America. GULFNET is a regional GPS network created in 1997 in the northern Gulf

Coast to measure strain across this active “passive margin”. This network consists of

16 continuously operating GPS sites established 2-4 years ago by a number of

different agencies, principally for navigational purposes, 3 continuous sites established

this year by LSU, and 24 episodic sites first occupied in 1997-1998 by the author.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Preliminary results focusing on the continuous sites that have been operational for

more than two years suggests that deformation in selected locations is in accord with

predictions based on the regional paradigm for deformation, i.e. Gulfward motion

along normal faults. However, velocities are not yet significant at most sites at 95%

confidence. Many of the sites established for navigation and meteorological purposes

cannot be used as they are located in areas of well documented fluid withdrawal and

reflect local motion. Improving the understanding of deformation in the Gulf region

will require re-occupying the much denser network of 26 episodic GPS sites, which

are mostly located in central and southern Louisiana.

In Chapter 4 the methods and analysis used to investigate the rigidity of the North

America plate (which were already described in Chapter 2) are applied globally to

define and calculate angular velocities for all but three plates. This analysis results in

a new model for Recent (Holocene) time global plate ve/ocities (REVEL) that

describes the relative velocities of 14 plates. A detailed comparison of the predictions

made by REVEL and independently observed geologic estimates is made for most of

the plate pairs. For several plate pairs, statistically significant differences are observed

between the geodetically and geologically determined velocities reflecting changes in

plate velocity through time. Some of the rate differences reflect gradual slowing

associated with convergent plate boundaries, and may reflect increased resistance to

subduction or convergence associated with decreased slab pull and/or crustal

thickening. For example, Nazca plate motion relative to South America manifests a

directional change to a more easterly azimuth. This is consistent with pivoting of the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. plate about its southeastern boundary with South America, where the youngest

lithosphere is being subducted and slab pull is weakest.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2: RIGIDITY OF NORTH AMERICA

Introduction

The extent to which plate interiors are rigid is an important kinematic boundary

condition for a wide range of geophysical studies (for examples see Gordon, 1998). It

is also important for geodetic studies, where it is often useful to reference the motion

of one or a group of sites to an adjacent stable block or plate interior. Plate rigidity

also bears on the understanding of intraplate seismicity such as the enigmatic large

earthquakes during 1811-1812 in the New Madrid region of North America [Nuttli,

1982].

Argus and Gordon [1996] and Dixon et al. [1996] investigated the rigidity of

North America using space geodetic data from a limited number of sites. Since that

time, the number of semi-permanent, continuously operating Global Positioning

System (GPS) sites has greatly increased, and the accuracy of their site velocity

estimates has improved, thanks to longer time series and improved analytical

techniques. Several published studies on plate kinematics and plate rigidity are now

available, and demonstrate root mean square misfits between measured space geodetic

site velocities and predicted velocities from rigid plate models at the level of —1-2

mm/yr or better for most major plates [e.g., DeMets and Dixon, 1999; Kogan et al.,

2000; Beavan et a l, 2000]. This suggests that the rigid plate approximation is valid to

about this level, at least on the decadal time scale inherent in these geodetic

measurements. To the extent that this is true, one could simply assume plate rigidity,

and use deviations from rigid plate predictions to explore sources of noise in geodetic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. data, some of which are not well characterized. With this approach, however, it is

difficult to investigate possible deviations from plate rigidity, such as the intraplate

seismicity at New Madrid without circularity. If space geodesy is to be useful for

addressing true non-rigid plate behavior in the intraplate setting (e.g., New Madrid),

where strain rate magnitudes are probably low, at or below the detection threshold of

current space geodetic techniques, it seems appropriate to first assess all possible noise

sources in such measurements, as well as all other geophysical processes likely to

affect the data.

This study constitutes a step in that direction, and builds on earlier efforts at noise

characterization [Mao et al., 1999]. In contrast to that study, which focused on the

numerical analysis of individual time series, a more empirical approach is used here,

exploiting data from over 80 stations that are currently in operation on stable North

America, mainly for applications in navigation and meteorology. While not

specifically designed for high precision geodetic measurements, these stations may

nevertheless prove to be useful for geophysical applications, provided that their noise

characteristics are understood. In this study the impacts on data quality between

geodetic and non-geodetic quality stations is assessed utilizing the weighted root mean

square about the best fit velocity line.

These data are used to investigate a number of related issues, including data noise,

isostatic adjustment (e.g., rebound) from the last cycle of Pleistocene glaciation,

subsidence and growth normal faulting along northern coastal Gulf of Mexico, rigidity

of the Colorado Plateau and related extension across the Rio Grande Rift, and possible

differential motion across the New Madrid seismic zone.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Data Collection and Analysis

Observations from 129 semi-permanent, continuously recording GPS stations

distributed throughout North America were used in this study (Figure 2.1). The

stations are operated by various agencies for navigation, near-real time meteorology

and geophysical applications, and are generously made publicly available through the

efforts of many individuals and institutions.

Accurate coordinate time series are critical for a study like this. Uncertainty in the

positions of the GPS satellites is a major error source for such data. Beginning in

about 1993, the global tracking network for the GPS satellites became sufficiently

robust that satellite ephemerides (satellite positions as a function of time) became

significantly more accurate compared to earlier periods. The data used span the time

period from January 1, 1993 to June 1, 2000. To ensure consistency, all data were re

analyzed specifically for this study, resulting in a uniform set of site velocity and error

estimates. All the sites described in this chapter are continuous, in the sense that they

record data for 20-24 hours per day, typically for at least 300 days per year

(Appendix). For more than 98% of the continuous sites presented here, all known

existing data were processed. For the remaining sites, there may be additional data

that was not available at the time of this study. The processing strategy generally

follows procedures outlined in Dixon et al. [1993, 1997], although some of these have

been updated considerably. Salient points are listed below:

1) GIPSY-OASIS v5.0 software developed at the Jet Propulsion Laboratory (JPL) and

non-fiducial satellite orbit and clock files provided by JPL [Zumberge et al., 1997]

were used.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a Dom Margolin ■ non-Dom Margolin I Rigid Plate O Sites within 1,800 km of the center of Hudson Bay

Figure 2.1. Site locations and antenna, types (See Appendix for coordinates and site names). Projection is Lambert azimuthal equal area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2) Ionosphere-free combination of both undifferenced carrier phase and P code pseudo

range data (weights of 1 cm and 1 m, respectively), typically recorded at 30 second

intervals, with a 5 minute decimation rate were used.

3) A new comprehensive antenna height and type change file was created going back

to. 1993 for all stations, and used the model values of Mader [1999] for first order

antenna phase center corrections.

4) To improve the definition of horizontal atmospheric gradients and reduce their

effect on the final position estimates, an elevation angle cutoff of 10° (where data are

available) and the horizontal gradient model of Bar-Sever et al. [1998] were used. The

same random walk model for the zenith delay as Bar-Sever et al. [1998] (3 mm/Vhr)

was used, but a looser constraint for the two orthogonal horizontal gradients (5

mm/Vhr) was used. The mapping function of Neill [1996], which describes how the

average atmospheric path delay varies as a function of elevation angle was used.

5) All sites were corrected for ocean tidal loading, using the NLOADF program of

Agnew [1997], and the Schwinderski ocean tide model.

6) Carrier phase cycle ambiguities were estimated, not fixed.

7) Offset parameters at the date of each change of antenna height and model type to

correct for second order effects such as inaccuracy of phase center estimates and

incorrect records of changes in antenna height were estimated; these can exceed 1 cm

in the vertical component but generally are a few millimeters or less for horizontal

components.

8) Daily position estimates were generated with loose constraints [Heflin et al., 1992;

Blewitt et al., 1992], then transformed to International Terrestrial Reference Frame

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (ITRF-97) [Boucher et al., 1999] using up to 51 co-located sites whose positions are

defined in ITRF-97. The number of co-located sites depends on station availability.

For 1998 and later this usually exceeds 45, but in 1993 for example, it was typically

15-20. Transformation parameters are computed each day, hence each daily position

estimate is essentially independent.

Velocity estimates are based on a weighted least squares line fit to the daily

position estimates, including the offset parameters described above (Figure 2.2). In

some cases a station was moved or offset during the 1993-2000 period of this study

(the term “station” refers to a specific equipment installation, and “site” refers to one

or more stations in a given limited geographic region). When it was possible, a

publicly available vector tie information (“site ties”) was used to link the two time

series. When this was not possible the two time series were simply concatanated and

an offset, as described for antenna height and model changes, was calculated. Station

names for such linked time series are indicated by the letter “Z”, e.g., site RCMZ

represents the combination of time series from stations RCM5 and RCM6.

The velocity error estimates account for both white (uncorrelated) and coloured

(time-correlated) noise [Langbein and Johnson, 1997], and are modeled following

Mao et al. [1999], This approach is based on a numerical analysis of 23 globally

distributed GPS time series for the period 1994-1997, and avoids any assumptions

concerning fit of data to a particular geophysical model (e.g., a rigid plate model), and

thus provides independent error estimates. To account for improvements in analytical

techniques since 1997 and site-specific effects, the correlation between WRMS (the

weighted root mean square scatter of the daily position estimates about a best fit

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 80 L a titu d e

-40

Slope= 0.8 ± 03 mm/yr WRMS= 2.6 mm x j = 0.89 -80 — ^whiic= mm ^nicker= 3-1 mm . i i ” ] i j i

80 " Longitude

-40

Slope=-13.9 ±0.4 mm/yr WRMS= 4.5 mm y j. =0.79 * -80 ^whiic — nim ^nicker= mm

-40

-80 Slope= -0.4 ±1.1 mm/yr WRMS= 8.9 mm x? = 0-85 m • 5.8 mm ^flicker = 11.4 mm

i — 94jan01 95jan01 96jan01 97jan01 98jan01 99jan01 OOjanOl

Figure 2.2. Example coordinate time series for station GODE. Site velocity is given by slope of weighted least squares line fit through the data, and velocity error is slope uncertainty, accounting for uncorrelated and time-correlated error, time span of observation, and total observing time.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. straight line) and white and flicker noise amplitudes observed in the data o f Mao et al.

[1999] is exploited, as outlined in Dixon et al. [2000].

The site velocity data with respect to ITRF-97 is then inverted to derive best fit

angular velocities for North America, termed here ITRF-97 angular velocities

(Appendix), minimizing the weighted, least squares misfit to the data for the plate as

described by Ward [1990] and Mao [1998]. Relative angular velocities for pairs of

adjacent plates (e.g., Colorado Plateau-North America) are then derived by

differencing the ITRF-97 angular velocities, with appropriate error propagation.

A plot of site velocities relative to ITRF-97 (Figure 2.3) bears a superficial

resemblance to the velocity field predicted by the NUVEL-1 No-Net-Rotation

(NUVEL-1-NNR) velocity field [Argus and Gordon, 1991] or its updated version,

NUVEL-1 A-NNR. Inasmuch as the NUVEL-1 A geologic model may be a biased

representation of Recent plate motions for some plate pairs, as described earlier, it may

be inappropriate to use NNR as a reference frame for geodetic observations. Previous

versions of the ITRF have been strongly tied to NNR, although recent versions of the

ITRF are believed to be free of biases associated with any particular plate motion

model [Sillard et al, 1998]. In this approach, ITRF-97 is treated as an arbitrary initial

reference frame, differenced out in the subsequent calculation of relative angular

velocities. Hence any bias in the ITRF should not influence the results. The success

of this approach can be assessed in two ways:

1) The orientation of residual velocities on a given plate should be random, and their

average magnitude should be similar to the average uncertainty of the site velocities,

assuming that the chosen sites truly represent the stable plate interior, far from

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. <3,

Stable plate site -*— 10 mm/yr

o e O o S>oo o CN CN o

Figure 2.3. Site velocities relative to ITRF-97 for 81 sites on rigid North America. Earthquake epicenters (magnitude > 4.5, depth < 50 km, between 1973-2000) suggest plate boundaries. Projection is Lambert azimuthal equal area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deforming plate boundary zones, and assuming that no biases have been introduced by

this procedures (e.g., Figure 2.12);

2) The predicted motions from the estimated relative angular velocity along a given

plate boundary can be compared to independent geologic and geodetic data.

The first test is applied in this chapter, and in Chapter 4 both test are applied and

suggest that.this approach is valid. Briefly, residual velocities for all plates are

randomly oriented in azimuth and have magnitudes that are near the expected error

level given observational error.

Unless otherwise stated, random walk noise is set to zero in the calculation of the

velocity errors and all uncertainties in the tables and text represent one standard error,

while all error ellipses in figures represent two-dimensional 95% confidence regions

(2.45 times the one-dimensional one standard error).

Results and Discussion

Several versions for defining the angular velocity of rigid North America, based on

various data subsets, are defined in this section. Rigid North America is defined by

omitting all stations that might be contaminated by non-rigid plate behavior. In this

study all stations west of the Rio Grande Rift are excluded because of possible

ongoing extension and Pacific-North American related deformation, all stations along

the Northern Gulf of Mexico are excluded because of possible subsidence effects and

Gulfward directed transport due to growth normal faulting, and all stations within a

radius of 1,800 km of the center of Hudson Bay are excluded since these are most

likely to have significant glacial isostatic adjustment effects. A series of definitions

that utilize different groupings of these stations are used throughout this chapter. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. models are evaluated with a standard statistical measure of data misfit, X2 per degree

of freedom (hereafter X 2), which in this case describes the misfit of the velocity data

to a given rigid plate model:

where O,- is a velocity observation (north or east), C,- is the calculated velocity at the

same site based on a given angular velocity, Oi is the velocity error, N is the number of

data, and 2N-3 is the number of degrees of freedom (number of data minus number of

adjustable parameters, 3). The minimum Xv2 indicates the best fit model. Values of

Xv2 —1.0 indicate a good fit of data to model and suggest that error estimates are

reasonable. X 2 < 1.0 suggest that errors are overestimated, while X 2> 1.0 suggests

either that errors are underestimated or that a given model poorly fits the data.

The best definition for the angular velocity of rigid North America incorporates 64

sites and results in X 2 — 1.04 (close to 1.0) and a mean rate residual of 1.03 mm/yr

(about the mean error level), suggesting that to a first approximation, the error model

is approximately correct and the velocity data are adequately fit by the rigid plate

model. The site coordinates, velocities with respect to ITRF-97, and residual

velocities with respect to the 64 site core definition of rigid North America are given

in the Appendix.

GPS velocity error and random walk noise. Since GPS velocity errors are a

strong function of total observing time [Mao et al., 1999], one way to check this

assumption that the velocity residuals mainly reflect observational error is to restrict

the data set to sites with progressively longer time spans, and investigate the effect on

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. residuals. For example, using only time spans of 7.0 years or longer (8 sites) gives a

mean rate residual of 0.67 mm/yr. The trend of decreasing rate residuals with longer

data time span supports the inference that the rate residuals reflect GPS velocity errors

rather than true non-rigid plate processes. Thus the rate residuals represent an upper

bound to the actual level of plate rigidity. Rate residuals are positively biased (see

also Argus and Gordon [1996] and Argus et al. [1999]), implying that the actual level

of plate non-rigidity that can be demonstrated with a given data set will be higher than

the mean rate residual. Presumably this bound will be better constrained in future

years as GPS velocity errors decline further as a result of longer observation times.

Figure 2.4 groups the sites on stable North America by amount of observing time

and separates them into two groups: One set includes only sites included in the core

definition of North America (64 sites) (Circles), and the second site includes also those

sites within a radius of 1,800 kilometers of Hudson Bay (Diamonds). When the trend

o f X v2 values is examined, the expected trend of decreasing X v2 with increased

observing time is not seen. The stations with the longest observing times and the

smallest rate residuals tend to have Xv values » 1.0, e.g.,Xv = 2.52 for the solution

based on 8 stations with more than 7.0 years of data. This suggests that the velocity

errors for these stations have been underestimated, the effect of which is presumably

“swamped” in the general solution because most of the stations have observing times

of less than four years. Increasing the average velocity error by about a factor of 1.6

results in Xv —1.0. Mao et al. [1999] pointed out that for the GPS position time series

they analyzed, the effect of random walk noise, which has a time dependence such that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Time (yrs) 0 1 2 3 4 5 6 7 8 1.5 1.0 2.0 0.5 0.0

Time (yrs) 0 1 2 3 4 5 6 7 8

1.5 1.0 1.0 3.0 2.5 2.0 0.5 0.0 Figure 2.4. sites from the 64 site model, and meanGIA affectedrate residuals sites versusexcluded. total observing Diamonds time includes for sitesGIA affectedlocated sites,on rigid81 North site model.America. Circles include only

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the power spectra in the frequency (f) domain is inversely proportional to f 2, will only

be apparent for time series longer thian about 4 years. Prior to that time, the velocity

errors are dominated by white noiise (no time dependence) and flicker noise (1/f

dependence). It is important to mote again that the velocity errors listed in the

Appendix are calculated assuming tlnat random walk noise is zero. Setting the random

walk noise magnitude in equation (20) of Mao et al. [1999] to 1.2 mm/Vt, where t is

time, has the effect of increasing the average velocity error for a 7.0 year time series

by about 0.3 mm/yr, from ~0.5 mm/yr to -0.8 mm/yr, and gives X 2 -1.0 for the 8

station solution.

Spurious local motion of the gecodetic mark (ground noise) is an important noise

source for displacement-type geodetnc techniques such as GPS, and has been modeled

as a random walk process [Johnson ^and Agnew, 1995; Langbein and Johnson, 1997].

Monument noise may be especially significant for the data considered here, because

the majority of sites do not include deeply anchored monuments or piers, which can

reduce this noise [Langbein et al., 1995]. In a study employing decade-long time

series of two-color EDM data, Langbein and Johnson [1997] estimated random walk

noise magnitudes of 1-4 mm/Vyr, and related this to monument noise. However, the

monuments of the 8 GPS stations considered here are not necessarily better in terms o f

reduced monument noise, compared to the monuments studied by Langbein and

Johnson [1997], because a dedicated numerical analysis of these 7 year time series has

not been done, i.e., simultaneously solving for white, flicker and random walk noise

magnitudes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Antenna effects. The data used in this study derive from stations which have a

wide variety of equipment types, local site characteristics, and applications ranging

from navigation to meteorology to high precision geophysical applications. An

important question is whether this variety impacts data quality and velocity error in

any significant way.

With so many variables, it is difficult to come up with objective tests that

effectively isolate the effect of one variable from all the others. The antenna type was

chosen as the principle variable to examine, because this information is readily

available. The Dorn Margolin antenna with “choke ring” backplane for multipath

suppression is a widely used antenna, and is believed to be superior in some respects

compared with other designs for high precision applications. To examine the effects

of antenna type under a variety of conditions stations located anywhere on the North

American continent were included that met the following criteria: 1) stations that were

operational between July 1, 1997 and July 31, 2000 (all data collected before and after

these dates was excluded); 2) stations that maintained the same model antenna during

this period; 3) stations that were operational for at least 20 hours a day. This resulted

in a total of 120 stations that met the three criteria.

