A Seismological Study of the Las Vegas Basin, Nevada: Investigating Basin Depth and Shear Velocity Structure

UNLV Retrospective Theses & Dissertations

1-1-2005

A seismological study of the Las Vegas basin, Nevada: Investigating basin depth and shear velocity structure

Darlene J McEwan University of Nevada, Las Vegas

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Repository Citation McEwan, Darlene J, "A seismological study of the Las Vegas basin, Nevada: Investigating basin depth and shear velocity structure" (2005). UNLV Retrospective Theses & Dissertations. 1886. http://dx.doi.org/10.25669/l65y-4koz

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This Thesis has been accepted for inclusion in UNLV Retrospective Theses & Dissertations by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. A SEISMOLOGICAL STUDY OF THE LAS VEGAS BASIN, NY

INVESTIGATING BASIN DEPTH AND

SHEAR VELOCITY STRUCTURE

Darlene J. MeEwan

Bachelor of Science State University of New York at Buffalo 2002

A thesis suhmitted in partial fulfillment of the requirements for the

Master of Science Degree in Geoscience Department of Geoscience College of Sciences

Graduate College University of Nevada, Las Vegas December 2005

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SEPTEMBER 8 ______20JÎ5 _

The Thesis prepared by

______DARLENE J . MCEWAN

E n titled

A SETSMOLOGICAL STUDY OF THE LAS VEGAS B A SIN . NV______

INVESTIGATING BASIN DEPTH AND SHEAR VELOCITY STRUCTURE

is approved in partial fulfillment of the requirements for the degree of

m a s t e r o f SCIENCE DEGREE IN GEOSCIENCE

Examination Committee Chair

{SV^i2’ Examinutt ee M ember Dean of the Graduate College

Examimiion Committee Member

Examination Committee Member

Graduate College Faculty Representative

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

A Seistnological Study of the Las Vegas Basin, NV Investigating Shear Velocity Structure and Basin Depth

Darlene J. McEwan

Dr. Catherine M. Snelson, Examination Committee Chair Assistant Professor of Geoscience University of Nevada, Las Vegas

This study examines the earthquake data recorded in Las Vegas, Nevada by the Las

Vegas Valley Broadband array. Teleseismic P-wave arrivals were used to calculate travel

time delays at basin sites relative to a hard-rock site. Delays up to 0.45 s were observed

within the basin and correspond to thicknesses up to 1.52 km based on an average P-wave

velocity of 4.37 km/s. Basin depths are shallower than expected and attributed to the

upper unconsolidated basin fill. Regional earthquakes were used to calculate Rayleigh

wave interstation group velocities along basin paths. Interstation group velocities range

from 0.25 km/s to 2.14 km/s over periods of 1.3 s to 4.0 s. Shear velocities, calculated

through inversion, range from 0.28 km/s to 2.85 km/s for the basin sediments and are

attributed to the clays and unconsolidated materials within the upper basin. The shallow

shear velocities determined through this method correlate well with geotechnical surveys

and offer greater depth of penetration making this method a non-invasive means for

calculating shear velocity at the hasin scale.

iii

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

ABSTRACT...... iii

LIST OF FIGURES...... vi

LIST OF TABLES...... viii

ACKNOWLEDGEMENTS...... ix

CHAPTER ONE INTRODUCTION ...... 1

CHAPTER TWO GEOLOGICAL BACKGROUND...... 7

CHAPTER THREE GEOPHYSICAL BACKGROUND...... 15 Site Response Studies ...... 15 Basin Geometry Studies...... 17 Shallow Shear Velocity Studies ...... 18

CHAPTER FOUR LAS VEGAS VALLEY BROADBAND ARRAY...... 19 Locations ...... 19 Instrumentation and Array Installation ...... 20 Data Availability...... 21

CHAPTER FIVE DATA ...... 25 Teleseismic Data...... 25 Regional Data ...... 27

CHAPTER SIX METHODS...... 39 P-wave Travel Time Delay Methods...... 39 Interstation Group Velocity Methods ...... 41

CHAPTER SEVEN RESULTS...... 51 P-wave Travel Time Delay Results...... 51 Interstation Group Velocity Results ...... 54

CHAPTER EIGHT INTERPRETATIONS...... 89 P-wave Travel Time Delay Interpretations...... 89 Interstation Group Velocity Interpretations ...... 93

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER NINE DISCUSSION AND CONCLUSIONS...... 103 P-wave Travel Time Delay Discussion...... 103 Interstation Group Velocity Discussion ...... 105 Summary...... 108

APPENDIX ...... on CD-ROM Appendix A Teleseismic Earthquake List for the LVVBBl ...... Appendix B Unfiltered and Filtered Seismograms for Teleseismic Earthquakes ...... Appendix C Regional Earthquake List for the LVVBBl ...... Appendix D Regional Earthquake List for the LVVBB2 ...... Appendix E Travel Time Residuals to SGS ...... Appendix F Back-azimuth and Residual Plots ...... Appendix G Travel Time Residuals to F02 ...... Appendix H Unfiltered and Filtered Seismograms for Regional Earthquakes ......

REFERENCES...... 110

VITA...... 115

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

Figure 1. Location map of the study area and local faults of the Las Vegas Valley 4 Figure 2. Regional map depicting the proximity of the Las Vegas Valley to the Nevada Test S ite...... 5 Figure 3. Location map of the Las Vegas Valley Broadband array ...... 6 Figure 4. Regional tectonic map of the Las Vegas Valley ...... 11 Figure 5. Structural models of the formation of the Las Vegas hasin ...... 12 Figure 6. Stratigraphie column of late Mesozoic and Cenozoic deposits thought to comprise the Las Vegas basin ...... 13 Figure 7. Distribution of near surface deposits of the Las Vegas basin from 12 well logs ...... 14 Figure 8. Vault set-up for stations of the second deployment of the Las Vegas Valley Broadband array...... 24 Figure 9. Global map of the teleseismic earthquake locations used in the travel time delay calculations...... 32 Figure 10. Maps of regional earthquake locations examined for the surface wave dispersion analyses...... 35, 36 Figure 11. Comparison of the 1-D IASP91 earth model with the classic 1-D earth model by Jeffreys and Bullen (1940)...... 45 Figure 12. Example of the cross-correlation of P-wave arrivals using between a basin site and hard-rock site for a teleseismic event ...... 46 Figure 13. 2-D model of P-wave velocity of the Las Vegas basin and surrounding region ...... 47 Figure 14. Example of the difference between group and phase velocity for surface wave dispersion ...... 48 Figure 15. Example of the multiple filter technique for a regional earthquake ...... 49 Figure 16. Example of a phase matched filtered seismogram ...... 50 Figure 17. Graph of the travel time residuals calculated with respect to SGS and with respect to F02 ...... 63 Figure 18. Bar graph of the average travel time residuals calculated with respect to SGS and with respect to F02 and associated errors ...... 64 Figure 19. Map of the Las Vegas Valley showing back azimuths to event 2002.290.04.23 as well as the residual gradient with respect to SGS ...... 66 Figure 20. Map of the Las Vegas Valley showing back azimuths to event 2002.311.15 .14 as well as the residual gradient with respect to SGS ...... 67 Figure 21. Map of the Las Vegas Valley showing hack azimuths to event 2002.321.04.53 as well as the residual gradient with respect to SGS ...... 68 Figure 22. Map of the Las Vegas Valley showing hack azimuths to event 2002.331.01.35 as well as the residual gradient with respect to SGS ...... 69