Figure 2.5 shows the WRMS scatter for Dorn Margolin versus non-Dorn Margolin

time series for the three orthogonal positions. There is a clear tendency for time series

of sites using Dorn Margolin antennas to exhibit lower scatter. Similarly if the data set

is restricted to only those stations that lie on rigid North America (81 sites) 50 possible

sites are available and these show a similar distribution (Figure 2.6). However,

inspection of the station locations for these time series indicates a strong geographic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 - N = 76 DM N = 5 4 non-DM M =2.4 North M =2.6 North 20 : o 16 - c o 12 : zE 8 - 4 :

0 : 1 2 3 4 5 1 2 3 4 5 WRMS (mm) WRMS (mm)

24 n N = 76 DM N = 5 4 non-DM 20 M = 4.4 East M =4.7 East o 16 - j- . c n

= 8 Z 4

0 - 2 4 6 4 6 WRMS (mm) WRMS (mm)

N = 76 DM N = 5 4 non-DM M =8.3 Vertical M =8.9 Vertical

i11111111 ■111111111 ■ i 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16 WRMS (mm) WRMS (mm)

Figure 2.5. Histogram of WRMS values for north, east and vertical positions for all sites on North America (for a time-series from 1 July, 1997 to 31 July, 2000, and each station must have maintained the same antenna model during this time frame) and separating the sites between those with a Dorn Margolin antenna and those without (non-Dorn Margolin) antennas. N is number of sites, M is mean.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 N =23 DM N = 27 non-DM 12 - M =2.4 N onh M =2.6 North 3 10 - c J 6- 4 -

7 -

m m ., 1 2 3 4 1 2 3 4 5 W RMS (mm) W RMS (mm)

N =23 DM N = 27 non-DM M =4.5 East M =4.7 East 6 -

I i 111 4 6 2 4 6 WRMS (mm) W RM S (mm)

10 N =23 DM N = 27 non-DM M =8.4 Vertical M =9.2 Vertical

o ■£ 4H

2 - 0 ' ' ' i'' ' i ■'11’ it m , 0 2 4 6 8 10 12 14 o 2 4 6 8 10 12 14 16 WRMS (mm) WRMS (mm)

Figure 2.6. Same as Figure 2.5. but for sites on the rigid North American plate including sites within a radius of 1,800 km of the center of Hudson Bay (58°N, -83°E) (Sites with circles or diamonds around them on Figure 2.1).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. bias: in this data set, the majority of stations equipped with non-Dorn Margolin

antennas are located in a band from the Great Lakes to Texas (Figure 2.1). It is

possible that meteorological conditions rather than antenna type explains the

difference.

To test this, pairs of stations in the same geographic area were compared, where

one of the stations used a Dorn Margolin antenna and the other used an antenna of

different design. From the potential pool of 120 stations, 24 pairs of stations were

identified that were grouped into pairs within 0-100 km, 101-200 km, and 201-300 km

of one another. Figure 2.7 shows the WRMS scatter of the vertical velocity for the

different pairs and again a clear bias is seen towards lower WRMS values for stations

equipped with Dom Margolin antennas.

An additional test was done to evaluate if different monument types produced

differences in WRMS values at sites equipped with Dom Margolin antennas. The

sites were separated according to two different types of monuments: sites were the

antenna was attached to a building or a pillar/rod, and sites which used a braced

monument (Langbein et al., 1995). The sites were limited to those located in the dry

deserts of the southwestern US, to reduce both meteorological and geographic effects.

There are 32 sites that meet these criteria in this area. Figure 2.8 shows the results of

this test, which indicate that given the short three year time series evaluated here it is

not possible to differentiate any reduction in noise between such monument types.

In conclusion, significantly improved solutions are seen at those sites using Dom

Margolin antennas, although the contribution of monumentation could not be assessed

in this study. Non-Dom Margolin antenna equipped sites yield useful data, especially

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 s s , c3 14 e e 43 C < 12

10 o CO s 8 ^ 0 < pair < 100 km □ 100 < pair < 200 km £ A 200 < pair < 300 km 6 8 10 12 14 16 WRMS DM Antenna (mm)

Figure 2.7. WRMS plot for pairs of stations, one with a Dom Margolin, antenna and one with a different antenna, grouped by distance between stations.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

8 8 10 Vertical WRMS (mm) I111!I 1.111111111111111 111. I! II1111111! 0 East I liSMi WRMS (mm) TTTTTTTTTTTTTTTT 0 North | I I I I I I I I I | I T'l I I | I II | I I I T'l I I I I I I I I I I WRMS (mm) Braced N=21 Building N=11 1 M I I M 1 H 1 M I I M 1 0 8 8 - 0 —* 4 ■ I 4 I Z, * Figure 2.8. building WRMS or pillar. plot N foris numberstations of stations.in the southwestern USA separated into sites with braced monuments and sites attached to a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in the horizontal, although it tends to be noisier. It is therefore useful to weight the

velocity error at a specific site by its WRMS to obtain a more representative error.

Glacial isostatic adjustment. The North American lithosphere is isostatically

adjusting from the mass loading effects of the last glaciation, reflecting the delayed

response of the earth’s mantle associated with viscous flow. This adjustment imparts

both vertical and horizontal motions to the surface velocity field [e.g., Peltier, 1998b]

and needs to be considered when using geodetic data to constrain plate rigidity,

because the velocity effects measured over a decade may not be representative of

longer term behavior. That part of the surface velocity field due to glacial isostatic

adjustment (GIA) is a function of both the ice loading history and the mantle viscosity

structure, neither of which is well known, and corresponding model predictions can

vary significantly [e.g., Argus et al., 1999; Milne et al., 2000; Mitrovica et al., 2000].

However, most models agree on the region of maximum glacial isostatic effect, even if

they do not agree on the magnitude or direction of predicted motion. For North

America, rather than correcting each site by a specific model prediction, a conservative

approach is taken, and the subset of sites most likely to be affected by GIA are

excluded when defining the “core” solution. The motion of other sites (including

those likely to be affected by GIA) relative to this core solution can be evaluated to see

if a pattern exists that may plausibly be ascribed to GIA. In North America a broad

area around Hudson Bay is the region o f maximum glacial isostatic adjustment-related

motion. The ICE-4G model [Peltier, 1994] was used to identify sites with GIA-related

horizontal rates > 0.7 mm/yr, and this resulted in a geographic area that encompassed a

1,800 km radius from the center of Hudson Bay. Seventeen sites are located within

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. this GIA affected circle: ALGO, CHB1, CHUR, DET1, DUBO, FLIN, KEW1, MILL

NRC1, SAG1, SCH2, STB1, STP1, WHPI, WIS1, YELL, YOU1. Of these 17 sites, 8

exhibit statistically significant uplift (rebound), and 5 exhibit uplift rates greater than

3.0 mm/yr (Appendix). Sites influenced by GIA may also subside as a result of the

collapse of a peripheral bulge, but associated horizontal velocities are predicted to be

small compared to the observational error (e.g., less than 0.5 mm/yr in ICE-4G). The

center of Hudson Bay is close to the ice thickness maximum, and thus likely close to a

rebound maximum. Figure 2.9 shows the vertical velocity of GPS sites as a function

of distance from central Hudson Bay and it is clear from this figure that this process

affects the vertical velocities. Figure 2.10 plots the measured vertical velocities

against predicted vertical velocities from two recent GIA models [Peltier, 1994; Milne

et al., 2000]. While neither model exactly matches the data, there is a strong

correlation.

Figure 2.11 shows the horizontal residuals for the 17 stations that lie within a

radius of 1,800 km of Hudson’s Bay. While there is a general tendency for these

velocity residuals to exhibit the expected general pattern of outward flow, a good

match between these data and either of the two GIA models mentioned above is not

seen. One problem with a comparison of this type is that the GPS residuals indicate

differences from the core solution, which is itself derived from site velocities that may

include a small GIA velocity component (this procedure reduces but does not

eliminate the problem). Also, the predicted horizontal motions associated with GIA

are small and near the noise level of current GPS velocities.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

1,000 1,000 1,200 1,400 1,600 1,800 Distance from center ofHudson Bay (km) 0 200 400 600 800 800 600 400 0 200 8 6 2 0 4 -2 -4 -6 -8 14 12 10 -10 O o

copyrightowner. Further reproduction prohibited without permission. 16 p

C3 14 c c 4—*

Qi s 10 o CO s 8 + 0 < pair < 100 km Pd □ 100 < pair < 200 km £ A 200 < pair < 300 km 6 8 10 12 14 16 WRMS DM Antenna (mm)

Figure 2.10. Measured vertical velocity versus predicted vertical velocity for sites within a radius of 1,800 km of the center of Hudson Bay (58°N and -83°E). The predicted vertical velocities were obtained by interpolating the contours on Figure 3 of Peltier [1998a] to the site locations and from Milne et a l (2000).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

} $ J A. Residual velocity Residual 2m m /yr $ 40° Figure 2.11. Site velocities relative to rigid North America for sites within a radius of 1,800 km ofthe center of Hudson Bay (58°N and -83°E). Projection is Lambert azimuthal equal area. 10 VO

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. To see if the GIA sites affect the model for defining the rigid North American plate

the X 2 using all 81 sites was compared with a model which excluded the 17 GIA sites,

namely a 64 site model. Excluding the 17 GIA affected sites resulted in a reduction of

Xv2 from 1.34 to 1.04, clearly indicating that the solution is sensitive to GIA. The

residual velocities for the 64 site solution is shown in Figure 2.12, and the vectors

show a clear random pattern as would be expected if the sites truly lie on a rigid plate.

In contrast the GIA affected site vectors show an outward directed pattern from

Hudson Bay (Figure 2.11).

Rio Grande Rift and Colorado Plateau. The origin of the Rio-Grande Rift is

generally thought to be related to the clockwise rotation of the Colorado Plateau with

respect to rigid North America (Hamilton, 1981; Gordon et al., 1984; Bryan and

Gordon, 1986; Steiner, 1986). However considerable controversy remains on both the

position and rate of the angular velocity of the Colorado Plateau with respect to North

America and if extension is currently active (Tweto, 1979; McCalpin, 1982; Seager et

al., 1984; Chapin and Cather, 1994). Combining 5 GPS and 3 VLBI sites that lie on

the Colorado Plateau results in a X 2 = 1.3, strongly suggesting that the Colorado

Plateau is rigid. A simple F-test (Stein and Gordon, 1984) to compare if the Colorado

Plateau can be resolved as a separate block from rigid North America fails. This

implies that within the uncertainties of the measurements it is not possible to observe

any currently active extension across the Rio Grande (See the Appendix for residual

velocity values with respect to rigid North America for Colorado Plateau sites).

New Madrid Seismic Zone. A variety of mechanisms have been proposed to

explain the New Madrid seismic zone that include both plate scale compressive

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Residual velocity 2m m /yr A ------>

Figure 2.12. GPS site velocities relative to stable North America excluding sites within 1,800 km of the center of Hudson Bay (58°N and -83°E). (See Appendix for velocities). Projection is Lambert azimuthal equal area. stresses as well as local sources of stress. Both plate scale and local mechanisms have

been tested using GPS (Liu et al., 1992; Dixon et al., 1996; Weber et al., 1998;

Newman et al., 1999). Local GPS surveys suggest that strain accumulation may be

occurring at very low rates whereas plate wide GPS studies are unable to detect any.

All studies recognize that strain may be accumulating but at rates below the limit of

the GPS resolution at the time of the studies. Similar results are seen in this study

when examining plate-wide sources of deformation. An F-ration test [Stein and

Gordon, 1984] was done comparing the Xv values that results from separating rigid

North America into two blocks both along the northeast-southwest trend of the strike

slip faults and northwest-southeast along the strike of the inferred thrust fault at New

Madrid. Both groupings result in block motion that is indistinguishable from zero at

99% confidence.

Conclusions

The robustness of the GPS velocity estimates has reached a level where they are

now sensitive to GIA. Sites located in areas where GIA models have predicted the

greatest horizontal and vertical velocities should be excluded from definitions of rigid

plates. This is indicated by the improvement in the Xv2 from 1.34 to 1.04 for rigid

North America by excluding sites within a radius of 1,800 km of the center of Hudson

Bay. The best angular velocity solution for North America using a total of 64 sites has

a mean rate residual of approximately 1 mm/yr (1.03 mm/yr).

It is not possible to identify any far field effects that may influence the New

Madrid seismic zone. The Colorado Plateau appears to be rigid, and no motion can be

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. resolved with respect to rigid North America, suggesting that the Rio Grande Rift is

not currently extending.

The error model used in this study (Mao et al., 1999 and Dixon et al., 2000)

underestimates the random walk component for longer time series, however this was

indirectly taken into account in its original formulation by a slight increase in the

estimates of flicker and white noise components. There is a significant improvement

in those sites using Dom Margolin antennas, although it is not possible to assess the

contribution of monumentation. Non-Dom Margolin equipped sites yield useful

velocities, especially in the horizontal, although they tend to be noisier and therefore it

is important that the velocity error model used takes into account the WRMS of each

site. For sites equipped with Dom Margolin antennas improvements in data quality

reflecting the use of braced monuments was not detected. However, it is important to

remember that this study only evaluates a three year time series, and it is expected that

the benefits of braced monuments will become apparent in studies conducted over

longer periods of time.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3: GULFNET AND REGIONAL DEFORMATION OF THE NORTHERN GULF OF MEXICO

Introduction

Although the Gulf region is topographically subdued and almost aseismic, it has

been long held that it currently deforms by active normal faulting and salt migration.

Detailed subsurface structure mapping, stratigraphic correlation, and reflection

seismology have been used to achieve a detailed understanding of the structural and

sedimentary evolution of the region over geologic time scales (e.g., Fisk, 1944;

Atwater and Formann, 1959; Murray, 1961; Martin, 1978; Anderson, 1979; Salvador,

1991; Worrall and Snelson, 1988; Saucier, 1994). What is still not understood is the

yearly to decade-long behavior of faults and salt movement that appears to control

much of the strain that occurs in the region.

Progradation of five major delta complexes over the past 7000 years are

responsible for the formation of coastal Louisiana [Fisk, 1944; Kolb and Van Lopik,

1958; Frazier, 1967]. This long period of sedimentation has resulted in the formation

of vast expanses of marshlands separated by active and abandoned distributaries.

Since the early 1900s, land building processes have gradually diminished and land loss

has resulted in the net removal of 1,500 square miles of land [Britsch and Dunbar,

1993; Gagliano et al., 1981]. Peak rates of land loss [42 square miles/yr] occurred

between 1958 and 1974 and have now decreased to 25-29 square miles/yr [Dunbar et

al., 1992; Britsch and Dunbar, 1993].

The current public policy debate on solutions to the problem of wetland loss is in

great need of accurate and precise modem rates of subsidence. Growth faulting, salt

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. domes, compaction, and crustal flexuring resulting from sediment loading are likely to

be the main factors responsible for wetland loss [Boesch et al., 1983, 1994]. Despite

the long standing recognition of the role that subsidence plays in wetland loss [Fisk,

1960], evidence of the amount of subsidence remains poorly constrained, and comes

principally from tidal gauge stations located along the coastal edge, with limited

information inland.

In 1997, a regional high-precision GPS network GULFNET was established to

measure horizontal and vertical strain across the Northern Gulf of Mexic, focusing on

southern Louisiana. GULFNET is a joint project of Louisiana State University and the

University of Miami with funding from the National Science Foundation and

Louisiana Board of Regents. The geometry of the network was guided by the geologic

paradigm for regional deformation, i.e., the distribution of active normal faults and salt

structures (Figure 3.1). The GPS network will provide the first comprehensive data set

on the amount and distribution of subsidence across the whole region. This

information can be integrated with other data sets to help assess existing and future

remediation and coastal stabilization programs, as well as improve the understanding

how different man-made and natural processes contribute to land and wetland loss and

gain. In addition the strain measurements can be integrated with simple models of

elastic strain accumulation or aseismic creep.

Previous Studies

Most areas currently monitored with GPS are seismically active, characterized by

rapid horizontal motion with a much smaller vertical motion. Geology textbooks

commonly describe plate margins, such as the Gulf Coast of North America, as

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ,:-«v . . A L L A A B A M A ''N, MISSISSIPPI . .

i. AMANSAS / * > ^ V « v > upu^ s /4 / / m M ^ 5 X.'X X.'X ^ m T E E T

r w 'p ' . •„ •„ . •vSjr OKLAHOMA r28* ./<> r28* L> L> Figure 3.1. Regional tectonic map ofthe northern Gulfof Mexico from Worrall and Snelson, 1988. 0\ OJ

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. “passive”, or quiescent, based on the lack of earthquakes (Forlich, 1982; Strover et

al., 1988) and dynamic topography. Such characterizations are incorrect because they

ignore geologic and geodetic evidence of dynamic lateral and vertical deformation.

There are at least 4 major processes that contribute to surface displacements in the

Gulf region: 1) growth normal faulting, resulting in generally southward displacement

of coastal regions toward the Gulf; 2) the visco-elastic response due to lithospheric

loading of the delta region by increased sediment influx over the last 6,000-8,000

years associated with glacial retreat, resulting in depression of the delta and uplift of a

peripheral bulge; 3) sediment compaction, caused by dewatering and diagenesis of

Holocene sediments (older sediments also undergo these processes, but at slower

rates); 4) fluid withdrawal, (oil, gas, water) from anthropogenic activities, resulting in

localized regions of subsidence. Because GULFNET was designed to avoid the

effects of fluid withdrawal, it can provide insights into the first 3 processes.

The Gulf area is well suited for examining the question of active growth normal

fault behavior because it is the classic locality for such faults [Ocamb, 1961; Hardin

and Hardin, 1961; Seglund, 1974; Bally et al, 1981; Jackson and Seni, 1983; Xiao and

Suppe, 1989; Lopez, 1990; Nelson, 1991], Over 70 years o f oil and gas exploration

and production have resulted in a clear understanding of the three dimensional

structure and sedimentation associated with these faults. However, there are very few

constraints on the deformation rates characterizing this process, and it is also uncertain

if growth normal faulting is invariably aseismic. Characterizing the current

deformation rates and their spatial variations will permit reexamination of the

understanding of the processes that have over time created the geologic record.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Surprisingly, few studies characterize the magnitude of slippage along active

growth normal faults and their local surflcial effects with the exception of the

preliminary work of Thomsen, 1963; Hanor, 1982; Norman, 1994; and Lopez, 1996,

Lopez et a l, 2000. Only four faults, Lake Charles, Baton Rouge, Lake Pontchartrain

and Lake Hatch, have been studied. The Lake Charles fault has received limited

attention associated with a magnitude 3 earthquake which occurred in 1983 [Stevenson

and Agnew, 1988]. Existing conventional leveling data across the Baton Rouge fault

suggest that present-day slip is 3-4 mm/yr [Wintz, et al., 1970; Kazmann, 1970;

Whiteman, 1980]. On-going movement, apparently aseismic, along the Baton Rouge

fault is well documented by the partial collapse of a local high school gym located

adjacent to the fault on the footwall [McCuIloh, 1991] and in areas to the east (Saucier.

1994). Slippage along the Lake Pontchartrain fault has been linked to offsets in the

railroad bridge between Slidell and New Orleans [Lopez, 1996; Lopez et al., 1997].

Lopez et al. [2000] concluded that these vertical offsets suggest slippage at rates of 5-8

mm/yr. The Lake Hatch area has been studied by Keucher [1994].

Southeastern Texas, central Louisiana, and Mississippi are thought to reflect

modem uparching of the Gulf coastal plain in response to sediment loading [Jurkowski

et al., 1984; Sawyer, 1985; Dunbar and Sawyer, 1987; and Nunn, 1985, 1990; Sawyer

et al., 1991; Butler et al., 1995; Howell et al., 1995]. Currently, the only data

available on rate and location of crustal uplift is from regional leveling of the National

Geodetic Survey, who last surveyed the Gulf Coast in 1969 [Jurkowski et al., 1984;

Holdahl and Morrison, 1974]

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Most, if not all, estimates of subsidence in the Gulf Coast are based on the 85 tide

gauge stations located along the coast, 45 of which have records dating back more than

25 years [Penland and Ramsey, 1990; Turner, 1991]. Subsidence estimates from tide

gauge stations can be estimated by subtracting the global mean sea-level rise signal [1-

3 mm/yr; Pirazzoli, 1993], from the local measured relative sea level at a particular

tide gauge station. The “correction” results in an estimate of subsidence in Louisiana

o f —1 to 15 mm/yr [Penland and Ramsey, 1990; Turner, 1991; Paine, 1993].

GULFNET

GULFNET consists of a combination of episodic and continuous sites whose

placement and geometry were guided by the geologic paradigm for regional

deformation, i.e., the distribution of active normal faults and salt structures. In the

Northern Gulf Coast there are 16 continuously operating GPS sites that have been

established by a number of agencies for navigation and meteorological purposes.

Three additional high-quality geodetic sites have been recently established by this

author as part of GULFNET. O f the 19 continuous sites, 8 are located inland and the

rest are located within 150 km of the coast (Figure 3.2).

To create a denser network at low cost a series of episodic sites were established

using existing monuments. An episodic site is occupied only 2-3 days a year every 1

to 2 years for 8-24 hrs a day. Episodic site locations were chosen with the aim of

forming four north-south transects: 1) from Leesville to Lake Calcasieu; 2) from

Alexandria to Franklin; 3) from Baton Rouge to Cocodrie; 4) from Slidell to Venice.

Monuments were identified by searching the National Geodetic Survey (NGS)

benchmark database for all Class A benchmarks (NOAA, 1978). In the transect areas

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Florida 0 0 50 100 Alabama M O B I f t NDBC Mississippi LUMN WNFL Louisiana Episodic GPS Stations ♦ Select Tide Gauge Stations a ■ Continuous GPS Stations (USCG and USACE) Q New Continuous GPS Stations (LSU) PATT LKHU HOUS A R P 3 A U S 5 Texas ANTO CORC Figure 3.2. Location ofcontinuous and episodic GPS sites in the northern Gulfof Mexico. Projection is Mercator, O

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where less than 5 Class A benchmarks were found in the database, the search was

widened to include Class B benchmarks (NOAA, 1978). In both cases, only those

benchmarks that consisted of steel rods driven to refusal of at least 10 meters depth

were considered. Approximately 400 potential sites were identified. With the

descriptions of the location of the benchmarks, the laborious task of recovering them

and assessing a benchmarks suitability for GPS data collection was carried out in late

1996 and early 1997. For a recovered benchmark to be suitable for GPS data to be

collected it had to meet the following criteria:

1) The monument had to have an unobstructed view of the sky with no nearby trees,

buildings, or utility posts within 50 m, or fences higher than 1 meter.

2) The monument had to be undamaged and not in an area subject to erosion.

3) The monument had to be located within 100 m of an access road.

4) The monument had to be located > 5 km from any water and/or oil and gas wells.

The combination of non-recovery of sites and non-suitability of a site resulted in

less than 45 sites meeting all the requirements. Of these, 26 were incorporated in

GULFNET, complementing the 19 continuously operating sites (Figure 3.3). Table

3.1 lists the sites, their location and a brief monument description. The resulting

network has a geographical distribution that extends from the Mississippi delta

southeast of New Orleans, across to the Atchafalaya River delta and west to the east

side of Lake Calcasieu. It extends north into the Toledo Bend flexure area in Texas,

Louisiana and Mississippi.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 100 km 50 271'

~ 0 GRAN 0 ^ A GALL* INDP A hammq slid \ Ndbc HOUM 269° 270°

BSOR ERWI A FRAN ^ HEND a 268° O OPEL laff 267° Episodic GPS Stations CAMP ROCK A A ■ Continuous GPS Stations (USCG and USACE) O New Continuous GPS Stations (LSU) 29° 30° Figure 3.3. Location ofcontinuous and episodic GPS sites in southern Louisiana. Projection is Mercator, to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3.1. Episodic site coordinates, monument description, PID Number and year established.

S ite P o sitio n R od S le e v e NGS M a r k in g Y ea r ID N E len g th len g th PID E stab . m m

BSOR 30.13 -91.32 M* N o n e BJ0831 G293 1976 CAMP 30.01 -93.13 19.50 N o n e BK2288 LSUS9 1980 COLU 39.25 -85.90 BRONSOM 1986 COC1 2 9 .2 6 -90.66 26.80 N o n e A U 3558 B391 1993 EMPI 2 0 .3 8 -89.60 18.30 N o n e A T 1391 1207B 1985 ERWI 30.50 -91.32 12.20 N o n e BJ3695 L378 1986 ESTE 2 9 .8 5 -92.22 24 .4 0 N o n e A V 0572 F380 1986 FERN 31.18 -90.47 28 .9 0 0 .9 0 B W 2580 M 361 1992 FRAN 2 9 .8 0 -91.53 14.70 0 .9 0 A U 3537 Z 388 1993 FRIE 3 2 .2 4 -92.92 15.70 N o n e CQ 2461 V 330 1977 GALL 2 9 .4 5 -90.26 25 .0 0 0.91 A U 3376 FAAL49B 1991 GRAN 2 9 .1 9 -90.09 19.50 6 .0 0 A U 2047 U 358 1982 HEND 30.31 -91.79 24 .4 0 6 .1 0 B J3212 U 360 1982 HOUM 2 9 .5 7 -90.76 18.30 N o n e A U 2085 T 359 1982 INDP 30.65 -90.50 14.70 None BJ1744 K179 1959 LAPL 30.10 -90.44 18.30 None BJ3738 F381 1986 LEES 3 1 .1 7 -93.34 18.90 0.91 BX3184 FAALA24A 1991 LHER 2 9 .5 6 -89.88 22.00 N o n e AT1392 1602C 1985 LIVI 30.69 -95.02 22 .8 0 0.91 BL2336 FAA00RA 1991 OPEL 30.53 -92.06 14.60 9 .1 0 B K 2405 R378 1986 ROCK 29.69 -92.85 30.50 N o n e A V 0534 LSUBMGM9 1981 SLID 3 0 .3 4 -89.83 14.60 N o n e B H 1869 X 382 1986 VENI 2 9 .2 8 -89.36 30.50 6.10 AT0737 R367 1984 WOOD 31.13 -92.50 33.00 None BX1117 V262 1969

* Brass cap embeded in a massive lock structure.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Data Collection and Analysis

During 3 field campaigns in the spring of 1997, 1998, and 1999 episodic site data

were collected. The equipment used in all 3 campaigns were Trimble SSI receivers

with Dom Margolin choke ring antennas. Louisiana State University equipment was

supplemented by equipment from the University NAVSTAR Consortium (UNAVCO),

and the University of Miami Rosenstiel School of Marine and Atmospheric Sciences.

The basic methodology for the field campaigns was as follows:

1) Tripods equipped with tribrachs and optical plummets were centered directly over

the dimple of the benchmark.

2) Antennas were then attached to the tribrach and optical plummet mounted tripod

and oriented with magnetic north. The leveling and centering were verified and

adjusted as necessary.

3) The antenna height was measured with a height stick from the dimple in the

benchmark to the bottom of the phase plane of the choke ring at three different points

approximately 120 degrees apart. These measurements had to be within < 1 mm of

one another and if they were greater than 1 mm the antenna was re-centered and re

leveled.

4) At regular intervals during the period of data collection the centering of the antenna

was checked. If the antenna was not centered the session was terminated and the data

discarded.

5) When data collection was completed, the antenna centering was verified, and if the

antenna was not centered, the data collected was discarded. The antenna height was

also re-measured and if the three antenna height measurements differed by more than 1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. mm, the data was discarded. If the heights at the start and end of the session differed

by more than 1 mm the data was discarded.

Data collection was begun by 0800h or earlier and continued until 1800h local

time and was repeated for at least 2 days and often 3 days. For sites operated by the

author ( > 85% of the total sites) the data were usually collected in a single continuous

24-72 hour session starting at 0800h. In 1997 data was collected at 12 sites (CAMP,

COCO, ERWI, FRAN, HEND, HOUM, INDP, LEES, SLID, VENI, WOOD). In

1998 the 12 original sites and an additional 13 sites (BSOR, COLU, EMPI, ESTE,

FERN, FRIE, GALL, GRAN, LAPL, LHER, LIVI, OPEL, ROCK) were occupied. In

1999 the original 12 sites, excluding WOOD, which was destroyed, and one new site

(COLU), were occupied.

The data collected during each of these campaigns has been archived at UNAVCO.

Both the episodic and continuous data was processed at University of Miami using the

same methods as described in Chapter 2. Unless stated, all uncertainties in the tables

and text represent one standard error, while all error ellipses in figures represent two-

dimensional 95% confidence regions (1.7 times the two-dimensional one standard

error).

Results and Discussion

No discussion is given of the 11 original episodic sites whose positions were

measured in 1997, 1998 and 1999 as none have velocities that are significant at 60%

confidence. The residual velocities for the 16 continuously operating sites with

respect to the rigid North American plate (64 site solution) is shown Figure 3.4 and

listed in the Appendix. These sites provide limited information due to the fact that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Florida Alabaama MLFt ► --- MOB1 % Tennessee ---- NDBC ------270° Mississippi 2,3 mm/yr A Residual velocities with respect to rigid North America V IC l Louisiana Arkansas ft WNFL LKHU & PATT GAL1 HOUS ARP3 Oklahoma CORC 260° Texas Mexico 30° Figure 3.4. Residual velocities with respect to rigid North America for continuously operating GPS sites in the northern Gulf of Mexico. Projection is Mercator.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. most of the coastal sites are located in areas of known water withdrawal (Poland,

1984), and have high, > 5 mm/yr, negative vertical velocities (BEA5, CORC, HOUS,

LKHU). This suggests that the horizontal residual velocities may not reflect regional

strain effects, but rather local ones, and therefore are not discussed further. Of the

remaining 5 coastal sites (ARP3, ENG1, GAL1, MOB1, NDBC) that have vertical

velocities < 2 m m /yr, 4 of the sites ENG1, GAL1, NDBC and MOB1 show horizontal

residual velocities that are perpendicular and in a down dip direction to the strike of

adjacent normal faults. Although none of these sites show residual velocities that are

significant at 95% confidence, the direction of motion is compatible with slippage

along nearby normal faults that are sub-parallel to the coast. It should be noted that

MOB1, located at the entrance to Mobile Bay, has motion that is compatible with

slippage on the eastern, north-south striking, normal faults of Mobile Bay that that are

downthrown to the west [Jones and Freed, 1996]. ARPS, located in Aransas Pass,

Texas, has a horizontal residual velocity (7.2±1.0 mm/yr) and azimuth (269±4.7°) that

is perpendicular to the strike of nearby eastwardly dipping normal faults. Since this

velocity is significant at 95% confidence, it may reflect that ARPS is moving on an

antithetic fault, although this has not been verified by examining local seismic

reflection data.

Of the landward sites, 2 ANTO and PATT are used in this study to define the

angular velocity of the rigid North American plate (See Chapter 2 for a listing of sites

used to define the rigid North American plate). AUS5, located in Austin, Texas shows

a larger residual velocity than ANTO and PATT and its WRMS in all 3 components

are at the highest end of the histograms shown in Figure 2.5 (north 3.1 mm, east 6.5

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. mm, and vertical 16.8 mm). This suggests that this station is probably noisy due to

local effects, possibly reflecting poor site location. For this reason it is not discussed

further. WNFL, located in Winnfield, Louisiana, is a relatively recent site (2.6 years)

and its horizontal velocities are not yet significant at 60%. VIC1, located in Vicksburg,

Mississippi shows a residual velocity of 2.69±0.5 mm/yr at an azimuth of 164±18°.

Both WNFL and VIC1 show positive vertical velocities (11.20±5.0 mm/yr and

3.70±1.8 mm/yr respectively) that are significant at 95%. Both sites lie at the northern

end of the predicted zone of crustal flexure and therefore their upward motion is

consistent with this interpretation. MLF1 shows a relatively large residual velocity

4.9±1.0 mm/yr at an azimuth of 326±13.5°, therefore at this time there is no clear

explanation for its residual velocity. The lack of nearby stations prevents us from

understanding if this is a regional or local effect.

Conclusions

The current distribution of continuous sites that have been operation for more than

3 years provide only limited information on regional Gulf Coast deformation. Many

of the sites are located in known areas of extensive water withdrawal and therefore

reflect local rather than regional motion. The remaining sites show motion that is

parallel with slippage along normal faults, which is consistent with the regional

paradigm for deformation. However, the velocities for all but one site, ARP3, are not

significant at 95% confidence. The key to better constraining the regional deformation

lies in the 11 episodic GPS sites that were first occupied in 1997. After data is

collected in 2001, resulting in a 4 year time series, these episodic sites should provide

velocities that are significant at 95% confidence.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4: REVEL A GLOBAL PLATE VELOCITY MODEL

Introduction

The present-day velocities of the earth’s lithospheric plates are an important

kinematic boundary condition for many geological and geophysical studies, including

regional neotectonics, seismogenic zone processes, and earthquake hazards. Currently

the most comprehensive picture of geologically young plate motion comes from the

global geologic model NUVEL-1A [DeMetset al., 1990, 1994], a significant update

of earlier global models [Chase, 1972, 1978; Minster et al., 1974; Minster and Jordan,

1978]. NUVEL-1A is based in part on mid-ocean ridge spreading rates dated from

magnetic anomaly 2A (approximately 3 million years ago, or mid-Pliocene time) and,

thus, describes relative velocities for most plate pairs averaged over Pliocene to Recent

time. NUVEL-1A is a robust model based on a large data set, but it nevertheless has

some deficiencies. First, some smaller plates (e.g., Amurian) are necessarily omitted

due to lack of data. Second, the geologic model may be biased due to poor or

insufficient kinematic data, e.g., North America-Pacific relative motion [Demets,

1995; DeMets and Dixon, 1999], and motion of the Caribbean plate relative to North

and South America [Dixon et al., 1998; DeMets et al., 2000; Weber et al., 2001].

Third, the 3 million year average velocity predicted by NUVEL-IA or any geologic

model may yield biased estimates of present day velocity for some plate pairs because

the plates are speeding up, slowing down, or changing direction, e.g., Nazca-South

America [Norabuena et al, 1998, 1999; Angermann et al., 1999].

Space geodesy has the potential to measure relative plate velocities directly over

periods of just a few years, as demonstrated by Satellite Laser Ranging (SLR) [Smith

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. et al., 1990; Robbins et al., 1993; Cazenave et al., 1993; Sengoku, 1998]. Very Long

Baseline Interferometry (VLBI) [Argus and Gordon. 1990; Robaudo and Harrison.

1993; Ryan et al, 1993; Sato, 1993]; Doppler Orbitography and Radiopositioning

Integrated by Satellite (DORIS) [Cazenave et al., 1992; Soudarin and Cazenave, 1993,

1995; Cretaux et al., 1998] and the Global Positioning System (GPS) [Dixon et al.,

1991a; Dixon, 1993; Argus and Heflin, 1995; Larson et al., 1997; Dixon and Mao,

1997]. Most studies to date suggest that plate velocities estimated from space geodesy

are consistent with the NUVEL-1A model within 95% confidence, but uncertainties in

the geodetic estimates have been large enough that important differences could be

missed. The true uncertainty level of space geodetic data has also proven difficult to

quantify. Therefore, it has been difficult to address an important tectonophysical

problem, namely, the extent to which individual plate velocities may be accelerating or

decelerating over the last few million years, and to what extent such changes, if they

occur, can be understood in terms of simple plate driving forces.

New space geodetic data and new analytical techniques now permit a significant

refinement of the description of present-day plate motion. In this chapter a

comprehensive velocity model for most major and several minor plates, based

primarily on GPS is presented. This study differs from previous studies in several

respects:

1) A very large geodetic data set is now publicly available, through the efforts of many

individuals, institutions and geodetic agencies, permitting a more accurate and more

comprehensive geodetic plate motion model. This data set represents both a large

number of sites, with a generally good geographic distribution, as well as long, nearly

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. continuous time series at individual sites, resulting in precise site velocity estimates.

All but 2 major plates (Cocos and India) now have at least 2 GPS sites, the minimum

number to determine a plate’s angular velocity.

2) Velocity estimates for the Caribbean, Philippine, Amurian and Nubian plates.

While these and other plates and continental blocks have been the subject of prior

local studies, previous global geologic and geodetic models have omitted one or more

of these plates, or approximated Nubia (west Africa) by combining it with Somalia

(east Africa).

3) A rigorous, independent estimate for GPS velocity errors (described in Chapter 2).

This permits simple, objective tests of whether the GPS site velocity for a given

location is consistent with rigid plate behavior, and whether plate motions averaged

over the last few years differ significantly from motions averaged over the last few

million years.

The velocity predictions of geological plate motion models are sometimes termed

“present-day” or “current” because they describe geologically young plate motion, and

are derived from the youngest easily identified magnetic anomaly, typically 2A (~3

Ma, or mid-Pliocene) [Minster and Jordan, 1978; DeMets et al., 1990, 1994] to, in

some cases, anomaly IN (-0.8 million years, or mid-Pleistocene [e.g., DeMets, 1995].

The geodetic plate motion model presented here is valid over a very different time

span, roughly the last decade. It may also be representative of plate motions over the

Holocene or Recent epoch (last -10,000 years), to perhaps, the Late Pleistocene epoch

(last few hundred thousand years) provided that strain effects related to the seismic

cycle and isostatic effects associated with the last glacial cycle are accounted for, or

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. avoided. The former can impact site velocities, especially in the vicinity of active

plate boundary zone faults, while the latter may impact site velocities in parts of North

America, Eurasia, Greenland and Antarctica. To emphasize the time span over which

this model may be valid, it is termed it “REVEL” (for Recent ve/ocity) with the suffix

2000, to indicate the last year of data included in. the model. It is expected that the

model can be improved significantly in subsequent years as additional data are added

and time series lengthen.

Data Collection and Analysis

The data described in this chapter span the time period January 1, 1993 to June 1,

2000. To ensure consistency, all data were re-analyzed specifically for this study, as

described in the data analysis section of Chapter 1. The great majority of sites used in

this study are continuous, in the sense that they record data for 20-24 hours per day,

typically for at least 300 days per year (Appendix). For more than 98% of the

continuous sites presented here, all known existing data were processed. For the

remaining sites, there may be additional data that was not available at the time of this

study. In some cases (e.g., the Philippine and Caribbean plates) continuous site

distribution is very limited. To augment these data episodic sites, occupied usually

every year or two, were included. A total of approximately 2x105 station-days of data

were analyzed for this study, most of which are used to estimate plate velocities

(Appendix). Unless stated, random walk noise is set to zero in the calculation of the

velocity errors and all uncertainties in the tables and text represent one standard error,

while all error ellipses in figures represent two-dimensional 95% confidence regions

(2.45 times the one-dimensional one standard error).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. For high precision GPS data, the most important influence on GPS velocity errors

is simply the time span of observation. Appendix and Figure 4.1a show the time span

of data for the sites used in this study. The great majority of sites have recorded more

than 300 days per year for more than 2.0 years; the mode (most common value) is

between 4 and 5 years (Figure 4.1a). Figure 4.2 shows the expected evolution of

velocity error as a function of time for a typical station that operates essentially

continuously, recording 300 days per year. On average, the velocity errors for the

horizontal components of a 4 year time series will lie in the range 0.5 mm/yr to 1.5

mm/yr. However, in order to obtain adequate geographic coverage stations with time

spans as short as 1.3 years have also been used. This kind of station will have

horizontal component velocity errors of about 2-4 mm/yr. Figure 4.1b shows the

available data per plate. Four plates (Amuria, Anatolia, Arabia, and Philippines) have

less than 10 velocity data (i.e., less than five sites, with two horizontal velocity

components per site).