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 23. Map of the Las Vegas Valley showing back azimuths to event 2002.285.20.09 as well as the residual gradient with respect to F02 ...... 70 Figure 24. Map of the Las Vegas Valley showing back azimuths to event 2002.286.20.55 as well as the residual gradient with respect to F02 ...... 71 Figure 25. Map of the Las Vegas Valley showing hack azimuths to event 2002.297.03.34 as well as the residual gradient with respect to F02 ...... 72 Figure 26. Graph comparing the average residual observed with respect to SGS with the average calibrated residual to SGS ...... 73 Figure 27. Map of the Las Vegas Valley showing the location of the interstation paths used in the shear velocity inversion ...... 74 Figure 28. Path F20-F04 showing Vs profile and model fits ...... 79 Figure 29. Path F20-F04 showing Vs profile and resolution matrix ...... 80 Figure 30. Path SQP-CHY showing Vs profile and model fits ...... 81 Figure 31. Path SQP-CHY showing Vs profile and resolution matrix ...... 82 Figure 32. Path SQP-F23 showing Vs profile and model fits ...... 83 Figure 33. Path SQP-F23 showing Vs profile and resolution matrix ...... 84 Figure 34. Path VAH-F04 showing Vs profile and model fits ...... 85 Figure 35. Path VAH-F04 showing Vs profile and resolution matrix ...... 86 Figure 36. Path F04-N06 showing Vs profile and model fits ...... 87 Figure 37. Path F04-N06 showing Vs profile and resolution matrix ...... 88 Figure 38. Cross-section of the Las Vegas Valley depicting incoming P-waves from teleseismic earthquakes ...... 99 Figure 39. Comparison of calculated hasin depths with the Langenheim et al. (2001a; 2001b) model ...... 100 Figure 40. 3-D slice of the Las Vegas Valley showing basin contacts ...... 101 Figure 41. Map of the Las Vegas Valley depicting the depth to the water table 102

Vll

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

Table 1. Station information for the first and second deployments of the Las Vegas Valley Broadband array...... 22 Table 2. Legacy stations reacquired by the Las Vegas Valley Broadband array ...... 23 Table 3. Teleseismic earthquakes used in travel time delay methods ...... 30, 31 Table 4. Regional earthquakes for the interstation group velocity methods ...... 33, 34 Table 5. Interstation paths examined for the interstation group velocity methods.... 37, 38 Table 6. Average residuals, calibrated residuals and estimated basin depths ...... 65 Table 7. Source-receiver group velocities ...... 75, 76 Table 8. Interstation group velocities ...... 77, 78

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

I would like to thank my advisor, Dr. Catherine M. Snelson, for her guidance and

encouragement throughout this projeet. Without her incentive, I would not have

experienced a wide variety of research opportunities throughout my time here. I would

also like to thank the members of my committee, Drs., Andrew Hanson, Wanda Taylor,

Barbara Luke and Artie Rodgers for their time and support. Special thanks to Dr. Robert

Herrmann, Dr. Hrvoje Tkalcic, and Dr. Randy Keller for their expertise and direction.

Thank you to Voss Lytle for his computer assistance and Kai Watson for extracting the

earthquakes from the continuous data set of the LVVBB2. Thank you to Pat Lewis, Don

Rock, Duane Smith, John Sandru, and Aaron Hirsch for setting up the broadband

stations. Also, thank you to those who housed the instrumentation including the Las

Vegas Water District, the CCSN Cheyenne Campus, Nellis Air Force Base, the Las

Vegas Motor Speedway, the Clark County Fire Department, the City of Las Vegas Fire

Department, the Central Las Vegas Fire Department, and the Southern Nevada Water

Authority. Special thanks to Ron Lynn at the Clark County Building Department for

arranging the permitting at the local fire departments and Brian Wernicke at Caltech for

allowing us to deploy at their GPS station north of town.

Thanks to all my colleagues here at UNLV, including the Geophysics group, for their

constant support and friendship. Thank you to my family, especially my parents, Ed and

Stella, who have provided me with the education needed to complete this thesis. Finally, I

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thank my fiancé, Jonathan, whose love and support has been essential to the completion

of this thesis, as well as his initial encouragement to pursue this degree.

This thesis is part of the Las Vegas Valley Seismic Response Project and was funded hy

Lawrence Livermore National Laboratory, the University of Nevada Las Vegas, the

National Nuclear Security Administration, the UNLV Graduate and Professional Student

Association and the UNLV Geoscience Department.

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

INTRODUCTION

Las Vegas Valley (LVV), Nevada is a northwest-southeast trending broad

sedimentary basin located within the central Basin and Range province of western North

America. This fault-bounded asymmetric basin contains Miocene through Holocene

clastic deposits including Late Neogene alluvial deposits (Plume, 1989). Basement roeks

are classified as Precambrian through Miocene metamorphic, carbonate, clastic and

volcanic rocks (Plume, 1989).

Nevada is classified as the third most seismically active state following California and

Alaska (dePolo et al., 2000). In addition, the Federal Emergency Management Agency

(FEMA) characterizes southern Nevada as an area of high seismic risk hased on

annualized earthquake loss ratios (Federal Emergency Management Agency, 2000). A

recent hazard simulation using the HAZUS (Hazards U.S.) program, a risk assessment

software program for analyzing potential losses from floods, hurricane winds and

earthquakes, estimated economic losses totaling $11 hillion and over 10,000 casualties in

Clark Country, Nevada for a Mw 6.9 on a fault within the LVV (Perry and O’Donnell,

2001).

Young normal faults located within the region, including the Death Valley fault zone,

California Wash fault zone, the Black Hills fault, and the Frenchman Mountain fault zone

are potential sources of strong ground motion. Recent work has shown that at least eight

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. normal faults within the basin (Cashman fault, Eglington-Decatur fault. Frenchman

Mountain fault. River Mountains fault. West Charleston fault, Whitney Mesa fault.

Valley View fault, etc.) are tectonic in origin and not compaction driven as previously

believed (Slemmons et al., 2001) (Figure 1). These north-south striking faults are Late

Neogene in age and capable of producing M6.5 - 7.0 earthquakes within the Valley

(Slemmons et al., 2001). Recent seismological studies have shown that strong ground

motions in alluvial hasins from sources up to 300 km away can be overwhelmingly

destructive (e.g.. Su et al., 1998). Therefore, understanding the hasin's response to

ground motions is crucial for hazard mitigation and minimizing economic losses.

New research conducted by the Las Vegas Valley Seismic Response Project

(LVVSRP), including the focus of this thesis, attempts to characterize the Las Vegas

basin’s response to strong ground motions as well as possible future nuclear testing at the

Nevada Test Site (NTS) (Figure 2). The project was a collaborative effort hy engineers

and geoseientists from Lawrence Livermore National Laboratory (LLNL), the University

of Nevada Las Vegas (UNLV), the University of Nevada Reno (UNR) and the University

of California Berkeley funded by the National Nuclear Security Administration (NNSA),

the Department of Energy (DOE), US Geological Survey (USGS) and UNLV. Research

conducted as part of the LVVSRP, consists of geological and geophysical studies

charaeterizing the basin’s geometry, depth and near-surface geology; geophysical and

engineering studies resolving the velocity of basin sediments; and engineering studies

determining building and structural responses to strong ground motions. This thesis

utilizes earthquake data collected as a subset of this project. Data include loeal, regional

and teleseismic earthquakes acquired by two deployments of the Las Vegas Valley

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Broadband array (LVVBB) (Figure 3). These data provide new geophysical constraints

on the northeastern portion of the LVV, an area believed to overlie the deepest portion of

the basin (Langenheim et al, 2001a and 2001b; Snelson et al., 2004) and previously

unconstrained by earlier arrays (Figure 3).

The purpose of this research project was to examine new data acquired hy the

LVVBB over the understudied northeastern section of the basin. Seismic waves

generated from earthquake events offer insight to the geophysical properties of the upper

crust and basin. The study of these waveforms was used to better constrain the hasin’s

depth and shear velocity structure as well as contribute to complete models of Las Vegas

basin providing further understanding of the basin’s response to ground motions.

This study examines both teleseismic events in addition to regional and local

earthquakes. Primary or P-waves are examined from teleseismic events to calculate

differences in travel-times from the earthquake’s source to a hard-rock site when

compared to hasin sites. This analysis provides estimates of hasin-fiU thicknesses and is

compared to current hasin models. Surface waves, in particular Rayleigh waves, were

studied to evaluate the shear velocity structure of the hasin fill. For regional earthquakes,

short-period, fundamental mode Rayleigh wave group velocities were calculated using a

multiple filter technique (Dziewonski et al., 1969). These group velocities are calculated

along two-station paths which criss-cross the deepest part of the basin. This analysis

contributes 1 -D profiles of the shear velocities of the hasin fill sediments through

inversion. Shear velocities offer insight to the shear stiffness of shallow basin sediments,

an important factor in controlling ground motion.

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

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Figure 1. Regional fault map of LVV with annotated features. Inset of a Nevada state map showing the study area in black upper left. Valley faults consist of east-dipping late Neogene normal faults, west-dipping normal faults and the inactive Las Vegas Valley Shear Zone. CF: Cashman fault; EDF: Eglington-Decatur fault; FF: Frenchman Mt. fault; RMF: River Mountains fault; WCF: West Charleston fault; WMF: Whitney Mesa fault; VVF: Valley View fault; LVVSZ: Las Vegas Valley Shear Zone. Adapted from Slemmons et al. (2001). Also shown are the contours of depth to basement (contour interval = 1 km) from Langenheim et al. (2001a; 2001b). The deepest portion of the basin is found northwest of Frenchman Mountain with depths up to 5 km.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. OR M VADA N\

NTS

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Figure 2. Regional map depicting the proximity of the LVV to NTS with annotated features. Inset of a Nevada state map (upper right) showing the location of the map outlined in black, with the study area in the solid black box. Gray lines indicate city roads and highways.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. i t I \ Shoii-poriod SciMiiomeler lAA'Ulil broadband SoiMiionioicr 1.\ \1JR2 Uroadbaiid Seisniomcler Illume and Ashociales

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Figure 3. Map of Las Vegas Valley showing the station locations and names of LVVBBl in blue (September 2002 - January 2003) and LW BB2 in yellow (July 2003 - present). The Blume and Associates array is shown as black circles for reference (e.g. Murphy and Hewlett, 1975; Su et al., 1998; Rodgers et al., 2004). Also shown are the contours of depth to basement as gray lines (contour interval = 1 km) from Langenheim et al. (2001a; 2001b). Light gray lines indicate major highways.

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

GEOLOGIC BACKGROUND

The western margin of North America has experienced convergent, divergent and

transform plate boundary interactions and combinations thereof throughout its

development (e.g., Burchfiel et al., 1992). Rifting, magmatism, and the initiation of the

Cordilleran passive margin occurred about 600 Ma (e.g., Burchfiel et ah, 1992). Marine

and non-marine sedimentation resulted in a westward thickening Paleozoic depositional

sequence that was deposited on Archean and Middle Proterozoic crystalline basement

rocks that comprise the North American eraton (e.g., Burchfiel et ah, 1992). The Antler

orogeny in the Late Devonian and the Sonoma orogeny, beginning in the latest Permian,

marked the end of the quiescent passive margin setting (e.g., Burchfiel et ah, 1992).

During the Mesozoic, western North America developed a convergent plate margin

associated with subduction of oceanic crust beneath the continent (e.g., Burchfiel et ah,

1992; Taylor, 1996). East and southeast compression associated with convergence

attributed to the Nevadan, Sevier and Laramide orogenies resulted in accreted terrains in

the west and thrust faulting farther inland (e.g., Burchfiel et ah, 1992). Mesozoic thrust

faults seen in southern Nevada related to the Sevier orogeny include the Keystone, Gass

Peak, and Muddy Mountain thrusts (Figure 4) (e.g., Burchfiel et ah, 1974; Tabor, 1982;

Taylor, 1996). These thrusts typically place Neoproterozoic to Paleozoic rocks over

upper Paleozoic or Mesozoic age units (e.g., Taylor, 1996). The Cenozoic marks the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. initiation of the San Andreas transform plate boundary around 30 Ma (e.g., Burehfiel et

al., 1992; Taylor, 1996). At this time. Basin and Range extension and related strike-slip

faulting became widespread (e.g., Wernicke, 1992; Taylor, 1996).