From the site velocity data listed in the Appendix, the best fit angular velocity

vector (also termed an Euler vector) is derived describing the relative motion of

adjacent plates (the relative velocities of non-adjacent plates can also be derived, but

the results are more difficult to test against geological data). First, sites thought to be

representative of stable plate interiors are selected, using standard geographic,

geologic and seismological criteria. For example, for the Eurasian plate, all data likely

to be influenced by deformation associated with the northward motion of Africa,

Arabia or India, and all sites west of 120° East longitude are omitted because they may

lie in the deforming zone between Eurasia and North America, or lie on the North

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40-1 (a) 3 0 -

2 0 - X3u 3 2 10-

vswa 1 23456789 Length o f time series (Years)

1 5 0 - (b) C3 1 2 0 -

O 9 0 - O 6 0 - 2 ££2* 3 0 -

af.ysW: o - % ^ ^ ^ ^ \ ^ ^ ^ Plates

Figure 4.1. Histograms of (a) total length of time series at each site and (b) number of data for each plate (See Appendix for abbreviations).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

6 South America North America 5 White + Flicker Noise Model 4 3 [2000]. Solid lines show north (N), east (E) and vertical (V) error for a Time (years) Time et al. 2 0 3 2 4 0 [1999] as modified by Dixon et al. Figure 4.2. Estimate of GPS velocity error as a function of time for the stations used in this study, calculated according to the m odel o f M ao typical North American station, based on the mean WRMS for each component for the stations listed in the Appendix (3, 5 and 10 mm). Dashed lines show corresponding curves based on mean South American station. U\ Ul

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. American plate. Second, the site velocity data is inverted to derive best fit angular

velocities for each plate, termed here ITRF-97 angular velocities (Table 4.1),

minimizing the weighted, least squares misfit to the data for a given plate as described

by Ward [1990] and Mao [1998]. Relative angular velocities for pairs of adjacent

plates are then derived by differencing the ITRF-97 angular velocities, with

appropriate error propagation.

One advantage of this approach is that GPS site velocities can easily be compared

to independent space geodetic techniques such as SLR, VLBI and DORIS, whose

solutions are also available in ITRF-97. In addition, for plates with limited data, it is

straightforward to augment the GPS data with data from these other techniques [e.g.,

Norabuena et al., 1998; 1999]. This approach is used for the Arabian, Anatolian, and

Nazca plates. One disadvantage of this approach is that three steps are required to

derive a relative angular velocity (transform to ITRF-97; derive ITRF-97 angular

velocities; difference two ITRF-97 angular velocities), whereas it is possible to go

directly from loosely constrained (pre-ITRF) position estimates to a plate velocity-

minimized frame, deriving a relative angular velocity in only two steps [e.g., Kogan et

al, 2000].

As this new velocity data set incorporates a longer time span, additional sites, and

improved analytical techniques, the angular velocities presented here are more precise

than previous published estimates for individual plate pairs [Dixon and Mao, 1997;

Dixon et al., 1998; DeMets and Dixon, 1999; Norabuena et a l, 1999]. However the

major conclusions of those studies remain unchanged.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Results

The results of this study are summarized in the Appendix and Tables 4.1-4.3 and

Figures 4.4-4.23. The Appendix lists the individual site velocities in ITRF-97,

grouped by plate, as well as site positions, total time span of data, the number of data

(number of daily position estimates), the weighted root mean square scatter (WRMS)

of the data about a best fit line which is an approximate measure of data quality, and

the residual velocities predicted from the best fitting ITRF-97 angular velocity for

each plate (Table 4.1). The Appendix and the site map figures include sites that are in

the deforming plate boundary zone or other deforming zone, and are not used in the

estimation of the angular velocity for that plate. The residual velocities for these sites

indicate motion relative to the stable plate interior. Table 4.1 lists the best fitting

angular velocity with respect to ITRF-97, the root mean square (RMS) of the velocity

residuals, and the X 2 for each plate.

Relative angular velocities for pairs of plates sharing a common boundary are

listed alphabetically in Table 4.2. To facilitate comparison to the NUVEL-1A model,

the plates in a given pair in Table 4.2 are with the same conventions as in Table 2 of

DeMets et al. [1994].

The velocity residuals are displayed in the site map figures (e.g., Figures 4.4 and

4.5), along with seismicity (magnitude > 4.5 and depth < 50 km) for the period from

1973 to 2000, obtained from the National Earthquake Information Center which

generally outlines the appropriate plate boundary. The great majority of the sites are

located more than 100 km from a major plate boundary, or more than 100 km from

significant plate boundary

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

O(o 0.011 °/Myr 0.776 0.023 0,003 0.002 0,004 0.005 0.068 0.005 0.059 O -7 c 34 75 -52 0.053 -70 0.026 -80 -64 0.003 -31 -82 -21 1.9 1.1 -49 0.6 0.3 0.3 0.4 37 0.6 6 0.011 0.4 0.9 Omin 1.7 1.1 1.5 3.3 0.4 1.8 0.9 0.8 0.4 36.4 Oiunj 12.9 (0 0.309 1.119 0.626 0.9 0.278 4.0 0.199 0.647 3.1 0.654 °/Myr 4>" °E 38.7 -90.5 -79.1 -99.5 -78.0 0.252 1.6 1.0 -81 113.0 -89.4 0.355 -133.9 0.224 -102.9 0.252 -135.5 0.108 11.5 4.8 A.1’ °N 59.3 -119.3 57.5 34.8 51.6 44.4 43.6 -64.2 -24.8 1.1 1.1 1.3 58.6 1.4 1.1 0.9 1.6 -42,2 -35.1 Rale" Rcsid, 3 7 3 3,5 40.6 26.3 1.063 5.4 3 1.5 49.9 7.0 0.545 7 5 1.6 No. 19

62 1.0 -2.6 Sites 2 1.19 1.11 1.81 5 2.6 0.19 2.12 0.95 6 1.3 37.5 0.75 6 0.87 4 0.77 X, rate residual, and angular velocity for each plate with respect to ITRF-97. X 2, Plate is the latitude in °N is andthe <|) longitude in °E ofthe pole ofrotation. ofthe pole error ellipse. Amurian Eurasia Arabia Australia 1.55 11 Antarctica 1.03 Anatolia Caribbean N. America 1.07 Pacific 1.40 Nazca Nubia Philippine S. America Sundaland 0.21 2 0.6 X Table 4.1. a Mean a rate residual in b mm/yr 0 2-Dimensional0 1-sigma lengths in degrees of the semi-major semi-minor

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4.2. Relative angular velocities for adjacent plate pairs.

C _ C Plate* A.b G) Omaj ^min tfo. Pair °N °E °/Myr o °/Myr

Am-Eu 48.07 -173.34 0.071 109.8 10.7 -81 0.102 Am-Ph 50.84 155.87 1.312 4.1 1.0 60 0.142 At-Ar 2 9 3 3 39.98 0.562 23.5 1.3 -28 0.764 At-Eu 28.08 33.02 1.019 12.6 0.6 -1 0.764 An-Pa 66.01 -84.97 0.854 0.7 0.4 87 0.012 An-Sa 85.50 -125.04 0337 5.4 1.8 -78 0.013 Ar-Eu 26.12 24.84 0.464 2.3 0.7 83 0.032 Ar-Nu 30.60 31.85 0.432 2.0 1.0 -82 0.033 Au-An 14.75 40.11 0.652 1.6 0.7 -25 0.004 Au-Eu 12.97 46.47 0.643 1.1 0.4 -50 0.004 Au-Nu 14.88 52.18 0.622 1 3 0.6 -57 0.004 Au-Su 9.05 55.46 0.712 73 0.9 -79 0.051 Ca-Sa 5 2 3 5 -65.79 0372 63 1.9 -8 0.023 Eu-Na 67.44 13733 0340 1.5 0.9 -31 0.004 Eu-Pa 63.36 -78.37 0.897 0.7 0.4 84 0.006 Eu-Ph 50.75 154.10 1345 2.4 0.5 38 0.062 Na-Ca -7531 33.04 0.185 11.0 1.4 85 0.010 Na-Pa 50.38 -71.95 0.753 0.7 0.4 -79 0.005 Na-Sa 12.38 -49.48 0.170 4.2 1.9 -7 0.015 Nu-An 2.65 -31.36 0.133 7.2 2.7 -16 0.006 Nu-Eu -13.14 -18.51 0.068 8.2 3.4 14 0.005 Nu-Na 78.52 96.86 0311 2.0 1.3 -73 0.004 Nu-Sa 61.31 -39.41 037 7 4.5 1.5 4 0.009 Nz-An 35.10 -89.32 0.455 4.4 1.1 15 0.016 Nz-Ca 48.90 -107.53 0.376 6 3 3.3 -19 0.020 Nz-Pa 55.39 -8732 1.266 1.7 0.5 18 0.008 N z-Sa 52.12 -90.93 0.631 3.6 1.7 5 0.011 Pa-Au -61.39 -173.30 1.078 0.5 0.5 -28 0.008 Ph-Pa -8.55 -43.06 1.093 1.7 0.6 9 0.068 Su-Ph 54.51 162.03 1324 5.9 0.8 58 0.063

a The first plate rotates counter-clockwise with respect to the second. Plate abbreviations are given in the Appendix b X is the latitude in °N and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. zone-related seismicity. In this way significant seismic cycle effects will hopefully be

reduced, which in some cases can extend far from faults and long after an earthquake

due to viscous relaxation in the lower crust and upper mantle [e.g., Pollitz, 1992;

Pollitz and Sacks, 1994], and thus relate the measured decadal velocities to longer

term plate velocities. In cases where this criteria is violated (e.g., a given site is

included to obtain necessary geographic “spread”) it has been verified that the site is

not overly sensitive to first order strain effects via simple elastic half space models.

Discussion

Plate rigidity. In order for the angular velocity estimates to be valid descriptions

of plate motion, it is important that the plates behave rigidly over the time period of

observations. Rigid plate behavior on time scales of several million years is

demonstrated by the success of rigid plate geological models, and the exceptions are

well described and spatially limited [Gordon, 1998]. Space geodesy can also test the

assumption of rigid plates [Argus and Gordon, 1996; Dixon et al., 1996]. A

fundamental assumption of this study is that that plates behave rigidly over periods of

several years. However, the existence of intraplate seismicity such as the New Madrid

seismic zone [Nuttli, 1973; Schweig and Ellis, 1994; Weber et al., 1998; Newman et

al., 1999] argues that some local intraplate deformation must occur, at least on short

time scales. Plate rigidity at the level of a few mm/yr over a several year time span is

suggested by the generally good agreement between geological models and space

geodetic estimates of plate motion [Smith et al., 1990]. On the other hand the spatial

sampling afforded by techniques such as VLBI and SLR is limited, and to date the

precision of GPS, which in principle can provide better spatial sampling, has also been

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. limited in terms of the constraints it can provide on plate-wide rigidity. Dixon et al.

[1996] noted that with data then available, GPS data lacked the sensitivity to detect a

clear post-glacial rebound signal (also termed glacial isostatic adjustment because

some parts of the crust undergo subsidence), and provided only a crude upper limit to

plate rigidity, as apparent deviations from a rigid plate model (e.g., the velocity

residuals in the Appendix and Table 4.1) mainly reflect GPS velocity errors rather than

true non-rigid plate processes. While the current data set is a significant improvement

relative to data available in 1996, and apparently has some sensitivity to glacial

isostatic adjustment (see next section), the same limitation still applies, although a

tighter upper bound on plate rigidity from the GPS data can now be placed. Useful

constraints are available in the current data set for the North American, Eurasian, and

Australian plates. For the remaining plates, the number of data are too small to derive

rigorous estimates of plate rigidity, although it should be noted that the velocity data

fit the rigid plate model at about the expected level for most of the plates studied.

Using all 81 available sites to define stable North America, comprising sites on the

plate interior that have observations spanning as short as 1.3 years, the mean rate

residual is 1.12 mm/yr (below a solution using 64 stations is described that eliminates

sites that are sensitive to glacial isostatic adjustment, with a mean rate residual of 1.03

mm/yr). In either case the residual magnitude is roughly the level of the GPS velocity

error, suggesting that the rigid plate model is appropriate, and that the residuals likely

reflect GPS observation errors rather than non-rigid plate processes, as found in prior

studies [e.g., Dixon et al., 1996]. Since GPS velocity errors are a strong function of

total observing time [Mao et al., 1999] one way to check this conclusion is to restrict

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the data set to sites with progressively longer time spans, and investigate the effect on

residuals. If the angular velocity estimate for North America is based on the 39 sites

in this data base with time spans of 4.0 years or longer, the mean rate residual is 0.80

mm/yr. Using only time spans of 6.0 years or longer (8 sites) gives a mean rate

residual of 0.67 mm/yr. The trend of decreasing rate residuals with longer data time

span supports the inference that the rate residuals reflect GPS velocity errors rather

than true non-rigid plate processes. Rate residuals are positively biased (Axgus and

Gordon [1996] and Argus et al. [1999] give a complete discussion) implying that the

actual level of plate non-rigidity will be higher than the mean rate residual. If it is

assumed that an upper bound on plate rigidity is three times the mean rate residual,

then the GPS data set comprised of 6 year or longer time series suggests that the stable

interior of North America is rigid to better than 2.0 mm/yr. Presumably this bound

will be better constrained in future years as GPS velocity errors decline further, and

new sites are added. Of the 14 plates examined here, all but 2 (Anatolia and Nazca)

have mean rate residuals less than 1.6 mm/yr (Table 4.1), about the level expected

given the velocity error. The main point for this paper is that the velocity residuals,

whether reflecting true non-rigid plate processes, local site effects, or (the preferred

explanation) GPS velocity errors, are small enough that the rigid plate approximation

can be assumed valid for the data set under consideration.

If the error model is approximately correct, X 2 for the individual plates should

approximately equal 1.0, provided that the plate is rigid, that the selected sites truly

represent the stable plate interior, and that the data set is large enough and statistically

representative. The latter criteria is met for the North American plate (128 data; X 2 =

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.03) and the Eurasian plate (38 data; X 2 =1.11). These values are sufficiently close

to 1.0 that the assumption of a rigid plate criterion is satisfied. For the remaining

plates, the number of data are small enough that larger deviations from 1.0 are

expected due to statistical fluctuations even if the plate is rigid. Plates with small

numbers of sites are also more sensitive to outliers or blunders, the effect of which is

diminished when a large number of sites is available. All but two plates (Amuria and

Nazca) have Xv2 <1.6 (Table 4.1, Figure 4.3). Amuria, with only 3 sites, has a X 2 >

2.0, suggesting that these data are not well fit by the rigid plate model. Possible

explanations include:

1) one or more site velocities have a systematic error not reflected in the error model;

2) the choice of sites is inappropriate, and one or more sites lies in a deforming

boundary zone rather than the rigid plate or block interior;

3) the sites have a poor geographic distribution;

4) the plate is not rigid.

Factor (3) is probably the most important cause for the misfit, but factors 1 and 2

cannot be excluded. Significant improvement in the velocity description of this plate

is likely in the future.

Glacial isostatic adjustment. The lithosphere in North America and Eurasia is

isostatically adjusting from the mass loading effects of the last glaciation, reflecting

the delayed response of the earth’s mantle associated with viscous flow. This

adjustment imparts both vertical and horizontal motions to the surface velocity field

[e.g., Peltier, 1998a] and needs to be considered when using geodetic data to derive

plate motion models, because the velocity effects are not representative of longer term

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4.3. Histogram showingXv for each plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. motions. That part of the surface velocity field due to glacial isostatic adjustment is a

function of both the ice loading history and the mantle viscosity structure, neither of

which is well known, and corresponding model predictions can vary significandy [e.g.,

Argus et ah, 1999; Mitrovica et aL, 2000]. However, most models agree on the region

of maximum glacial isostatic effect, even if they do not agree on the magnitude or

direction of predicted motion. For North America and Eurasia, rather than correcting

each site by a specific model prediction, a conservative approach is taken by simply

excluding the subset of sites most likely to be affected by glacial isostatic adjustment.

For North America, a broad region around Hudson Bay is the region of maximum

glacial isostatic adjustment-related motion. The ICE-4G model [Peltier, 1994] is used

to define sites with horizontal rates due to glacial isostatic adjustment >_0.7 mm/yr,

and compare solutions with and without this data subset. Thus, a solution for North

America using all available data (81 sites, Xvz = 1.34) versus a model that omits 17

sites most affected by GIA (ALGO, CHB1, CHUR, DET1, DUBO, FLIN, KEW1,

MIL1, NRC1, SAG1, SCH2, STB1, STP1, WHP1, WIS1, YELL, YOU1) giving a 64

site model with XZ = 1.04) results in an improved solution. O f these 17 sites, 8 exhibit

statistically significant uplift (rebound), and 5 exhibit uplift rates greater than 3.0

mm/yr (see Appendix). Sites may also subside due to glacial isostatic adjustment,

associated with collapse of a peripheral bulge, but associated horizontal velocities are

thought to be small compared to the observational error (e.g., less than 0.5 mm/yr in

ICE-4G).

For Eurasia, the solution using all available sites (23 sites, Xv2 = 2.39) versus a

solution that omits 4 sites on the Fennoscandian platform (KIRU, METS, ONSA, and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TROM) most likely to be affected by glacial isostatic adjustment are compared, using

the same criteria for horizontal motions [Peltier, 1998b], approximately equivalent to

eliminating sites with vertical motion > 3 mm/yr, as predicted by the uplift model of

Davis et al. [1999]. In this case the fit also improved significantly (19 sites. Xv2

= 1. 11).

For Antarctica, where the ice loading history is relatively straightforward

incorporating published corrections for glacial isostatic adjustment results in a

significantly reduced misfit (see Antarctica section below). Therefore this global data

set has the sensitivity to detect glacial isostatic adjustment signatures in North

America, Eurasia and Antarctica, and that ignoring this effect could bias space

geodetic-based plate motion estimates when comparing to longer term geological data.

In the remaining discussion, only the solutions that omit sites with high sensitivity to

glacial isostatic adjustment for North America and Eurasia, but exploit the published

corrections for Antarctica. This approach, suggests that this study’s plate velocity

model, while based on data recorded over less than a decade, should be representative

of longer term motions.

Comparisons to Geologic and Geodetic Data

In this section the criteria for inclusion or omission of key sites by plate (listed

alphabetically) are discussed, and the results compared to independent data, and some

geological implications are discussed. Omitted from this discussion are the Anatolian

plate [Reilinger et al., 1997; McClusky et al., 2000] and the Sundaland plate

[Walpersdorf et al., 1998; Rangin et al., 1999; Chamot-Rooke and Le Pichon, 1999]

where publicly available and/or continuous data are very limited. The velocities of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. these plates are better described by local (often campaign GPS) data, but they are

included in Tables 4.1 and 4.2 for completeness.