During the Cenozoic, strike-slip structures such as the LVVSZ and Lake Mead fault

system (LMFS) developed in southern Nevada (e.g., Guth, 1981; Tabor, 1982;

Duebendorfer and Wallin, 1991; Duebendorfer and Black, 1992; Duebendorfer and

Simpson, 1994; Campagna and Aydin, 1994; Taylor, 1996; Duebendorfer et al., 1998;

Slemmons et al., 2001) (Figure 4). Several studies of these stmctures offer insight to

basin genesis based on structural interactions, however these interactions have largely

been left unresolved (e.g., Guth, 1981; Duebendorfer and Wallin, 1991; Duebendorfer

and Black, 1992; Duebendorfer and Simpson, 1994; Campagna and Aydin, 1994;

Duebendorfer et al., 1998; Langenheim et al., 2001a; 2001b). It has been hypothesized

that the LVVSZ in combination with other strike-slip faults in the region accommodates

differences in degrees of extension in different areas (e.g., Guth, 1981; Duebendorfer and

Simpson, 1994). Others suggest that the LVVSZ and the LMFS form kinematically

linked structures accommodating areas of extension and shortening (Duebendorfer et ah,

1998). Several studies have associated strike-slip faulting with the creation of pull-apart

basins in LVV, and not the accommodation of regional stresses (Campagna and Aydin,

1994; Langenheim et al., 2001a; 2001b) (Figures 5a and 5b).

In addition to the strike-slip faults, several normal faults cut across the Valley

(Slemmons et ah, 2001). Models by Langenheim et al. (2001a; 2001b) suggest that strike-

slip faulting in combination with normal faulting has played a significant role in the

formation of Las Vegas basin, forming pull-apart basins trending northwest-southeast

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. across the Valley with the deepest sub-basin northwest of Frenchman Mountain (Figure

5b). Studies suggest that these north-south striking normal faults have a tectonic origin

with a minor component of slip related to subsidence within the LVV (Slemmons et ah,

2001; Langenheim et al. 2001a; 2001b) (Figure 1). These normal faults are potential sites

of M6.5-7.0 earthquakes generating ground motions within LVV, therefore threatening

the greater Las Vegas community (Slemmons et ah, 2001) (Figure 1).

Mountains west and north of LVV include the Spring Mountains, Sheep Range and

Las Vegas Range. These mountains are primarily composed of Paleozoic marine and

nonmarine units over Mesozoic eolian sandstone (e.g., Burchfiel et al., 1974; Tabor,

1982). Frenchman and Sunrise mountains border the LVV on its eastern edge and are

composed of sedimentary rocks of the Grand Canyon supergroup consisting of mostly

Paleozoic marine sediments underlain by Precambrian granite and schist of the Vishnu

Group (e.g., Tabor, 1982). South of the LVV are the McCullough Range and River

Mountains containing Miocene volcanic and intrusive rocks including a quartz-

monzonite exposed in the McCullough Range (e.g.. Plume, 1989).

The Valley fill is composed of clastic deposits and younger coalescing alluvial fans

(Tabor, 1982) (Figure 6). The Miocene Horse Spring Formation is exposed in the

southeast part of the Valley and volcanic rocks in the southwest (Plume, 1989; Tabor,

1982). The Thumb member of the Horse Spring Formation consists of mostly clastic

rocks with freshwater limestone, landslide breccia, evaporites, and igneous intrusives

(Tabor, 1982) (Figure 6). The lower and upper members of the Horse Spring Formation

are described as conglomerates with interbedded sandstone and siltstone; the middle unit

contains tuff, tuff breccia and voleaniclastic rocks (Tabor, 1982) (Figure 6). The Muddy

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Creek Formation is Miocene to Pliocene in age and makes up most of the upper

consolidated rocks beneath Late Neogene deposits (Tabor, 1982; Plume, 1989) (Figure

6). The Muddy Creek Formation contains a lower member of sandstone and

conglomerate and an upper member of sandstone and siltstone with some ealeium

carbonate horizons (Tabor, 1982). Drill hole logs suggest that the upper member of the

Muddy Creek Formation composes a substantial portion of the basin fill (Tabor, 1982).

During the Pliocene and Quaternary, fluvial and alluvial fans shed from nearby

mountain ranges graded sediments basinward coalescing in towards the center of the

LVV (Tabor, 1982) (Figure 6). Thick lenticular gravels, sands, and silts with some lake

deposits comprise the stratigraphy (Tabor, 1982).

Recent work by Taylor et al. (2004), as part of the LVVSRP, has characterized the

near surface deposits of the basin from over 1100 well logs with an average depth of 165

m and classified the basin sediments into three domains. The three spatial assemblages

are the western, central - Las Vegas Wash, and eastern sections (Taylor et al., 2004). The

western province covers a broad extent with primarily coarse-grained deposits associated

with alluvial fan deposition that interfinger with clay deposits in a wide zone toward the

center of the basin (Taylor et ah, 2004) (Figure 7). The central - Las Vegas Wash region

is dominated by clay-rich sediments that interfinger with coarse-grained materials in a

narrow zone on its eastern margin (Taylor et al., 2004) (Figure 7). The eastern zone is

dominated by a mix of coarse to fine gravels and clays (Taylor et al., 2004) (Figure 7).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I lAts li'nas I alley

Keystone - Muddy Mountain Gass Peak - Wheeler Pass Glendale thrust plate thrust plate

Contact - Red Spring - Autochthon Wilson Cliffs thrust plate

Basin fill deposits

Figure 4. Regional tectonic map of LVV and surrounding areas adapted from Taylor (1996). Marked are the major Mesozoic thrust faults in the area, including the Wheeler Pass - Gass Peak, Keystone - Muddy Mountain - Glendale - Mormon, and Contact - Red Spring - Wilson Cliffs thrusts. Cenozoic strike-slip faults include the Las Vegas Valley Shear Zone and Lake Mead fault system. FM = Frenchman Mountain, GPt = Gass Peak thrust, Kt = Keystone thrust, LM = Lake Mead, LMFS = Lake Mead Fault System, LVR = Las Vegas Range, LVVSZ = Las Vegas Valley Shear Zone; MMt = Muddy Mountain thrust, WCT = Contact - Wilson Cliffs - Red Spring thrust, WPt = Wheeler Pass thrust.