Amuria. Amurian plate angular velocities have been reported by Zonenshain and

Savostin [1981], Wei and Seno [1998], Heki et al. [1999] and Holt et al. [2000].

Three sites are used to define the Amurian plate, SUWN, DAEZ and VLAD (Figure

4.4). The limited geographic distribution results in correspondingly large uncertainties

in the angular velocity. XIAN, SHAO and TOHK are not used; SHAO lies southeast

of the accepted southern boundary of the plate and on the east side of the Tan-Lu fault;

TOHK’s and YSSK’s residual direction, west-northwest, reflects elastic strain

accumulation from the subducting Pacific and Philippine plates.

Relative motion between Amuria and Eurasia results in the formation of the Baikal

rift. This angular velocity predicts that Amuria moves southeast relative to Eurasia at

about 5-6 mm/yr. The velocity of IRKT, a site near the rift on the Eurasian plate is

3±2 mm/yr to the northwest with respect to Amuria (Figure 4.4). At 55°N, 113°E in

the northern Baikal rift, close to where Sankov et al. [2000] estimate a minimum

extension rate of 3.2±0.5 mm/yr at azimuths between 130° and 155° based on offset

geological features, this study predicts 6±2 mm/yr at an azimuth of 160±50° (Amuria

relative to Eurasia). At 51.8°N, 107.6°E, close to where Calais et al. [1998] measured

a local velocity with GPS campaign data of 4.5±1.2 mm/yr with azimuths ranging

from 080° to 140°, this study predicts 6±2 mm/yr at an azimuth of 150±50°. Similarly

at 47.5°N, 106.5°E, close to where Calais and Amaijargal [2000] measure a velocity of

6.4±1.6 mm/yr at an azimuth of 125±30° from continuous GPS data, this study

predicts 6±3 mm/yr at an azimuth of 150±40°. This agreement with several

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. S ubic plate site residual velocity 2_5 mm/yr t s - Eu Boundary zone site residual velocity 5 mm/yr ^

take Baikal

. Am

yssk A

VLAD

40'

Ton-Lu, fault / TAEJVp tohk

30'

110° 120 ° 130° 140°

Figure 4.4. Mercator projection showing space geodetic site locations for the Amurian plate, and earthquake epicenters (magnitude > 4.5, depth < 50 km, between 1973- 2000) suggest plate boundaries. Sites labeled in upper case letters are assumed to lie on the rigid plate interior and are used to define the plate's angular velocity (Table 4.2). Solid arrows at these sites indicate residual velocities, for clarity no error ellipses drawn. Sites labeled with lower case lie in deforming plate boundary zones or other deforming areas, and are not used in the rigid plate definition; their velocities (open arrows) indicate velocities relative to the rigid plate interior (e.g., irkt moves northwest at 3 mm/yr with respect to Amuria, reflecting extension across the Baikal rift), 2D and 2 standard error ellipses drawn. See Appendix for plate abbreviations and velocity magnitudes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. indipendent results is encouraging, given that this angular velocity for Amuria is the

most poorly determined one in this study (Table 4.1).

Antarctica. The Antarctic plate is defined using 7 sites (CAS1, DAV1, KERG,

MAW1, MCM4, SYOG, and VESL) (Figure 4.5). OHIG and PALM are excluded due

to their proximity to the complex plate boundary with the Scotia plate [Pelayo and

Wiens, 1989; Klepeis and Lawyer, 1996]. Previous GPS-based angular velocity

estimates for this plate included OHIG due to the paucity of data on the plate

available at that time [Larson and Freymueller, 1995; Larson et al., 1997; Yamada et

a l, 1998].

The present-day velocity field for Antarctica is affected by glacial isostatic

adjustment (GIA). James and Ivins [1995, 1997, 1998] have modeled these effects

using a number of different data sets. Their revised CLIMAP model, D91, predicts

that the area of maximum horizontal velocity from glacial isostatic adjustment should

be in the Ross Ice Shelf region. Their model can be tested in two ways with the

current data set. First, the fit of this velocity data to a rigid plate model {X 2 =1.36 for

uncorrected velocities) should improve when site MCM4 is excluded, located on

McMurdo Island near the edge of Ross Ice Shelf. The misfit does in fact decrease

(Xv2 = 1.18). Second, this studies velocities (including MCM4) can be corrected with

the predicted glacial isostatic adjustment velocities of James and Ivins, [1997] (Table

6, D91). This results in a best fit 7 site model ( X 2 = 1.03). Therefore the GPS

velocity field calculated in this study in Antarctica is sensitive to glacial isostatic

adjustment, and that model D91 of James and Ivins [1997] provides a reasonable

correction for these effects. Table 4.3 compares several ITRF-97 angular velocity

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0"

Sa ;>v Nu

VESL [SYOG !ohig KERG oo oo palm M AW I X

(Nz DAVI An

Ross Ice CAS! Shelf M CM 4 / Au

Pa Stable plate site residual velocity 2_5 mm/yr Cs- Boundary zooe site residual veioaty € 5

Figure 4.5. Similar to Figure 4.4, for the Antarctic plate (Sc is Scotia plate).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

0.017 0.012 0.012 0.011 °/Myr Gmjn Gmjn C O 34 34 34 d d 1.2 1.1 _ fTpiin

1.9 1.2 33 0.012 1.2 1.2 1.7 Ocnnj i CO 0.229 0.236 0.231 1.9 °/Myr [1998]. “Antarctic” is the same as in f °E -136.3 0.210 2.3 1.4 48 -133.4 -132.4 -133.9 0.224

58.6 58.1 59.0 -132.1 59.0 58.6 Jamesand Ivins 1.5 1.4 1.4 1.3 Rateb Resid. °N 7 7 1.4 7 7 No. Sites

1.36 1.38 1.35 1.03 x \ GIA* Models GIA velocities from models listed in their Table 2; Antarctic-6 omits site MCM4. o fthe pole error ellipse. Antarctic-6 1.18 6 Antarctic Antarctic-1CE3G Antarctic-D91 Antarctic-LC79 a Velocitiesa for rigid Antarctic plate as listed in the Appendix were adjusted with predicted a Meana rate residual A is b inthe mm/yrlatitude in °Nis theand (f> longitude in °E o fthe pole o frotation. c 2-Dimensionalc 1-sigma lengths in degrees ofthe semi-major semi-minorGmnjand axes d £d is the azimuth ofthe semi-major ellipse axis in degrees clockwise from north. Tabic 4.3. Zv2, rate residual, and angular velocity for the Antarctic plate with respect ITRF-97 for different glacial isostatic adjustment models.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vectors for various mixes of sites and glacial isostatic adjustment corrections. While

all the vectors agree within uncertainty, the best fit angular velocity for Antarctica,

based on GPS site velocities corrected for glacial isostatic adjustment with the James

and Ivins [1997] D91 model is used in the subsequent discussion.

. The Antarctic-Pacific and Antarctic-Australia boundaries are spreading ridges, and

their velocities are well determined in geological models such as NUVEL-1A. The

Pacific and Australia velocities are also well determined in REVEL (Tables 4.1-4.2),

hence any difference in velocity between the two models probably indicates velocity

changes over the last 3 million years. Figures 4.6 and 4.7 indicate that the rates and

azimuths predicted by the two models for both plate pairs are virtually identical

through about 160° of longitude. For Australia-Antarctica, REVEL and NUVEL-1

have a mean misfit of 0.2 mm/yr in rate for the entire plate boundary, implying a

remarkable steadiness of plate motion over this time interval.

Arabia. Reches and Schubert [1987], Hempton [1987] and Coleman [1993]

discuss the Arabian plate and young deformation on its boundaries. The Arabian plate

is defined using a combination of 2 GPS sites (AMMN, BAHR) and 1 SLR site (7575)

(Figure 4.8). While the data set is small, the relatively low values of Xv and mean rate

residual suggest a good fit of data to model (Table 4.1). The accuracy of the angular

velocity for Arabia-Nubia can be tested by comparing it to measured slip rates across

the Dead Sea fault, separating Arabia and Nubia. Young (< 140 Kyr) offset

geomorphic features along the north-striking Dead Sea fault at 30.8°N, 35.4°E indicate

a slip rate of 4±2 mm/yr [Klinger et al., 2000] which compares reasonably well to the

predicted velocity at this location, 3±1 mm/yr at an azimuth of 359±13°. If motion of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

160 one standard errors. ± Au-An 13763669 Longitude (°E) REV EL-2000 -i | i | i |- ■ Transform fault (Bathymetry) © Transform fault (Altimetry) 60 80 100 120 140 - - 10 50 - 30 - -30 -30 "I"" NUVEL->A -10 o o I N ■g 2 & £ U 5 iH • < it Au-An i" r i" 1994], and other models shown in legend. Bathymetric measurements are mainly from et al, -- NUVEL-IA — REVEL-2000 k Scafloor spreading rate Longitude (°E) 80 100 120 140 160 80 50 [1990] and altimetry measurements are from Spitzak and DeMets [1996] both are.shown with et et al. (this study), NUVEL-1A [DeMets Figure 4.6. Measured seafloor spreading rates and transform fault azimuths along the Australia-Antarctica plate boundary compared to the predicted rate Dand eM etsazimuth at Greythe shadingsame indicateslocation ± usingone standard the errorangular for REVEL velocity prediction. for the ,' ‘ ° REVEL-2000 u>

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. An-Pa Longitude (°E) T 160 160 180 200 220 240 ■ Transform fault (Bathymetry) ® Transform fault (Altimetry) — REVEL-2000--- NUVEL-1A

- - -

150 150 - 140 140 - 130 130 - 120 110 100 zmt ( W rm N) from CW ( Azimuth REVEL-2000 NUVEL-1A Scafloor spreading rale Longitude (°E) I i | i | i [— 180 180 200 220 240 An-Pa 100 h V Figure 4.7. Similar to Figure 4.6, for Antarctica-Pacific.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V 40' At 7575

f ? ( Eu Nu ta AMMN

30' .rc Ar BAHR

Stable plate site residual velocity 1 mm/yr A Boundary rone site residual velocity 20 ‘ Nu 5 mm/yr ^

40“ 50“

Figure 4.8. Similar to Figure 4.4, for the Arabian plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Arabia relative to Nubia has been steady over the last few million years it is expected

that there should be close agreement between the REVEL prediction and the rate of

seafloor spreading across the Red Sea. However, measured spreading rates averaged

over the last 3 Ma [Chu and Gordon, 1998], as well as the model rates of Jestin et al.

[1994] based on a similar time period are both systematically higher than REVEL

(Figure 4.9). Similarly, NUVEL’s Axabia-Eurasia rate is significantly higher than

REVEL. This suggest that both of these rate differences reflect gradual slowing of the

Arabian plate as it moves north and collides with Eurasia. Associated crustal

thickening forms the Zagros [Alavi, 1994] and Caucasus Mountains and increases

gravitational body forces that opposes Arabia’s northward motion.

Australia. The stable Australian plate is defined using 11 sites located on

Australia and Tasmania (ALIC, CEDU, DARW, HOB2, JAB1, KARR, PERT, TID2,

TIDB, TOW2, and YAR1). NOUM and AUCK have velocities relative to stable

Australia that are larger than the corresponding velocities of stable interior sites

(Figure 4.10), suggesting that they should not be used for the Australian plate

definition.

Separate Indian and Australian plates have been recognized for some time [Wiens

et al., 1985; DeMets et al., 1988; Gordon and DeMets, 1989], and a Capricorn plate

has also been proposed in the central-western Indian Ocean with a broad, diffuse

boundary [Royer and Gordon, 1997; Gordon et al., 1998]. A relative angular

velocities for any of these plate pairs cannot be calculated due to insufficient geodetic

data. However, the Xv test can be used to evaluate the fit of the velocity data for

COCO (Cocos Island, on the southern edge of India-Australia boundary), DGAR

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 Ar-Eu REV EL-2000 N U VEL-1A Latitude (°N) - 36 - 32 - 24 - 20 Ar-Nu 1 1 | i | i T " 1994 ------

1 ------1 etal, Latitude (°N) ------18 18 20 22 24 26 Scafloor sprd. rate (Chu and Gordon, 1998) a --- Chu and Gordon, 1998 —i —i 1 — REVEL-2000■ - Justin - 8 8 - 6 - 4 - 18 18 - 12 12 - 10 - 14 14 - 16 16 -

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure Figure 4.10. Similar to Figure 4.4, for the Australian plate (So is Somalia plate).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Diego Garcia, near the inferred boundary between the India and Capricorn plates) and

IISC (Bangalore, India) to the rigid Australian plate model. X 2 for the 11+3 sites is

9.97, i.e., a very poor fit. If IISC (11+2 sites) is excluded, X 2 decreases to 1.63,

consistent with a separate Indian plate. However, when either DGAR or COCO are

excluded in an 11+1 site solution for Australia, a negligible improvement occurs

(excluding DGAR gives X 2 = 1.67, excluding COCO gives X 2 = 1.58; the original 11

site solution has X 2 = 1.55). The negligible improvement from excluding COCO or

DGAR reflects the fact that these two sites have velocities that are negligibly different

from stable Australia, and is consistent with the slow relative motion predicted

between Capricorn and Australia [Royer and Gordon, 1997].

Caribbean. The Caribbean plate has been the focus of kinematic studies for at

least 25 years [e.g., Jordan, 1975; Stein et al., 1988; Deng and Sykes, 1995].

Geological estimates of the velocity of the Caribbean plate with respect to its

neighbors are hampered by the paucity of relevant data (especially rate data from

spreading centers) and the tectonic complexity of the boundary zones around the plate

(see discussions in DeMets et al. [1990] and Dixon et al. [1991b]). The velocity data

set used here is identical to that presented in Weber et al. [2001], although a slightly

more conservative approach is used, omitting 2 sites in Puerto Rico (ISAB and PUR3)

because of the possibility of independent motion of the Puerto Rico block [Jansma et

al., 2000] (Figure 4.11). The resulting Caribbean-South America angular velocity is

nevertheless essentially identical to that presented in Weber et al. [2001]. The

Caribbean-North America angular velocity is similar to the estimate presented in

DeMets et al. [2000] although the current vector is constrained by additional position

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

AVES Stable plate site residual velocity residual velocity lioundary zone site SANA Cayman Trough Figure 4.11. Similar to Figure 4.4, for the Caribbean plate (EPGfz is Enriqillio-Plantain Garden fault zone, Gmp is Gonave microplate, Ofz is Oriente fault zone, SPfz is Septentional fault zone, SIfz is Swan Island fault zone).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. data at SANA, ROJO, and CROl which improves their velocity estimate, as well as

new sites at BARB and TDAD. As pointed out in Dixon et al. [1998], Pollitz and

Dixon [1998], DeMets et al. [2000] and Weber et al. [2001], the motion of the

Caribbean plate with respect to both North and South America is considerably faster

than predicted by NUVEL-1A, presumably reflecting systematic errors in the

geological model.

With the benefit of hindsight, it is clear why NUVEL-1A systematically

underestimates the speed of the Caribbean plate (the predicted azimuths for NUVEL-

1A and REVEL are very similar along much of the Caribbean boundary). Sykes et al.

[1982], Rosencrantz and Mann [1991] and Mann et al. [1995] discuss the importance

of the Gonave microplate, which separates the North American and Caribbean plates.

This microplate or block is defined by the Cayman spreading center on the west

[Macdonald and Holcombe, 1978; Rosencrantz et al., 1988], the Oriente-Septentrional

fault zone on the north, and the Enriquillo-Plantain Garden fault zone on the south

[Leroy et al., 2000; Pubellier et al., 2000] (Figure 4.11). Dixon et al. [1998] estimated

8±4 mm/yr of motion along the Enriquillo fault zone in the Dominican Republic, a

significant fraction of overall North America-Caribbean motion. NUVEL-1A

underestimates Caribbean-North America (and by implication, Caribbean-South

America motion) by roughly the slip rate on this fault, because this rate represents that

portion of plate motion not accommodated on the Cayman spreading center, NUVEL’s

only Caribbean rate datum. REVEL provides a more accurate estimate of Caribbean-

North America and Caribbean-South America motion, not only for the decade time

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. scale, but perhaps for the last few million years as well, because it represents the total

relative plate motion, not just motion accommodated on the spreading center.

Eurasia. The Eurasian plate is defined using 19 stations (Appendix; Figure 4.12).

All stations south of significant seismicity reflecting the Nubia-Eurasia collision are

excluded (e.g., south of the Pyrenees VILL and CASC [Riberio et al., 1996; Herraiz et

al., 2000]), all sites in or south of the Alps and south of the Carpathians (e.g., BUCU

and SOFI [Calais, 1999, 2000; Grenerczy et al., 2000]) are excluded, all sites in

Central Asia south of 44°N are excluded because they may be affected by the India-

Eurasia collision (e.g., KIT3 and POL2), and all sites east of 140°W that may be on

the North American plate or in the deforming zone between North America and

Eurasia (e.g. BILI) are excluded. In addition IRKT is excluded because it is less than

100 km from the active Baikal Rift (Doser, 1991; Delvaux et al., 1997) (see Amurian

section). YAKZ is also excluded due to problems with its time series that are not

understood (the time-series for this site shows a large seasonal variation in all three

components). HERS located in southwest England is excluded based on the regional

stress indicators compiled by Mueller et al. [1997] that suggest compression in this

area. A 20 site solution including HERS results in a Xv2 =1.36 compared to the 19 site

solution Xv2 = 1.11. Also omitted are KIRU, METS, ONSA, and TROM sites due to

sensitivity to glacial isostatic adjustment as discussed earlier.

Following Kogan et al. [2000] the site BILI in northeastern Russia can be tested to

see if it lies on the North American or Eurasian plate by comparing the size of the

corresponding velocity residual. BILI has a residual rate of 9±3 mm/yr with respect to

Eurasia and 5±3 mm/yr with respect to North America, suggesting that BILI either lies

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. KSTU SUMK> Stable plate site residuni residuni velocity residual velocity Boundary zone site Boundary zone site $ ZWEN Figure 4.12. Similar to Figure 4.4, for the Eurasian plate (kit3 and po!2 are omitted for clarity see Appendix for their velocities).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. on the North American plate, or on the diffuse boundary between the 2 plates

[Chapman and Solomon, 1976; Zonenshain and Savostin, 1981; Cook et al., 1986].

The Rhine Graben, an active rift in Late Pliocene time [Meghraoui et al., 2000], is

currently the locus of significant seismicity, and might separate Eurasia into eastern

and western plates. An F-test [Stein and Gordon, 1984] to assess the reduction of

misfit when Eurasia is separated into 2 plates along the Rhine Graben was done. The

two-plate hypothesis fails at 99% confidence, and also fails if the plate is split along

the Ural Mountains, suggesting that the current data lacks the sensitivity to resolve

motion across these features.

A test of the accuracy of this study’s angular velocity estimate for Eurasia-North

America can be done by comparing the predicted velocity with measured spreading

rates along the Atlantic mid-ocean ridge and associated transform fault azimuths.

Figure 4.13 shows this comparison and also the predictions from Kogan et al. [2000].