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

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Figure 5. a. Geometry of the pull-apart basin model by Campagna and Aydin (1994). b. Structural model of Las Vegas basin from Langenheim et al. (2001a). A, B, and D mark elongated pull-apart basins; C marks a sub-basin not defined as a pull-apart basin.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thick Age Fm. ness Section Lithology (m) OOOOOOOOOOOOOOOOQ Unconsolidated gravel, sand, silt i g .% H °o o o o and clay from river beds, washes, « fans, dunes and talus slides Sandstone with some interbedded .goooooooooooooooa. kra > - ?.9.9,9.P.P.?° conglomerate, siltstone, and clay s i ^ooooooo 3 O O O O O oc i o 0» • • O • ♦ 0*0 • • • Gravel and sand o»o* • o • • *o O * *o COO“I (DCO 3 o O o O o OO Late ^ o o o o q o c Neogene •0*00*0 0*000 Q » O Gravel and sand Deposits • • *0 * 0 • • o ♦ • »o 0 » • • O • • 0 * 0 • • • • o *o • • o • • »o 0) O* * *0 • *o*o* • • "C O O o O ° % • • 0*0 * * Carbonate cemented gravel, sand, o and silt 0) o c N 0) CO Sandstone, siltstone and clay. o o II Muddy + Z 0) C reek Fm. o o o o a Fm. 00 ^ o o o O O OO some sandstone and siltstone with an intermediate limestone o mem ber CO OOOOOQOOOQOOOOOOO CM DOOOOOOOOOOOOOO Upper ooooooooooooooooo Some voleaniclastic rocks and Thumb pumice Mbr. OOOOOOOOOOOOOOQO Clastic rocks: sandstone, siltstone, o- ''OooooqoooO tJ OOOOO Lower conglomerate, lenses of landslide Thumb breccia Mbr. Fresh water limestone § Some lava flows and volcanic ash f i , ■ o o OO a o ~

o œJ O O i o - o np o o P o O O o o q o CO 'o O O o o o o o o o o Conglomerate po O o OooC i^o o o o _oo _oO OO o

4 P Conglomerate Breccia irfi Siltstone mnnn Unconformity Pebble prrnTTq r, , . p conglomerate Sandstone r Shale Volcanic ash

Figure 6. Stratigraphie column of late Mesozoic and Cenozoic strata thought to compose the basin fill. Adapted from Tabor (1982).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■D O Q. C g Q.

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C/) C/) W 664 8 ci' Feet Meters O-rO 250 660 709 250- 500 (J O , jÿ , o , 3 0 -O r-O 707 3 " - 100 CD ,704 o o .-o'. CD 250- ■D Q ÿ j O ÿ O , 0 . 0.0 O 500- OW

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Figure 7. Near surface deposits of the Las Vegas basin from 12 well logs (Log number indicated at top of section) located along a W- E transect across the center of the L W along Charleston Boulevard (Taylor et al., 2004).Yellow colors represent coarse- and mixed- grain size deposits. Grays represent clays. Blues represent carbonate: bedrock, cement or caliche. Orange represents the Jurassic Aztec Sandstone, a bedrock unit. Tops of wells are at their respective elevations given a 3Ox vertical exaggeration (from Taylor et al., 2004). CHAPTER THREE

GEOPHYSICAL BACKGROUND

A limited number of geophysical studies have been conducted within the LVV. Early

work consisted of seismological studies associated with monitoring nuclear testing from

the NTS for the Atomic Energy Commission and the Department of Energy (e.g.,

Bennett, 1974; Murphy and Hewlett, 1975). These projects largely consist of site

response studies to measure the amounts of ground motion in the LVV (e.g., Bennett,

1974; Murphy and Hewlett, 1975). In addition, gravity studies have been conducted to

better define the basin geometry for hydrological modeling and understand the geological

evolution of the basin (Plume, 1989; Campagna and Aydin, 1994; Langenheim et al.,

2001a; 2001b). Recent work conducted as part of the LVVSRP includes crust refraction

studies, shallow shear velocity studies, as well as seismological studies to further

understand the basin’s response to ground motions and possible future nuclear testing at

NTS (Snelson et al., 2004; Taylor et al., 2004; Rogers et al., 2004; Tkalcic et al., 2003;

Lin et al., 2005; Scott et al., 2005).

Site Response Studies

Early geophysical work consisted of strong ground motion and site response studies

examining basin amplification. Amplification is a measure of the increase or decrease in

shaking at a particular site due to local conditions relative to a hard-rock site. Areas

located on “soft-rock” sites (unconsolidated basin sediments) experience substantial

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. surface wave amplification in comparison to areas located on “hard-rock” (bedrock) sites

(e.g., Wen et al., 1992).

Early site response studies in the LVV utilized data acquired by the Blume array (e.g.,

Murphy and Hewlett, 1975; Su et al., 1998; Rodgers et al., 2004). The Blume array

consisted of at most 33 three-component seismometers located in present-day central Las

Vegas (Figure 3). Ground motion data were colleeted from regional earthquakes and

underground nuclear tests for the Atomic Energy Commission and the Department of

Energy from the early 1950s through 1992 (Rodgers et al., 2004). Each study assessed

amplification within the basin fill relative to a hard-rock site. Murphy and Hewlett’s

(1975) study examined data recorded from six underground nuclear tests at 26 stations

within central LVV. Results indicated a maximum amplifieation factor of 2.5 at 1.0

second period on sites located within the Valley (Murphy and Hewlett, 1975). Su et al.

(1998) examined data reeorded from the June 29, 1992 Little Skull Mountain (LSM)

earthquake recorded on 9 stations within the Valley. Their results suggested that site

amplification is five times higher at basin sites compared to hard-rock sites (Su et al.,

1998). These values are larger than previously estimated by Murphy and Hewlett (1975)

by a factor of two. This discrepancy was attributed to the reference site used in the

Murphy and Hewlett (1975) study, which was located on 400 m of alluvial fill,

suggesting that values previously estimated may be lower due to amplification at the

reference site (Su et al., 1998). A recent study by Rodgers et al. (2004) determined site

response and amplification using a more complete data set collected by the Blume array.

Data included the 1992 LSM earthquake and data from 13 nuclear explosions at the NTS

recorded at as many as 16 stations throughout the Valley including two hard-rock sites

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Rodgers et al., 2004). Results indicate a relatively long duration of shaking on alluvial

surfaces with amplifications reaching values of as much as 10 (Rodgers et al., 2004). As a

result, additional data are needed to further refine amplification values within the basin

sediments to resolve these diserepancies.