Both estimates show reasonably good agreement with the geological data along most

of the mid-Atlantic ridge in the north Atlantic. At 64°N, 19°W in Iceland this study

predicts spreading at a rate of 20±1 mm/yr at an azimuth of 102°, which compares

well with the local measured velocity using episodic GPS data of 21±4 mm/yr at

azimuth of 117°±11° [Sigmundsson et al., 1995].

The amount of total plate motion accommodated within Iceland can be tested by

comparing the velocity of HOFN in the southeastern comer of the island relative to

stable North America (20.7±0.9 mm/yr at 103±2°) with the total plate rate computed at

the same location (19.5±0.3 mm/yr at 104±1°) (the calculated value is very similar

whether or not HOFN is used in the definition of Eurasia). Thus it appears that within

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r ------1 ------1 ------1 1995 ------1 etal., 2000 ------1 et a I., a et ------1 Latitude (°N) (Bathymetry) Sigmundsson ------Transform fault 50 50 60 70 80 - Kogan - | - N U VEL-I A - REVEL-2000 - - -

140 140 - 120 130 130 - 110 100 zmt ( C fo N) from CW (° Azimuth __ r \ ------1 ------i ------1 Eu-Na ------1 1995 U i U ------1 N4 N4 2000 et al, et ------1 etal,, ------1 Latitude (°N) ------1 ------— Kogan A A Seafloor spreading rate O Sigmudsson •--- REVEL-2000 NUVEL-IA 40 50 60 70 80 - | - 15 - 15 1 0 - 25- 2 0 u

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. uncertainties HOFN lies on stable Eurasia. Similarly REYK in southwestern Iceland

moves at 19.1±0.6 mm/yr at 279±2° relative to stable Eurasia, compared to the

calculated plate velocity here of 19.7±0.3 mm/yr at 281±1°). This suggests that

virtually all of the plate motion is accommodated within the island, consistent with

earlier studies [Sigmundsson et al., 1995; Jonsson et al., 1997; Hreinsdottir et al.,

2000].

Nazca. Due to the paucity of GPS data on the Nazca plate (only 2 sites EISL and

GALA) all available space geodetic data are included, adding SLR (7097), and DORIS

(EASA and GALA, renamed here GALD) sites (Figure 4.14). The resulting estimate

of Nazca-Pacific rate is significantly slower than the NUVEL-1A estimate and the

geological data on which it is based (Figure 4.15), consistent with previous results

[Norabuena et al., 1998. 1999; Angerman et al., 1999]. Nazca-Antarctica also differs

significantly in rate (Figure 4.16). As has been shown in previous studies, both Nazca-

Pacific and Nazca-South America have been decelerating during the last 25 Ma years

[Tebbens and Cande, 1997; Somoza, 1998]. This deceleration is sufficiently rapid that

it is manifested as differences between geodetic plate motion estimates and NUVEL-

1A predictions once the geodetic estimates become sufficiently precise.

The REVEL velocity azimuths predicted for Nazca-Antarctica also differ

significantly from NUVEL-1A (Figure 4.16). Although the Nazca site distribution is

limited, this difference is believed to be real and reflects changing plate direction over

time, for two reasons. First, stage pole reconstructions back to 30 Ma [Tebbens and

Cande, 1997] allow a much longer time record to be evaluated, and indicate changes in

Nazca-Antarctica direction in the same sense as inferred from the shorter epochs

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GALD GALA Sa

- 10°

- 2 0 ° -**L. JP' * Jf*

. . . . £ a s a

* C.7097 1 EISL -30°

-40°

Stable plate site residual velocity 2-5m m /yr A ------

250° 260° 270° 280°

Figure 4.14. Similar to Figure 4.4, for the Nazca plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Nz-Pa Latitude (°N) ■15 ■15 -10 -5 --- NUVEL-1■ A Transform fault (Bathymetry) — REVEL-2000 - - -

100 120 110 zmt ( C fo N) from CW (° Azimmth -10 Nz-Pa -20 Latitude (°N) -30 NUVEL-1A ▲ ▲ Sculloor spreading rale REVEL-2000 120 140 160 150 (ij 130 Figure 4.15. Similar to Figure 4.6, for Nazca-Pacific.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Nz-An Longitude (°E) 250 250 260 270 280 ■ Transform fault (Bathymetry) © Transform fault (Altimetry) _ REVEL-2000 ... NUVEL-1A

80 90 70 60

110 zmt ( C fo North) from CW (° Azimuth Nz-An REVEL-2000 NU VEL-1A Seafloor spreading rate Longitude (°E) 265 265 270 275 280 60 55 50 65 45

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. “sampled” by NUVEL-1 A and REVEL (Figure 4.17). Second, there is a simple

physical explanation for such directional changes. The average age of the lithosphere

being subducted along South America changes from south to north, with younger

lithosphere being subducted in the south, as the Chile Rise approaches the coast of

South America. Younger lithosphere is less dense than older lithosphere, hence it

contributes less to slab pull, probably the major plate motion driving force [Forsyth

and Uyeda, 1975]. For the very young (< 10 Ma), and hence, very buoyant lithosphere

at the southeastern comer of the Nazca plate between 40°S to 45°S, slab pull probably

becomes negligible, compared to stronger slab pull in the northeastern part of the plate

where older lithosphere is subducted. This pivots the Nazca plate about its

southeastern comer, causing a change in subduction direction over time, from

northeasterly to more easterly. Coincident changes in spreading direction over time

occur along the Nazca-Pacific and Nazca-Antarctica boundaries (Figure 4.17).

North America. The rigid North American plate is defined using 64 sites

(Appendix). All sites located in the Northern Gulf Coast are excluded due to possible

subsidence, and all sites west of the Rio Grande rift and west of the Rocky Mountain

front due to possible tectonic effects. As discussed earlier, Chapter 2, 17 sites are

omitted that may be most affected by glacial isostatic adjustment in the northern part

of the plate.

Fairbanks (FAIR), Alaska has been used in previous definitions of stable North

America [e.g., Dixon et al„ 1996]. In the current velocity solution, it moves at 2.2±0.5

mm/yr to the west-southwest, somewhat faster than would be expected for a “stable

plate” interior site. For a detailed discussion of North America see Chapter 2.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

20 10 N U VEL-1A A 0 O Nz-Pa+ Stage pole Nz-An Stage pole A f t* REVEL-2000 90 90 H 80 80 H O 100 - & -V - l 20 NUVEL-1 A A + Nz-An Stage pole 0 Nz-Pa Stage pole A ft ★ REVEL-2000 10 Age (Ma) Age (Ma) 0 80 80 H 40 40 - 160 160 H 120 H u CD Figure 4.17. Spreading rates and transform azimuths predicted for Nazca-Pacific (calculated at 12°S, 250°E) and Nazca-Antarctica (calculated at 40°S, 268°E) plate pairs using stage pole data ofTebbens and Cande [1998].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Nubia. NUVEL-1A was unable to resolve the motion between Nubia (west

Africa) and Somalia (east Africa) plates and treated them as a single African plate.

The angular velocity for Nubia is defined using 6 sites (GOUG, HARZ. HRAO,

LAMP, MASP, SUTH). HRAO and HARZ in South Africa are located in an area of

intense micro-seismicity (Figure 4.18). A 4 site solution that omits these sites has a

Xv =0.71, negligibly different from the 6 site solution with Xv2 = 0.75.

For the Somalia plate only 2 sites (MALI and SEY1), one of which (SEY1) is very

noisy (Appendix), are available (Figure 4.18). Thus a robust angular velocity for

Somalia could not be defined. Nevertheless an estimate of the rate of extension across

the equatorial region of the East African Rift by comparing the observed velocity at

MALI with the predicted velocity for Nubia can be made. This gives a rate of 6±2

mm/yr at an azimuth of 118±10°, in approximate agreement with geologically based

angular velocity predictions [Jestin et al., 1992; Chu and Gordon, 1999].

Pacific. The Pacific plate is defined using 7 sites (CHAT, KOK1, KOKB, BCWJ1,

MARC, THTZ, TRUK) (Figure 4.19). Two sites (UPOl and MKEA) are excluded as

they are located on the big island of Hawaii and therefore may be affected by

deformation associated with the active Mauna Loa and Kilauea volcanoes. The results

of this study agree with the conclusions of DeMets and Dixon [1999] that the

NUVEL-1 A angular velocity for Pacific-North America significantly underestimates

present-day Pacific-North American motion, probably due to lack of data constraint in

the NUVEL model. REVEL's estimate of Pacific-North America motion is 1±1

mm/yr faster than the geologic estimate of DeMets and Dixon [1999], i.e., there is no

evidence for significant acceleration over the last 3 million years. However, it is

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 340' 0 ' 20“ 40“ 60“

Figure 4.18. Similar to Figure 4.4, for the Nubian plate (So is Somalia plate)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ° 270 ° 250 ° 230 tcsidunl velocity lltiuniiary /one site -*- ° a Z' 210 t h t i

k o k ° K O K B ^ A .UI 190 CHAT ° 170 MARC TRUK. ° 150 Figure 4.19. Similar to Figure 4.4, for the Pacific plate (guam is omitted for clarity see Appendix for its velocity).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. interesting to note that spreading in the southern Gulf of California shows a trend of

increasing rate since 2.5 Ma [DeMets, 1995] and this study’s prediction for Pacific-

North America is very consistent with this trend (Figure 4.20). These data suggest

that at the present time, that part of Pacific-North America motion in the vicinity of

the southern Gulf of California not accommodated by Gulf spreading is limited to 1-3

mm/yr.

Five Pacific sites near the plate boundary zone with North America (CICZ, FARB,

SCIP, SNI1, VNDP) have velocities slower than expected if they were on the rigid

Pacific plate. Dixon et al. [2000] and Beavan et al. [submitted] explain these

discrepancies by a combination of strain accumulation on locked faults of the San

Andreas system, plus slip on additional faults offshore to the west [e.g., Sorlien et al.,

1999]. Figure 4.20 suggests that such faults in the region of southern Baja California

slip at no more than about 1-3 mm/yr.

It is not possible to directly verify this study’s Australia-Pacific angular velocity

against independent geologic data due to the lack of transform faults or spreading

rates. Australia-Pacific stage poles suggest little change over the last 11 Myr

[Sutherland, 1995]. At a point on the Alpine fault in New Zealand (43.5°S, 170°E),

REVEL predicts 42.2±1 mm/yr at an azimuth of247±1°, giving 41±1 mm/yr of fault-

parallel slip (azimuth 236°) and 8±1 mm/yr of shortening. This is larger than the rates

measured with episodic GPS (29.7±1.4 mm/yr of fault-parallel slip and 9.9±1.8 mm/yr

of shortening; Beavan et al., 1999). However Beavan et al. [1999] and Sutherland et

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pa-Na 54 -

52 - revised geologic

50 -

46 - * REVEL-2000 T * Measured spreading rates T -10 12 Age (Ma)

Figure 4.20. Measured seafloor spreading rates, adjusted for outward displacement, at various times in the southern Gulf of California [DeMets, 1995], and predicted Pacific-North America total motion from REVEL calculated at 23.5°N, 251.5°E. Revised Pacific-North America geologic model from DeMets and Dixon [1999],

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. al. [2000] conclude that the Alpine fault and sub-parallel faults in New Zealand only

take up 70-80% of total plate motion.

Philippine. Estimates of the velocity of the Philippine plate by conventional

geologic approaches is problematic because no spreading ridges bound the plate, so

geologic rate information is not available. This problem has been attacked with

earthquake slip vector data [Seno et al., 1993] and GPS data [Kato et al., 1996, 1998;

Kotake et al., 1998]. Four sites (GSI1, GSI2, GSI3, and OKTO) are used to define the

rigid Philippine plate (Figure 4.21). To test if the two sites closest to the subducting

Pacific plate, GSI2 and HACI, are affected by elastic strain from the subducting

Pacific plate their residual velocity with respect to an angular velocity based only on

the 3 sites to the west: GSI1, GSI3, and OKTO was calculated. GSI2, 100 km from

the trench, has a residual velocity of 3.8 mm/yr at an azimuth of 104°. HACI, 200 km

from the trench, has a residual of 7 mm/yr at 108°. The eastward direction of the

residuals suggests that these sites are not strongly influenced by seismogenic coupling

with the subducting Pacific plate. HACI was excluded from this angular velocity as its

inclusion results in a significantly higher Xv~ (Xv = 2.2).

The velocity of GUAM with respect to the Philippine plate is a measure of

spreading across the Mariana Trough, a back-arc basin. The angular velocity

calculated in this study predicts a spreading velocity across the Mariana’s trough at

GUAM (13.6°N, 144.9°E) of 51±2 mm/yr at an azimuth of 090±2°. This velocity is

higher than the published rates (full-spreading rate) that range from 30-42 mm/yr

[Bibee et al., 1980; Hussong and Uyeda, 1981]. Part of this difference could sim ply

reflect the change in spreading rates with latitude, which increases north to south.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30°

OKTO 20 °

guamc- ’ j.- y l

10°

Stable plate site residual velocity 2.5 mm/yr £ r Boundary zone site residual velocity 5 mm/yr

120 ° 130° 140° 150°

Figure 4.21. Similar to Figure 4.4, for the Philippine plate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Convergence across the southern Mariana trench is estimated by GUAM’s velocity

relative to the Pacific, 63±2 mm/yr at an azimuth of 105±1°.

South America. The South America plate is defined using 5 sites ASCI, BRAZ,

FORT, KOUR, and LPGS (Figure 4.22). Site CORD is excluded, due to its proximity

the Andes Mountain front, as well as RIOG, as it lies on the diffuse zone of

deformation affected by the subducting Pacific plate and the Scotia-South American

plate interaction. The slightly low X 2 = 0.77 probably reflects the often intermittent

data availability for sites BRAZ and FORT and the fact that this data is generally

noisy. Additional data collected at other sites will be important in permitting a better

assessment of the X 2 in the future.

The predicted South America-Nubia velocity is not directly comparable to the

NUVEL-1 A model, since NUVEL-1 A is based on a composite African plate, while in

this study the a separate Nubian plate can be resolved. However, direct comparison to

the underlying geological data, and to the geological model is instructive. REVEL and

NUVEL-1 A azimuths in the South Atlantic are indistinguishable from each other and

from the geologic data (Figure 4.23). However, the REVEL and NUVEL-1 A rates

differ significantly, with REVEL slower than both NUVEL-1 A and the geologic data

upon which it is based by about 7 mm/yr through a large range of latitudes (Figure

4.23). The simplest interpretation is that this reflects true deceleration. Longer term

geologic data support this inference. Figure 4.24, modified from Cande and Kent

[1992], shows a remarkable coincidence between the REVEL geodetic rate and a

longer term trend of decelerating spreading in the south Atlantic going back to about

30 million years, roughly the time of initiation of Andean crustal shortening [e.g.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 ° FORT

1 ?

- 20°

- 40 ° Stable plate site residual vclocitv .L5mrn/yr 4= Boundary zone sue residual vclocitv 5mm/vr

280° 300 ° 320 ° 340 °

Figure 4.22. Similar to Figure 4.4, for the South American plate.

100

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. *u 10 -10 -30 Latitude (°N) ■ Transform fault (Bathymetry) 9 Transform fault (Altimetry) REVEL-2000 •---NUVEL-1A -50 Nu-Sa

80 60 90 70 100 North) from CW (° Azimuth -10 -30 Latitude (°N) ^ Scafloor spreading rate REVEL-2000 --- - NUVEL-1A -50 Nu-Sa 1 0 - 40- £* 30- S 20- Figure 4.23. Similar to Figure 4.6, for Nubia-South America.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Nu-Sa 50-

4 0-

REVEL-2000 N U V EL-1A 2 0 - Interval spreading rate

0 10 20 Age (Ma)

Figure 4.24. Nubia-South America spreading rates (full rate) versus age for last 25 million years, after Cande and Kent [1992] (shaded area shows approximate one standard error), compared to NUVEL-1A and REVEL model predictions (arrows) calculated at 28°S, 348°E. REVEL differs significantly from NUVEL-1 A, but agrees closely with an extrapolation of interval spreading rates to Recent time.

102

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Isacks, 1988; Sempere et al., 1990; Allmendinger et al., 1997]. It is proposed that the

same process that has slowed convergence between South America and Nazca, e.g.,

formation of the Andes and crustal thickening, which may some how impede

subduction, may also contribute to a gradual slowing of South America’s westward

motion and consequent slowing of spreading in the south Atlantic.

Conclusions

In this study a model for the relative motion of 14 lithospheric plates based on high

precision space geodetic data was derived. By excluding sites that may be influenced

by seismic cycle effects within the plate boundary zone as well sites affected by

glacial isostatic adjustment, this plate velocity model is believed to representative of at

least geologically Recent motions (last —10,000 years) and have termed it REVEL, for

/?ecent ve/ocity. This model includes a rigorous and independent estimate for GPS

velocity errors, facilitating comparison with plate velocities for different periods.

Statistically significant differences between REVEL velocity predictions and those of

the NUVEL-1 A geological model are recognized. These are associated with both

changes in velocity over the last 3 million years and systematic errors in the geological

model. Nazca-South America, Nazca-Pacific, Nazca-Antarctica and South America-

Nubia are slower than the 3 million year average. The differences are part of a longer

term slowing trend dating back 25-30 million years, possibly associated with on-going

construction of the Andes [Norabuena et al., 1999]. In addition changes in Nazca-

South America and Nazca-Antarctica direction consistent with pivoting of the Nazca

plate about its southeast comer, perhaps due to subduction of very young lithosphere

in this region are recognized. Nubia-Arabia and Arabia-Eurasia appear to be slowing,

103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. perhaps related to the collision of Arabia with Eurasia and consequent crustal

thickening and increased resistance to plate motion. Pacific-North America motion

and motion of the Caribbean plate with respect to North and South America are

significantly faster than NUVEL-1 A, presumably reflecting systematic errors in the

geological model due to sparse geological data in these regions.