Basin Geometry Studies

Plume (1989), Campagna and Aydin (1994), and Langenheim et al. (2001a; 2001b)

condueted studies defining basin geometry and depth to basement using gravity data

collected throughout the LVV. Plume’s (1989) study used gravity data to constrain the

stmctural geometry of the basin in an effort to develop a hydrological model of the

Valley’s ground-water system. Plume (1989) determined two sub-basins within the

western LVV; a gentle east-dipping surfaee was loeated along the western margin of the

Valley with the deepest portion estimated at 0.9 to 1.5 km depth. Work by Campagna and

Aydin (1994) documented a deep basin following the northwestern trend along the

LVVSZ. Langenheim et al. (2001a; 2001b) eonstructed a basin configuration model

beneath LVV from -2000 gravity values collected by the US Geological Survey. These

data were then constrained by drill hole and seismic reflection data (Langenheim et al.

2001a; 2001b). Results from Langenheim et al. (2001a; 2001b) suggest a series of

northwest trending sub-basins, with the deepest portion beneath the northeastern part of

the Valley, west of Frenchman Mountain (Figures 3 and 5b). Maximum depths within

this region are estimated up to 5 km (Figure 3). A recent seismic refraction project,

SILVVER 2003 (Seismic Investigation of the Las Vegas Valley: Evaluating Risks)

conducted as part of the LVVSRP, confirms basin geometries and depths as well as

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. images a large sub-basin defining the change from unconsolidated to consolidated

lithologies (Snelson et al., 2004). In addition, Snelson et al. (2004) suggest that high

velocity zones correspond to local faults mapped at the surface.

Shallow Shear Velocity Studies

Shallow surface wave studies considered part of the LVVSRP have been conducted to

characterize the shear velocity of the upper few hundreds of meters of the basin

sediments for seismic microzonation (Liu et al., 2005; Seott et al., 2005). Both active and

passive source studies were performed to integrate detailed shallow measurements with

greater depth of resolution (Liu et al., 2005). A passive source study by Scott et al. (2005)

determined the NEHRP (National Earthquake Hazard Reduction Program) Vs( 30 ) site

elassification, a measurement of shear wave velocities averaged to 30 m depth, along a 13

km transect. This study was able to correlate some shear velocity measurements to

mapped surfaee soils, however they had only partial success in extrapolating these

measurements basin wide. In addition, some correlation was found between stratigraphie

models and Vs( 30). Liu et al. (2005) attempted to diseera the relationship between Vs of

shallow basin sites and deep basin sites. Although deep basin sites typically had lower Vs

values than those over shallow basin sites, they concluded that this pattern was likely

attributed to Ethology (Liu et al., 2005).

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

LAS VEGAS VALLEY BROADBAND ARRAY

In the mid 1950s through the early 1990s, Blume and Associates deployed an array of

seismometers in the L W to monitor ground-motions generated from nuclear testing at

the Nevada Test Site (e.g.. Smith et al., 2001; Rodgers et al., 2004). The coverage area

was constrained by city limits during this era and was limited to present-day eentral Las

Vegas (Figure 3). However, there is a lack of data within the Valley’s recently urbanized

areas including the northeastern region; an area estimated to overlie the deepest portion

of the basin based on recent geophysieal studies (Figure 3) (Langenheim et al., 2001a;

2001b). In an effort to acquire new data in areas previously uneonstrained by the Blume

array, LLNL and UNLV deployed two arrays of broadband and short-period

seismometers in the northern region of the Valley (Figure 3). Twelve instruments were

deployed from September 2002 through late January 2003 (LVVBBl) (Figure 3; Table

la). Six instruments were redeployed in July 2003 and acquired data through September

2004 (LVVBB2) (Figure 3; Table lb).

Locations

Locations for both deployments of the Las Vegas Valley Broadband array (LVVBBl

and LVVBB2) were chosen based on accessibility, security, and geographic location

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Table la and Ib; Figure 3). Some stations were re-deployed at legacy sites, sites

previously oceupied by the earlier Blume array (Table 2; Figure 3).

The first array, LVVBBl, consisted of a mix of twelve broadband and short-period

seismometers with a roughly southwest-northeast geometry across the central and

northeastern section of the Valley. Acquisition for LVVBBl began in late September

2002 and ended in late January 2003. The second deployment of the Las Vegas Valley

Broadband array (LVVBB2) consisted of six broadband stations located primarily at

Nellis Air Force Base and loeal sehools. The LVVBB2 was deployed in July 2003 and

removed in mid-September 2004.

Instrumentation and Array Installation

For the twelve stations of LVVBBl, all were 3-eomponent sensors with three short-

period Teledyne Geoteeh S-13s, two broadband Guralp CMG3ESPs and seven broadband

Guralp CMG40Ts. All six sensors for the LVVBB2 were three-eomponent broadband

Guralp CMG40Ts. Sensors for both the LVVBBl and LVVBB2 were set to record with a

sample rate of 40 samples/see. Broadband sensors sample seismic waves over a broad

range of frequencies typically 0.01 Hz to 50 Hz and are capable of accurately recording

long period signals from teleseismic events. Short-period sensors typieally sample at

higher frequencies (greater than 1 Hz) and are ideal for regional networks. Sensor types

used in the two deployments were based on availability from LLNL. All stations used a

Reftek-08 DAS (data acquisition system) powered by a solar panel and car battery.

Seismie stations for LVVBBl were housed in a crude vault system with the sensors

located above ground. Proper vaults were prepared to house the instrumentation for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LVVBB2, consisting of a 3x2 ft lock-box containing the battery, digitizer and GPS clock,

a buried sensor and mounted solar panel (Figure 8). Each array consisted of sites located

within the northeastern portion of the basin with one exception; station SGS/OMS was

located at a “hard-rock site” (bedrock) near Frenehman Mountain.

Data Availability

Throughout both deployments of the LVVBB, data availability for each station varies

for any particular earthquake. If no data are available from any station for a particular

event, then earthquake size, depth and/or distance from the array is sueh that baekground

noise is greater than the signal and the event cannot be detected. The use of the

broadband seismometers has made teleseismic events with a minimum magnitude of Mw

5.5 deteetable in the LVV. However, these same events are difficult to detect with the

short-period seismometers used in the LVVBBl due to their instrument response. For

some earthquake records, seismograms are unavailable from one or two seismic stations

and attributed to problems related to low power supply, overheating, lack of disk space

and mechanical problems with the digital acquisition system.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table la: Station Information for LVVBBl Deployment dates: September 2002 - January 2003

Station ID Location Latitude Longitude Sensor %pe

ULV UNLY Campus 36.1083 -115.1411 CMG3ESP Broadband

LVW Las Yegas Water 36.1754 -115.1884 S13 Short-period District

CHY CCSN Cheyenne 36.2238 -115.1056 CMG40T Broadband Campus

YAH YA Hospital on 36J47 -115.0505 CMG40T Broadband Nellis AFB

LYM Las Yegas Motor 3&2S5 -115.0113 CMG40T Broadband Speedway

GPS GPS Station at 36.3192 -114.9318 S13 Short-period Apex Mine site

F20 Clark County Fire 36.2003 -115.0479 CMG40T Broadband Station 20

F23 Clark County Fire 36.2335 -115.0809 S13 Short-period Station 23

F02 North Nellis AFB: 36.2511 -114.9755 CMG40T Broadband Firestation 2

F04 East Nellis AFB: 36.2388 -115.0232 CMG40T Broadband Firestation 4

SOS Stewart Grant 36.1816 -115.0195 CMG40T Broadband Reservoir Frenchman Mtn.