104

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX: SITE COORDINATES, VELOCITIES AND RESIDUAL VELOCITIES

Site1 Position1” AT0 Totald Velocity' WRMSr Rate Resid. ID °N °E Yrs. Data North East Vertical N E V N E

Amurian Plate (Am) SUWN 37.28 127.05 2.66 838 -14.1*1.0 28.0*1.3 3.4*3.1 3.2 5.1 9.0 2.0 0 2 TAEJ 36.40 12737 4.72 1666 -172*0.7 27.8*0.9 3.8*1.9 4.0 5.8 11.1 -1.1 -0.1 VLAD 43.20 131.93 3.65 524 -163*1.1 25.5*1.2 1.5*2.6 4.5 5.9 10.0 0.3 0.1 irkt 52.22 10432 4.87 1729 -92*0.7 26.6*0.8 -1.6*1.8 4.4 5.4 10.3 2.9 -1.6 shao 31.10 12130 5.56 1686 -16.8*0.5 33.9*0.8 0.2*1.6 3.5 6.6 10.2 -1.5 4.1 tohk 35.49 133.70 3.37 1016 -15.9*1.0 23.7*1.4 -1.4*2.8 4.0 6.8 11.8 0.9 -3.4 xian 34.37 10932 3.48 982 -14.7*0.9 34.4*1.1 0.3 ±2.5 3.8 5.2 10.0 -1.5 3.5 yssk 47.03 142.72 1.09 186 -15.0*3.1 14.8*2.8 6.8*9.1 3.7 4.0 11.0 2.4 -72

Anatolian Plate (At) ANKR 39.89 32.76 5.12 1518 9.9±0.8 2.6±0.8 -I.7±1.6 4.8 6.1 8.9 -0.2 03 7580 3738 33.19 -- 12.0±4.0 5.2*3.9 -5.1*4.0 -- - 13 -23 7585 39.80 34.81 -- 17.7*4.3 3.2*4.3 -3.3±4J - - - 4.4 -5.9

Antarctic Plate (An) CAS1 -66.28 110.52 6.09 1657 -11.7*0.8 3.7*0.6 6-2* 1.7 5.5 5.5 13.3 0 2 0.3 DA VI -68.58 77.97 6.09 1677 -6.5±0.7 -2.2±0.8 3.7* 1.6 5.3 6.4 12.4 0.4 0.4 KERG -49.35 7036 5.73 1817 -5.2±0.7 5.3 ±0.9 6.9*1.6 5.1 7.1 11.6 0.1 0.3 MAW I -67.60 62.87 6.60 1732 -33*0.7 -3.6*0.8 4.3* 1.4 5.8 7.3 10.3 0.4 -0.2 MCM4 -77.84 166.67 5.53 1919 -I2.4±0.7 10.7*0.7 3.7* 1.9 4.5 5.6 14.7 -0.9 -0.4 SYOG -69.01 39.58 2.62 673 -12*1.5 -3.9*1.8 7.2*3.4 4.5 6.6 10.0 -2.8 0.6 VESL -71.67 357.16 1.99 544 8.7±2.2 -5.7*1.6 2.9±4.4 5.1 4.6 10.1 -1.4 -4.4 ohig -63.32 302.10 5.21 1277 103*1.0 14.7*0.9 8.7±2.0 6.1 6.6 13.1 -2.3 2.3 palm -64.78 295.95 2.06 714 15.8±2.3 11.8*1.9 4.4*4.1 5.8 5.5 9.9 3.6 -1.3

Arabian Plate (Ar) AMMN 32.03 35.88 1.25 229 17.3±2.2 22.3*2.3 10.5*6.3 2.8 3.9 7.5 -02 1.1 BAHR 26.21 50.61 4.12 1458 26.9*0.6 29.2*0.8 -0.7±2.0 3.0 4.9 9.3 0.0 -0.1 7575 37.92 40.19 -- 23.8*5.9 14.4*5.5 -7.7±5.8 --- 2.5 -2.3 katz 33.00 35.69 3.00 442 19.7*0.9 23.0*1.3 3.7*3.4 3.1 5.0 11.4 1.0 2.6

Australian Plate (Au) ALIC -23.67 133.89 6.05 1467 57.0*0.5 33.9*0.8 1.4* 1.6 3.2 6.3 11.8 0.0 -0.5 CEDU -31.87 133.81 6.23 772 57.5*0.5 31.8*0.7 3.4* 1.6 3.5 5.3 10.6 0.5 0.7 DARW -12.84 131.13 6.21 854 57.5*0.5 38.3*1.1 2.2* 1.7 3.5 8.7 12.5 0.3 0.1 HOB2 -42.80 147.44 6.09 1695 54.3*0.5 15.5*0.7 2.2* 1.5 3.7 5.8 10.7 0.1 -1.3 JAB1 -12.66 132.89 3.00 377 57.8*1.0 36.4*1.7 -2.1*3.3 3 2 6.4 10.5 0.7 -1.5 KARR -20.98 117.10 6.04 1488 56.1*0.5 41.8*0.9 2.2*1.6 3.4 7.2 11.7 0.0 0.6 PERT -31.80 115.89 6.96 2179 55.6*0.4 42.2*0.6 0.6* 1.3 3.3 6.3 11.2 -0.2 1.7 TID2 -35.40 148.98 4.60 1254 53.4*0.6 19.6*0.8 3.9±1.9 3.2 5.1 10.0 -0.3 -1.4 T1DB -35.40 148.98 7.60 2156 53.8*0.5 20.0*0.6 5.3*1.3 4.1 6.6 11.8 0.1 -1.0 TOW2 -19.27 147.06 5.56 1436 53.6*0.5 30.8*0.9 3.9±1.7 3.5 6.9 11.1 -0.7 -0.8 YAR1 -29.05 115.35 7.60 2406 55.2*0.4 42.2*0.6 3.6±1.2 3.4 6.2 11.4 -0.5 1.0 auck -36.60 174.83 4.89 1744 37.4*0.6 52*0.8 42*1.6 3.4 5.8 9.0 -2.3 -2.3 coco -12.19 96.83 4.15 1222 47.1*0.9 42.6*1.4 0.6±2.4 4.2 8.2 13.0 -1.5 -2.6 dgar -7.27 72.37 4.20 1415 29.9*0.8 45.6*1.6 2.7±2.5 4.2 9.3 14.1 -1.8 0.2 iisc 13.02 77.57 5.59 1564 31.7*0.6 42.4*1.0 0.0*1.8 3.9 8.1 13.0 -4.2 13.7 noum -22.27 166.41 2.60 864 42.9*1.0 22.2*1.8 2.7*3.3 3.2 6.8 10.1 -2.4 -1.4

Caribbean Plate (Ca) AVES 15.67 296.38 3.87 29 11.5*0.9 13.2*2.1 5.0±3.4 2.5 7.0 9.4 0.4 1.2 BARB 13.09 300.39 2.69 430 13.0*1.1 14.2*2.1 -0.1*3.8 3.3 7.4 11.5 0.4 0.9

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Site1 Position 11 AT1 TotaId Velocity' WRMSf Rate Resid.s ID °N °E Yrs. Data North East Vertical N E V N E

CROI 17.76 295.42 6.56 1696 10.9±0.5 10_5±0.8 2.8±1.6 3.5 6.8 12.9 0.2 -0.5 ROJO 17.90 288.33 635 21 6.2±0.9 1L5±L9 -5.5±3.7 3.0 9.0 18.4 -0.4 0.6 SANA 12.52 27837 6.25 29 4.8±0.9 14.1±1.2 0.8±3.8 4.1 6 3 20.8 1.17 1.2 TDAD 10.68 298.60 4.05 7 10.7±1.4 I5.9±33 5.9±4.1 3.2 8.1 9.4 -1.25 1.6 isab 18.47 292.95 4.18 26 9.3±1.4 11.1±2.0 -0.8±7.8 4.2 7.0 30.8 -0.44 0.6 pur3 18.46 292.93 3.16 1035 9.7±0.9 7.7±1.6 0.9±3.3 33 7.0 13.7 -0.04 -2.8

Eurasian Plate (Eu) ARTU 56.43 58.56 1.07 266 63±2.2 24.0±2.2 4.1 ±7.6 2.7 33 8.6 1.5 -1.0 BORI 52.28 17.07 5.86 2049 12.5±0.4 20.4±0.5 -2.2±1.3 2.7 3.9 8.1 -0.6 -0.1 BRUS 50.80 4.36 639 2173 13.8±0.3 I7.9±0.4 0.1±13 2.6 3.9 7.4 -0.6 -0.5 GLSV 50.36 30.50 2.45 808 11.7±1.0 22.0±1.0 03±3.2 2.9 3.7 8.4 0.7 -l.l GOPE 49.91 14.79 4.90 1695 I3.0±0.5 21.6±0.6 -53±2.5 2.7 4.0 17.4 -0.35 0.99 HOFN 64.27 344.80 2.86 949 153±0.8 11.0±0.9 3.5±2.6 2.7 3.7 7.6 0.15 135 JOZE 52.10 21.03 6.96 2329 12.4±0.4 213±0.4 -5.5±1.3 3.0 4.1 10.7 -0.11 0.11 KIRU 67.86 20.97 7.08 2372 13.3±0.5 15.9±0.6 4.9±1.4 4.1 5.9 12.5 0.79 -0.81 KOSG 52.18 5.81 7.60 2474 14.6±0.3 17.7±0.4 -0.9±1.0 2.5 3.9 7.4 0.33 -0.65 KSTU 55.99 92.79 2.92 727 -5.7±l.l 25.2±1.3 03±3.0 3.7 5.1 9.6 -1.63 -0.07 LAMA 53.89 20.67 5.63 1689 12.8±0.5 20.7±0.5 -2.3±1.4 3.0 43 8.1 034 0.01 NYAL 78.93 11.87 7.56 2106 14.4±0.4 103±0.4 4.8±I3 3.5 4.0 10.0 0.7 -0.4 PENC 47.79 19.28 437 1404 12.4±0.5 22.6±0.6 -7.0±2.4 2.7 3.7 133 -0.4 0.4 POTS 5238 13.07 5.85 1977 I3.4±0.4 20.0±0.4 -0.1±13 2.5 3.6 7.9 -03 03 SUMK 44.21 74.00 1.66 443 -0.1±1.6 26.6±1.8 6.2±5.5 3.0 43 10.6 -0.9 -0.9 TIXI 71.63 128.87 1.90 657 -12.5±1.8 14.8±1.4 7.7±4.1 4.1 4.0 8.5 -0.7 -1.5 TOUL 43.56 1.48 3.44 1093 15.0±0.6 18.8±0.8 -2.4±2.1 2.4 3.9 7.1 0.4 - 1.0 WTZR 49.14 12.88 4.58 1631 13.5±0.5 20.5±0.5 -0.8±1.6 2.5 3.4 7 3 -0.1 0.0 ZECK 43.79 41.57 2.89 972 9.5±0.8 26.6±1.0 3.3±2.7 2.6 4.1 8.4 0.7 1.0 ZWEN 55.70 36.76 533 1726 9.8±0.5 23.7±0.6 -4.8±1.6 3.1 4.4 93 0.0 0.9 case 38.69 350.58 3.49 911 15.1 ±0.6 15.6±I.O 13±2.1 2.6 4.8 7.8 0.0 -3.5 bili 68.08 166.44 0.97 268 -23.3±2.8 12.0±2.2 7.6±8.0 3 3 3.1 8.1 -8.3 3.0 bucu 44.46 26.13 1.49 451 9.1±1.5 27.1 ± 1.5 5.7±4.2 2.6 33 73 -2.6 3.6 hers 50.87 0.34 7.50 2226 15.2±0.4 15.8±0.6 0.5±1.2 3.3 6.1 10.0 0.5 -1.8 irkt 52.22 104.32 4.87 1729 -9.2±0.7 26.6±0.8 -1.6±1.8 4.4 5.4 103 -23 1.5 kit3 39.13 66.89 5.64 1503 2.3 ±0.6 29.1±0.8 -2.7±1.6 3.9 6.1 10.2 -0.4 1.4 mets 6032 24.40 7.60 2375 11.3±0.4 19.8±0.5 2.6±1.0 3.2 4.9 8.5 -0.7 0.1 onsa 57.40 11.93 7.60 2459 13.2±0.3 16.5±0.4 -0.6±1.1 2.8 4.3 9.0 0.0 0.0 pol2 42.68 74.69 531 1511 2.1±0.6 28.4±0.7 0.5±1.5 3.6 5.3 8.2 1.5 0.8 sofi 42.56 23.39 3.15 888 10.7±0.8 253±I.l -2.8±2.6 2.9 4.7 9.1 -1.5 1.7 trom 69.66 18.94 7.59 1807 153±0.5 13.9±0.5 2.0±1.1 4.2 4.9 9.0 2.5 -1.8 vill 40.44 356.05 5.71 1948 143±0.4 193±0.5 -2J±1.4 2.9 4.5 8.4 -0.6 -0.4 yakz 62.03 129.68 2.80 823 -9.3±2.0 23.1 ± 1.1 1.5±3.0 6.8 4.5 9.4 2.65 3.9

Nazca Plate (Nz) EISL -27.15 250.62 6.54 1789 -8.8±0.7 65.4±1.1 2.0±1.8 5.5 10.0 16.3 0.0 -2.5 GALA -0.74 269.70 4.51 876 9.7±0.8 53.1±1.3 2.4±2.1 4.3 7.8 10.9 1.5 2.1 GALD -0.90 270.38 -- 4.5±0.8 56.3±0.0 3.8±7.0 --- -4.5 5.2 EASA -27.15 250.62 -- -9.3±0.7 68.1 ±0.8 3.9±8.4 -- - -0.5 0.2 7097 -27.15 250.62 -- 9.3 ±0.8 68.1±0.9 3.9±0.9 -- - -0.5 0.2

North American Plate (Na) AML5 35.15 258.12 4.53 1436 -8.0±0.6 -123±0.9 -5.7±2.4 3.0 6.1 14.5 0.5 0.3 ANTO 29.49 261.42 4.53 1523 -7.0±0.6 -10.5±1.0 -3.3±2.6 3.1 6.5 16.8 0.4 0.6 AO ML 25.73 279.84 2.71 852 0.0±0.8 -8.9±1.7 -0.6±3.I 2.5 6.5 9.7 0.4 0.8 ARL5 32.76 262.94 4.53 1444 -6.4±0.7 -13.3±0.9 -5.7±2.4 3.7 6.0 15.1 0.4 - 1.1 ASHV 35.60 277.45 1.45 430 -1.0±1.4 -I4.3±2.5 -1.0±5.6 2.4 5.1 8.7 0.3 -0.7 BARH 44.40 291.78 1.85 666 3.1±1.0 -14.5±1.5 0.8±4.0 2.3 4.0 7.7 - 1.1 1.3 BRMU 32.37 295.30 7.38 2336 6.1 ±0.3 -12.4±0.5 0.1±1.2 2.7 5.1 10.4 0.6 -0.2

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Site1 Position11 AT Totald Velocity* WRMSf Rate Resid. tD °N °E Yrs. Data North East Vertical N E V N E

BRU1 43.89 290.05 4.57 1622 3.6±0.6 -153±0.8 0.9±2.0 3.1 5.2 11.0 0.1 0.5 CHL1 38.78 284.91 4.89 1663 0.7±0.5 -153±0.7 -0.2±2.0 2.8 4.9 12.2 -0.9 -0.8 DNRC 39.16 284.48 1.17 409 2.4=fcl.9 -15.9±3.2 -53±7.2 2.6 5.4 9.8 1.0 -1.3 DQUA 34.11 265.71 3.59 1218 -5.6±0.7 -12.5±1.0 7.1±2.8 2.8 5.0 13.1 03 0.2 DUCK 36.18 284.25 2.94 925 2.4±0.7 -15.6±1.I -10.6±3.2 2.6 4.7 11.7 1.1 -1.8 EPRT 44.91 293.01 1.85 600 4.7±1.0 -15.7±1.4 2.4±4.1 2 3 3.7 7.9 0.1 0.2 ERLA 39.02 27539 1.45 443 -1.0±1.4 -13.1±23 -3.7±5.0 2.4 4.5 7.1 l.l 1.5 GAIT 39.13 282.78 4.89 1470 0.5±0.4 -14.1 ±0.6 1.7±1.6 23 4.5 8.7 -03 0.6 GDAC 37.78 257.82 339 1258 -7.0±0.6 -14.1±0.9 2.2±2.2 2.4 4.7 8.9 1.6 -0.9 GODE 39.02 283.17 7.24 2370 0.8±0.3 -13.9±0.4 -0.4±L.l 2.6 4.5 8.9 -0.1 0.7 HBRK 38.30 262.71 531 1798 -6.6±0.4 -I33±0.6 -0.4±1.5 2.7 4.6 9.2 0.3 0.4 HKLO 35.68 264.14 5.31 1818 -6.1±0.5 -I3.1±0.6 1.3±1.6 2.9 4.5 9.5 0.3 -0.0 HVLK 37.65 260.89 4.19 1441 -7.6±0.5 -13.5±0.7 -0.4±1.9 2.7 4.5 9.2 -0.1 -0.1 JTNT 33.02 259.02 331 1122 -6.6±0.7 -13.5±1.0 1.6±2.4 2.7 4.5 8.4 1.6 -1.5 KAN I 39.13 264.60 3.16 1101 -5.8±0.8 -133±l.I 0.9±2.7 2.9 5.0 10.3 0.4 0.8 KELY 66.99 309.06 4.93 1494 10.1±0.5 -183±0.6 -3.9±1.6 3.1 4.3 8.7 -0.3 0.1 KULU 65.58 322.85 4.10 734 14.8±0.7 -15.5±0.6 -1.0±23 3.0 3.6 10.9 0.1 -0.2 KYW1 24.58 27835 3.13 738 -0.2±0.9 -11.9±1.6 2.2±3.0 3.1 6.8 113 0.8 -1.9 LMNO 36.69 262.52 5.46 1871 -7.1±0.5 -12.3±0.6 -1.9±1.6 3.1 5.1 10.5 -03 1.0 LUBB 33.54 258.16 4.53 1528 -9.0±0.7 -U .9± l.l 0.4±2.6 3.7 6.8 16.8 -0.5 0.1 MDOI 30.68 255.99 7.17 2360 -9.7±03 -113±0.5 0.8±1.I 2.7 5.0 8.4 -0.4 -03 MIAZ 25.73 279.84 4.89 1592 -0.7±03 -113±0.9 3.7±1.9 3.0 6.1 11.4 -0.3 -0.8 MNP1 41.07 288.14 332 1028 3.4±0.8 -15.9±1.0 2.7±2.6 3.2 4.9 9.7 0.6 -0.8 NDSK 37.38 264.36 3.92 1365 -6.0±0.6 -13.5±0.8 I.3±2.I 2.9 4.5 93 0.3 0.1 NJIT 40.74 285.82 1.18 408 3.3±2.05 -I4.0±2.9 -4.4±6.5 2.9 5.0 83 1.4 l.l NLIB 41.77 268.43 7.43 2445 -4.9±0.3 -14.0±0.4 -1.2± 1.0 2.6 43 8.2 -0.1 1.1 ODS5 31.87 257.68 4.53 1557 -9.4±0.6 -11.6±1.1 -l.0±2.5 33 6.9 16.4 -0.7 -0.1 OMH1 41.78 264.09 1.83 627 -53±1.5 -12.7±3.1 6.7±5.1 3.3 8.0 11.8 1.07 2.1 OROl 44.90 29133 2.68 684 4.4±0.8 -16.5±1.1 -0.5±3.0 2.4 4.2 8.3 0.4 -0.5 PASO 31.77 253.59 4.53 1521 -I0.8±0.6 -11.7±0.9 -53±2.2 3 3 5.7 13.3 -0.7 -0.6 PATT 31.78 264.28 331 1082 -5.4±0.7 -I2.6±1.3 2.5±2.9 2.6 6.0 11.7 0.9 -0.7 PLTC 40.18 255.27 5.46 1916 -10.4±0.4 -13.4±0.5 -23±1.4 2.6 4.0 8.3 -0.9 0.2 PNB1 44.45 291.23 1.33 464 5.0±1.6 -13.8±2.4 -73±6.8 2.5 4.7 11.0 1.0 2.1 POR2 43.07 289.29 332 1105 2.5±0.8 -16.0±1.0 5.9±2.7 3.1 4.9 10.8 -0.7 -0.4 PRCO 34.98 262.48 4.70 1584 -7.5±0.5 -12.5±0.7 -2.1±1.8 2.7 4.9 9.9 -0.5 0.3 PRDS 50.87 245.71 3.47 960 -13.9±0.6 -153±0.7 -3.7±2.1 2.3 3.4 7.1 -13 -0.6 PSUI 40.81 282.15 1.18 429 0.7±1.7 -15.2±2.7 -4.6±7.0 2.4 4.6 9.5 03 -0.0 RCMZ 25.61 279.62 4.94 1458 0.1±0.5 -9.8±0.9 -13±1.9 2.6 6.1 11.6 0.6 0.6 RED1 39.56 284.43 1.49 518 2.4±1.4 -13.1±2.5 0.1±6.2 2.5 5.4 11.5 1.0 1.7 RIC1 37.54 282.57 1.64 488 1.1±1.1 -12.1±1.8 -1.0±4.8 2.1 4.2 8.5 0.5 2.1 RWDN 40.09 259.35 1.32 422 -7.0±1.7 -12.8±2.4 8.4±5.7 2.7 4.6 7.8 1.1 1.1 SAL1 35.37 265.18 3.16 1090 -1.1 ±0.1 -123±l.l 3.3±2.8 2.5 5.0 10.9 -1.7 0.9 SAV1 32.14 278.30 1.68 500 -0.6±1.4 -13.5±I .9 2.5±5.2 2.8 4.6 10.2 0.4 -1.0 SOLI 38.32 283.55 4.87 797 1.1 ±0.5 -14.1±0.7 -3.5±2.0 2.5 4.7 10.5 0.1 0.3 STJO 47.60 307.32 7.60 2494 9.2±0.4 -I6.3±0.4 -0.7±1.0 3.3 4.8 8.5 -0.6 - 1.1 STL3 38.61 270.24 3.92 1313 -4.1±0.6 -13.7±0.9 0.4±2.4 2.7 5.2 11.5 -0.0 0.6 TCUN 35.09 256.39 2.60 862 -83±0.8 -13.7±1.1 4.8±3.0 2.6 43 8.4 0.9 -1.4 THU1 76.54 291.21 5.27 1662 3.7±0.5 -21.8±0.4 -0.2±1.6 2.9 3.1 9.1 -0.3 -03 TMGO 40.13 254.77 5.75 1788 -9.7±1.5 -I4.2±0.6 -0.8±1.3 0.6 5.3 8.2 0.0 -0.7 USNO 38.92 282.93 2.87 857 0.8±0.7 -I4.6±1.2 0.7±2.8 2.5 4.8 8.9 0.0 -0.0 UTEO 19.74 260.81 1.28 160 -6.6—2.5 -8.8±4.3 -4.3±7.9 3.4 6.9 10.9 1.0 -0.9 VCIO 36.07 260.78 5.31 1796 -6.0±0.5 -12.6±0.7 -13±1.7 3.0 5.3 10.6 1.6 0.4 WES2 42.61 288.51 7.48 2365 2.4±0.4 -15.4±0.5 -0.3±1.0 3.8 5.6 83 -0.5 0.1 WHNI 42.74 256.67 1.77 618 -8.1±1.0 -12.3±1.6 4.4±4.3 2.2 4.2 8.3 0.9 2.1 WILI 41.31 283.98 1.45 420 l.l±l.7 -15.0±23 -3.0±5.7 2.8 4.6 9.0 -0.1 0.3 WSMN 32.41 253.65 535 1796 -10.0±0.5 -11.9±0.7 0.6±1.6 3.4 5.3 10.2 0.1 -0.6 bill 68.08 166.44 0.97 268 -23.3±2.8 12.0±2.2 7.6±8.1 3.2 3.1 8.10 -3.3 3.9

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Site* Position1* AT1 TotaId Velocity* WRMSf Rate Resid.s ID °N °E Yrs. Data North East Vertical N E V N E

cicz 31.87 24333 5.38 3584 15.8±0.6 -39.1±0.4 -0.6±13 2.9 4.9 83 293 -29.0 drao 49.32 240.38 7.60 2512 -13.7±0.3 -12.1±0.4 -0.4±1.0 3.0 4.5 8.1 0.6 1.2 hofn 64.27 344.80 2.86 949 I53±0.8 11.0±0.9 3.5±2.6 2.7 3.7 7.6 -4.6 203 fair 64.98 212.50 7.60 2407 -22.7±0.4 -73±0.4 -2.2±1.2 4.0 4.9 11.8 -23 0.5 farb 37.70 237.00 3.68 517 22.5±0.6 -39.4±0.9 -0.8±2.9 2.5 4.2 12.4 37.8 -28.9 piel 34.30 251.88 7.60 2505 -ll.7±0.4 -1 1.7±03 1.6±1.0 2.6 4.7 8.0 -1.0 0.0 prds 50.87 245.71 3.47 960 -I3.9±0.6 -15.2±0.7 -3.7±2.1 23 3.4 7.1 -13 -0.6 reyk 64.14 338.04 4.55 1551 17.9±0.6 -I0.6±0.6 -33±1.7 3.0 4 3 8.1 -0.6 0.7 scip 32.91 241.51 2.14 684 20.4±0.9 -43.0±1.4 0.8±3.3 23 4.4 6.9 34.4 -32.9 snil 33.25 240.48 4.71 1015 20.1 ±0.5 -4l.4±0.7 -33±1.7 2.7 4.7 8.5 34.4 -31.4 vndp 34.56 239.38 7.58 2129 193±0.4 -42.0±0.5 -1.5±13 3.8 5.6 10.3 33.8 -31.8 whit 60.75 234.78 4.16 1447 -I6.1±0.7 -11.8±0.8 -0.7±1.8 3.6 4.6 8.1 2.2 -0.6 will 52.24 237.83 5.12 941 -13.9±0.4 -14.6±0.5 -1.1±1.5 2.2 3.6 7.0 13 -13

Sites within 1,800 km o f the center o f Hudson Bay algo 45.96 281.93 7.60 2515 -0.7±03 -16.3±0.4 3.0±1.0 2.7 3.9 7.6 - 1.1 0.2 chbl 45.65 275.53 4.89 1657 -l.5±03 -14.4±0.7 1.6±1.6 2.9 4.7 8.9 0.6 2.0 chur 58.76 265.91 5.12 1466 -6.5±0.5 -18.6±0.5 11.2±1.5 3.1 4.0 7.9 -0.8 0.1 detl 4230 276.90 4.89 1706 -3.4±0.6 -14.1±0.9 1.0±1.S 3.8 6.6 10.4 -1.8 1.5 dubo 50.26 264.13 3.80 1303 -6.0±0.7 -173±0.7 -3.7±2.2 3.0 4.0 9.5 0.4 -0.4 flin 54.73 258.02 4.16 1454 -10.2±0.6 -17.2±0.7 1.7±1.9 3.1 4.2 8.8 -1.6 -0.0 kewl 4733 27138 3.16 1105 -5.4±0.7 -18.1±1.0 2.9±2.6 2.8 4.6 93 -1.7 -1.4 mill 43.00 272.11 3.16 1099 -2.7±0.7 -17.4±1.0 0.6±2.6 2.8 4.4 93 0.7 -1.8 nrcl 45.45 284.38 5.12 1234 0.6±0.4 -16.O±0.6 4.3±1.6 23 4.1 8.6 -0.7 0.4 sagl 43.63 276.16 4.57 1570 -2.6±0.5 -18.0±0.8 1.8±1.9 2.8 5.2 103 -0.8 -2.1 sch2 54.83 293.17 2.93 10 17 5J2±0.8 -17.5±0.9 9.5±2.7 2.7 4.1 8.6 0.5 0.7 stbl 44.80 272.69 3.16 1074 -23±0.8 -163±1.1 2.9±2.5 3.1 5.1 8.6 1.0 -0.2 stpl 44.30 268.10 3.16 1103 -43±0.7 —16.04=1.1 -2.0±2.7 23 4.9 9.9 0.7 -0.3 whpl 46.77 275.04 4.02 1329 -2.8±0.6 -16.3±0.7 3.4±2.1 2.8 4.2 9.4 -0.5 0.4 wisl 46.71 267.98 3.16 1097 -4.8±0.7 -17.9±0.9 -1.7±2.5 2.5 4.0 9.0 0.2 -1.6 yell 62.48 245.52 7.60 2507 -13.3±0.4 -16.4±0.4 2.4± 1.1 3.2 4.2 93 -0.5 0.0 youl 43.23 281.03 3.16 1106 -l.7±0.8 -I4.7±1.2 0.6±3.0 3.2 5.5 11.8 -1.7 13

Sites on the Colorado Plateau cast 39.19 -110.68 3.56 796 -10.8±0.6 -14.3±0.9 -5.9±23 2.5 4.6 8.1 0.8 -1.7 cosa 33.57 -111.88 1.34 368 -10.8±2.2 -9.8±2.7 -0.1 ±63 3.4 5.1 93 1.1 1.2 cotl 3232 -110.97 1.80 150 -11.7±1.2 -I0.I±2.0 -2.8±4.6 2.1 43 6.9 -0.1 0.7 fern 3534 -112.45 1.38 393 -1 l.l±1.4 -11.5±2.1 -3.6±5.6 2.3 4.1 8.0 1.0 -0.1 piel 34.30 -108.12 7.76 2557 -11.7±0.3 -1 1.7±0.4 1.7±1.0 2.6 4.7 8.0 1.0 0.3 7290 40.33 -109.57 - -11.5±8.8 -133±5.0 13.4±9.0 --- -0.3 -0.2 7261 35.21 -111.63 - -11.8±7.0 -13.8±4.6 16.7±7.5 --- 0.1 -2.3 7234 34.30 -108.12 - -10.9±0.4 -13.0±0.3 0.7±0.4 - - - -03 -13

Sites in the Northern Gulf of Mexico anto 29.49 -98.58 4.53 1523 -7.0±0.6 -10.5±1.0 -3.3±2.6 3.1 6.5 16.8 0.4 0.6 arl5 32.76 -97.06 4.53 1444 -6.4±0.7 -13.3±0.9 -5.7±2.4 3.7 6.0 15.1 0.4 -1.2 arp3 27.84 -97.06 4.53 1594 -6.9±0.6 -17.8±1.0 -0.9±1.9 3.2 6.6 10.1 -7.2 1.0 aus5 30.31 -97.76 4.53 1516 -8.9±0.7 -8.5±1.0 -3.4±2.6 4.0 6.7 17.0 -1.8 2.9 bea5 30.16 -94.18 3.14 1019 6.4±1.2 -4.7±2.0 -14.4±4.2 4.6 8.6 19.4 12.1 6.8 core 27.74 -97.44 4.53 1529 -6.6±0.7 -10.2±1.1 -8.4±2.5 3.9 7.0 16.1 0.3 0.4 eng! 29.88 -89.94 4.66 1580 -5.5±0.7 -12.3±1.1 0.7±2.2 4.2 7.3 13.8 -1.4 -0.7 gall 29.33 -94.74 2.95 994 -6.4±0.8 -10.4±1.2 0.9±2.8 2.8 5.0 9.4 -0.5 0.8 hous 29.78 -95.43 3.16 1064 -5.9±0.9 -14.9±1.7 -9.5±3.8 3.4 7.7 17.2 0.3 -3.6 lkhu 29.91 -95.15 4.91 1185 -4.2±0.5 -1 1.4±0.8 -2.0±1.8 3.0 5.2 9.7 1.9 0.0 mlfl 32.09 -87.39 3.16 1040 0.9±1.0 -l5.1±l-2 2.8±2.7 3.6 5.6 10.4 4.1 -2.7 mobl 30.23 -88.02 3.16 1051 -3.7±0.7 -14.5±1.2 0.4±2.8 2.8 5.4 10.7 -0.3 2.7 ndbc 30.36 -89.61 3.95 1363 -4.6±0.5 -12.3±0.9 0.4±2.2 2.5 5.4 10.0 -0.6 -0.5

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Site1 Position1" AT TotaId Velocity' WRMSf Rate Resid.8 ID °N °E Yrs. Data North East Vertical N E V N E

vie I 32.33 -90.92 4.89 1646 -7.1 ±0.5 -11.6±0.9 3.7±1.8 2.8 6.0 10.9 -2.6 0.7 wnfl 31.90 -92.78 2.60 855 -5.0±I3 -10.7±1.7 113±5.0 3.7 6.4 19.0 0.2 1.4

Nubian Plate (Nu) GOUG -40.35 350.12 1.98 531 16.0±2.4 18.6±2.7 -13±5.2 5.7 7.3 13.1 -03 -2.3 HARZ -25.89 27.71 7.60 2237 17.4±0.7 173±1.0 4.8±1.5 6.0 10.4 163 0.6 -0.2 HRAO -25.89 27.69 3.86 960 15.6±0.9 18.7±1.4 1.0±2.4 4.2 7.6 11.0 -1.2 1.0 LAMP 35.50 12.61 1.37 445 I9.3±1.7 19.8±1.8 -0.6±5.7 2.7 3.7 8.2 1.9 1.8 MASZ 27.76 344.37 7.60 2397 15.4±0.3 15.5±0.6 -0.4±13 3.0 6.7 11.6 -0.0 -0.2 SUTH -32.38 20.81 2.31 780 16.0±1.6 17.4±2.1 0.4±3.9 4.5 6.8 11.1 -1 3 03 mali -3.00 40.19 4.72 1581 12.4±0.8 27.0±1.5 1.7±2.4 4.4 10.2 163 -3.0 5 3 seyl -4.67 55.48 4.44 306 12.0±1.3 40.3±3.2 20.9±3.8 6.1 16.5 21.4 -0.6 19.4

Pacific Plate (Pa) CHAT -43.96 183.43 4.84 1739 31.3±0.6 -39.5±0.8 4.0±1.7 3.7 5.8 9.2 1.5 0 3 KOKl 21.98 200.24 3.16 1086 323±1.0 -62.3±1.6 3.5±3.2 3.7 7.0 13.6 0.5 -0.7 KOKB 22.13 200.34 7.58 2444 30.7±0.5 -60.9±0.7 1.9±1.3 4.4 7.6 13.4 - 1.0 0.7 KWJl 8.72 167.73 4.39 1291 25.4±0.8 -67.7±13 1.1 ±2.4 4.3 8.0 14.2 -0.4 0.1 MARC 24.29 153.98 4.10 1103 193±1.1 -71.1±1.4 5.4±2.7 53 7.7 15.0 -1.4 -13 THTZ -17.58 21039 7.60 2006 31.4±0.7 -633±1.5 4.1±1.8 6.1 15.0 203 -0.1 0.7 TRUK 7.45 151.89 3.88 494 20.6±0.9 -69.1±1.6 6.1±3.0 3.7 7.7 14.0 0.8 -0.7 cicz 31.87 24333 538 3584 15.8±0.6 -39.1±0.4 -0.6±13 2.9 4.9 8.2 -8.3 5.9 farb 37.70 237.00 3.68 517 22.5±0.6 -39.4±0.9 -0.8±2.9 2.5 4 3 12.4 -3.7 1.8 guam 13.59 144.87 5.54 1913 03±0.6 -8.7±l.l 5.9±1.9 43 8.7 14.7 -16.4 613 mkea 19.80 204.54 3.85 1264 31.1±0.7 -62.4±13 I.7±2.3 33 6.4 10.5 -0.6 -0.9 scip 32.91 241.51 2.14 684 20.4±0.9 -43.0±1.4 0.8±3.3 2.3 4.4 6.9 -4.4 1.5 snil 33.25 240.48 4.71 1015 20.1±0.5 -41.4±0.7 -3.3±1.7 2.7 4.7 8.5 -5.0 3.0 upol 20.25 204.12 3.16 1094 34.7±1.4 -63.5±1.9 -4.5±3.4 5.2 8.5 14.5 3.0 -1.9 vndp 34.56 239.38 7.58 2129 19.2±0.4 -42.0±0.5 -1.5±1.2 3.8 5.6 10.3 -6.3 1.5

Philippine Plate (Ph) GSI1 25.83 13133 3.62 925 21.4±0.9 -37.8±13 6.2±3.0 4.0 5.8 14.3 -0.5 -1.3 GSI2 26.64 142.16 3.04 873 4.4±1.4 -30.7±2.0 4.0±3.6 5.1 8.4 14.9 -0.1 3.1 GSI3 25.95 13139 3.04 881 22.4±l.l -36.2±1.5 0.2±3.5 3.9 6 3 13.8 0.7 0.1 OKTO 20.43 136.08 3.91 14 14.0±1.5 -45.7±2.6 9.4±5.3 4.0 7.5 15.2 -0.2 1.1 guam 13.59 144.87 5.54 1913 0.3±0.6 -8.7±1.I 5.9±1.9 43 8.7 14.7 0.2 51.1 haci 33.07 139.82 4.43 932 6.6±0.8 -14.5±1.2 -2.0±2.3 4.2 7.1 12.5 -1.6 5.8

South American Plate (Sa) ASCI -7.95 345.59 4.30 1423 8.8±0.8 -5.3±1.5 1.1 ±2.4 4.3 8.9 13.7 -0.6 0.5 BRAZ -15.95 312.12 5.08 1027 11.3±0.7 -6.9±I.l 1.0±2.2 4.3 7.6 14.4 0.4 -2.2 FORT -3.88 321.57 7.19 2298 11.5±0.5 -6.7±0.9 0.5±1.6 4.4 8.6 16.3 0.6 -1.6 KOUR 5.25 307.19 7.58 2415 10.0±0.5 -3.5±0.8 2.2±1.5 4.5 8.9 16.3 -0.9 1.7 LPGS -34.91 302.07 5.10 1512 11.0±0.9 -1.6±1.2 5.7±2.1 5.4 8.2 14.5 0.3 1.2 cord -31.53 295.53 1.02 235 11.7±3.5 4.4±5.3 -3.0±9.2 4.1 7.2 10.8 1.4 6.9 nog -53.79 29235 1.44 454 13.0±2.9 -2.7±3.0 14.7±5.9 5.0 6.0 9.7 2.9 -3.0

Sundaland Plate (Su) BAKO -6.49 106.85 2.43 674 -8.4±1.4 24.5±2.4 -4.2±4.6 4.0 8.1 14.9 -8.4 24.5 NTUS 1.35 103.68 2.94 798 -63±13 27.5±2.2 -l.7±3.5 4.0 9.0 13.1 -63 27.5

- No value available. a Upper case sites are stable plate, and are used to calculate the angular velocity for the plate. Lower case sites are in boundary zone or other deforming zone. Position in decimal degrees.

126

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 Length of time series analyzed for this study. d Number of station days analyzed for this study. e Velocities in mm/yr are with respect to ITRF-97 and errors are ±1 standard error. f Weighted root mean square scatter in mm of the daily position estimates about a best fit straight line. g Residual rate in mm/yr. For lower case sites residual rate is with respect to plate interior, as defined by best fit angular velocity listed in Table 4.1.

127

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VITA

Giovanni Sella was bom on the 15th of November, 1967, in New York, New York

to Silvia and Francesco Sella. At age six he moved to Nairobi, Kenya, where he

remained till he was sixteen. This was followed by two years in Switzerland where he

completed his high school. He earned a bachelor of science degree in geology and

geophysics from the University of Wisconsin-Madison in 1990, followed by a master

of science degree in geology from Louisiana State University-Baton Rouge in 1994.

He married Silvia Dinale in 1999. In May of 2001 he received the degree of Doctor of

Philosophy in geology from Louisiana State University-Baton Rouge.

128

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Giovanni Federico Sella

Major Fields Geology

Title of Dissertation: Space Geodetic Constraints on Plate Rigidity and Global Plate Motions

Approved:

for Professor ani

ailuate School

EXAMINING CO! •EE:

Date of Examination;

11 December 2000

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.