SQP Squires Park Central 36.175 -115.1409 CMG3ESP Broadband Las Yegas Fire Department

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table lb: Station Information for LVVBB2 Deployment Dates: July 2003 - present

Station ID Location Latitude Longitude Sensor

F02 North Nellis AFB: 36.2512 -114.9753 CMG40T Broadband Firestation 2

F04 East Nellis AFB: 36.2383 -115.0236 CMG40T Broadband Firestation 4

N06 Southeast Nellis 36.2203 -115.0346 CMG40T Broadband AFB

MHS Mohave High 36.2546 -115.1383 CMG40T Broadband School

HMS Hyde Park Middle 36T608 -115.1938 CMG40T Broadband School

QMS O'Callahan Middle 36.1825 -115.018 CMG40T Broadband School

Table 2: Reacquired legacy stations by LVVBB

Blume array LVVBBI(LVVBB2) Location

LVWl LVW Las Vegas Water District

SE6 ULV UNLV Campus

SGS SGS(OMS) Stewart Grant Reservoir - Frenchman Mtn

SQP4 SQP Squires Park Central Las Vegas Fire Department

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8 Solar Pa ne (O'

Fiberglass Enclosure 3. 3 " 3 0 .0 " CD

■DCD Reftek Digitizer O CQ. a Battery o 3 ■D O &L Sels. Co\er

CD C o n c re te Q.

Guralp CMG-40T ■D CD Seismometer C o n c re te C/) C/) Bedrock where C o n c re te possible.

Figure 8. Vault set-up. Provided by Pat Lewis and Don Rock of Lawrence Livermore National Laboratory. CHAPTER FIVE

DATA

Data for the LVVBBl and LVVBB2 consist of continuously recorded local, regional,

and teleseismic earthquake events recorded with a sample rate of 40 samples/sec. Data

for the LVVBBl were offloaded and separated by event times off site at LLNL. For the

LVVBB2, data disks were changed monthly and offloaded at the UNLV geophysics

computer laboratory. A catalog was built for regional and teleseismic earthquakes using

events tables from the USGS NEIC (U.S. Geological Survey National Earthquake

Information Center) website to constrain event times and locations. The UNR Nevada

Seismological Laboratory (NSL) website was used to constrain event times and locations

for local and regional earthquakes. The data collected from LVVBB2 were saved on data

tapes and shipped to LLNL and IRIS DMC (Incorporated Research Institutions for

Seismology Data Management System) for archiving.

Teleseismic Data

Teleseismic data, recorded at source-receiver distances greater than 1000 km or 30° -

180°, were used to calculated differential travel-time residuals. Data utilized in this

portion of the study are from the first deployment of the LVVBB. Earthquake events

were extracted from the continuous data set based on event times and magnitudes derived

from a teleseismic earthquake search at National Earthquake Information Center (NEIC)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. webpage (http://neic.usgs.gov/neis/epic/epic.html). The search encompassed all

teleseismic earthquakes of magnitude (Mw - estimates of seismic moment; mb -

maximum amplitude of teleseismic body waves) 5.5 and greater occurring within the

time frame of the first deployment (Appendix A). Over 60 teleseismic earthquakes of

magnitude (Mw or mb) 5.5 and greater occurred worldwide during this period. Potential

events extracted from the continuous data set were visually inspected and determined if

viable based on signal-to-noise ratios. A collection of 27 earthquakes with good signal-

to-noise ratios and exhibiting strong P-wave arrivals after filtering were used in the

analyses (Table 3).

Data availability at each station varies per earthquake. Discrepancies in the data

availability for each event are attributed to a lack of recorded data due to finite disk space

or unusable records due to low signal-to-noise ratios. Seismograms from the three short-

period stations, F23, LVW and GPS, were not used in the analysis because these data

were of low quality with low signal-to-noise ratios.

For the 27 events used in the analyses, azimuthal coverage is good with the exception

of a gap located north-east and east of the LVVBB (Figure 9). The majority of the events

recorded were located in the South Pacific Ocean near the Fiji Islands, Tonga Islands, and

Samoa Islands. Earthquakes recorded west of the array have epicenters in Japan and

eastern Russia including the Kamchatka Peninsula and Kuril Islands. Events with back-

azimuths to the northwest are located in western North America near Alaska. Six events

were located in Mexico and western Central and South America (Figure 9). Back

azimuths sampled from all events are skewed to south, west, and north of the array. No

events of Mw 5.5 or greater occurred from locations east or northeast of the array during

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the time of the LVVBBl due to low seismicity in this region (Figure 9). A list of events

used in the analyses can be found in Table 3 and the unfiltered and filtered seismic

records in Appendix B.

Regional Data

Regional earthquake data are used to calculate interstation group velocities

determined from surface wave dispersion. Data utilized in this portion of the study are

from both the first and second deployments of the LVVBB. For data from the LVVBBl,

earthquakes were extracted from the continuous data set using an event table from a

circular area earthquake search centered at Las Vegas, NV conducted on the USGS NEIC

webpage (http://neic.usgs.gov/neis/epic/epic.html). Earthquakes were extracted from the

LVVBB2 using an event table from a rectangular earthquake search conducted on the

Nevada Seismological Laboratory (NSL) webpage

(http://seismo.unr.edu/Catalog/catalog-search.html). This search encompassed all

earthquakes within a 400 km by 400 km square region centered on the LVV.

LVVBBl Regional Data

Regional data extracted from the first deployment of the LVVBB 1 included events

within a 300 km radius of the LVV with Richter magnitudes (ML - Local “Richter”

Magnitude) of 3.0 and greater (Appendix C). Twenty-two events equal to ML 3.0 or

greater occurred within the region during this epoch (Appendix C). Potential events were

extracted from the continuous data set and reviewed. Of the available data, ten

earthquakes were examined in closer detail (Table 4a)(Figure 10a). Data availability

varies at each station per earthquake. As mentioned above, discrepancies in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. completeness of each dataset are attributed to finite disk space, mechanical problems or

low signal-to-noise ratios. In addition, the interstation group velocity method calculates

the Rayleigh wave group velocity along an interstation path for two stations located with

roughly (< 1° difference) the same back azimuth to the earthquake source (Table 5a).

Earthquake records from stations not lying along an interstation path were not used in the

analyses (Table 5a).

The majority of the events examined in the analyses are located in southern

California, south and west of the array (Figure 10a). Three events are located in Nevada

approximately 250 to 300 km northwest of the LVV (Figure 10a). The back azimuths

sampled as part of this dataset range from northwest to south-southwest of the LVVBB 1.

Due to the tectonics of the southwestern United States and parameters of the earthquake

search, most of the earthquakes were located near southern California’s transform plate

boundary as well as the Walker Lane shear zone in western Nevada.

Of the ten events examined in the interstation group velocity analyses, four

earthquakes are located in close proximity (<10 km) to the town of Lavic in southern

California. The remaining California events are located near Valley Wells, Big Bear

Lake, and Palm Springs. The Nevada earthquakes have epicenters north of Scotty’s

Junction and southeast of Tonopah. The largest regional earthquake recorded during the

LVVBBl epoch and used in the analyses was a ML 4.8 earthquake located approximately

10 km NNW of the town of Lavic and roughly 200 km from the LVV. Due to a variety of

data limitations (see results section) only two earthquake data sets recorded by the

LVVBBl were used in the analyses.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LVVBB2 Regional Data

Regional data extracted from the second deployment of the LVVBB included

earthquakes of ML 1.5 and greater within a 400 km by 400 km square centered on the

LVV (Appendix D). Roughly 200 earthquakes of ML 1.5 or greater occurred during the

deployment of the LVVBB2 (Appendix D). Potential events were extracted from the

continuous data set and reviewed. Events with magnitudes lower than ML 2.5 exhibited

extremely low signal-to-noise ratios and were not used in the analyses. Fourteen

remaining earthquakes were examined in greater detail (Table 4b). Data availability from

the six stations of the LVVBB2 varies for each event. In addition, available data were

utilized only from stations lying along an interstation path (Table 5b).

Regional data extracted from the LVVBB were located mostly in Nevada with a few

earthquakes originating in California near Death Valley (Figure 10b). Of the fourteen

earthquakes examined, five were located close to Alamo, Nevada, northwest of the LVV.

Two were located near Boulder City and Lake Mead. A ML 2.64 earthquake was located

within NTS in the Nellis Air Force Bombing and Gunnery Range. The largest regional

earthquake recorded during the LVVBB2 epoch was a ML 4.5 earthquake located near

the town of Alamo, roughly 130 km from the LVV. A variety of data limitations (see

results section) restricted the analyses to only one earthquake from the LVVBB2 regional

data set.

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WC/) o' 3 0 Table 3. List of teleseismic earthquakes recorded by the L W B B l and used in the P-wave travel-time delay methods 3 CD From the NEIC worldwide earthquake search (http://neic.usgs.gov/neis/epic/epic_global.html) 8 Event ID Date Origin Time Latitude Longitude Magnitude Depth Location ci' 3 " mm/dd/yy (GMT) (km)

2002.285.20.09 10/12/2002 20:09:11.46 -8.295 - 7 1 . 7 3 8 6 . 9 M w 534.3 Brazil/Peru border 1 3 CD 2002.286.20.55 10/13/2002 20:55:07.46 -14.596 -175.416 6 . 1 M w 1 0 Fiji Islands, South Pacific Ocean "n c 3. 2002.287.14.12 10/14/2002 14:12:43.75 4 1 . 1 7 4 1 4 2 . 2 4 9 6 .1 M w 61.4 Japan, North Pacific Ocean 3 " CD

2002.289.10.12 1 0 / 1 6 / 2 0 0 2 10:12:21.43 51.952 157.323 6 . 2 M w 102.4 Russia, Kamchatka Peninsula ■DCD O Q. 2002.289.14.13 10/16/2002 14:13:12.74 -15.676 -173.048 6 .1 M w 3 3 Samoa Islands, South Pacific Ocean C a O 3 O 2002.290.04.23 1 0 / 1 7 / 2 0 0 2 04:23:55.94 -19.842 -178.401 6 . 4 M w 627.6 Fiji Islands, South Pacific Ocean ■D O 2002.292.12.09 10/19/2002 12:09:05.38 44.297 149.96 6 . 4 M w 3 3 Russia, Kuril Islands, North Pacific Ocean

CD Q. 2002.295.11.39 10/22/2002 11:39:04.21 -20.633 -178.391 6 . 2 M w 549 Fiji Islands, South Pacific Ocean

2002.297.03.34 10/24/2002 03:34:26.73 48.264 154.383 5 . 6 M w 3 3 Russia, Kamchatka Peninsula, North Pacific Ocean ■D CD 2002.303.16.26 10/30/2002 16:26:34.18 -25.321 -175.638 5 . 6 M w 1 0 South of Tonga Islands, South Pacific Ocean

C/) C/) 2002.307.03.37 1 1 / 3 / 2 0 0 2 03:37:42.07 38.886 141.977 6 . 4 M w 3 9 Japan, North Pacific Ocean

2002.311.15.14 11/7/2002 15:14:06.76 51.197 179.334 6 . 6 M w 3 3 Alaska, Aleutian Islands

2002.313.00.14 11/9/2002 00:14:18.08 13.743 -91.187 6.0 M w 3 3 Offshore Guatemala, Pacific Ocean

2002.321.04.53 11/17/2002 04:53:53.54 47.824 146.209 7 . 3 M w 4 5 9 . 1 Russia, East of Sakhalin Island, Sea of Okhotsk CD ■ D O Q. C 8 Q.

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WC/) 3o" 0 3 Event ID Date Origin Time Latitude Longitude Magnitude Depth Location CD mm/dd/yy (GMT) (km) 8 2002.331.01.35 11/27/2002 01:35:06.29 54.671 -160.741 5.6 Mb 33 Alaska, Aleutian Islands, North Paeifie Ocean ci' 3" 2002.357.13.46 12/23/2002 13:46:11.36 16.957 -85.578 6.0 Mw 33 North of Honduras, Caribbean Sea 1 3 2002.358.12.48 12/24/2002 12:48:45.78 47.715 154.6 5.7 Mw 33 Russia, Kuril Islands, North Pacific Ocean CD

"n 14:43:07.07 50.007 5.5 mb 67 Russia, Kuril Islands, North Paeifie Ocean c 2002.358.14.43 12/24/2002 156.164 3. 3 " CD 2002.362.09.36 12/28/2002 09:36:08.48 51.429 -168.526 5.8 Mw 10 Alaska, Aleutian Islands, North Pacific Ocean ■DCD O 2003.004.05.15 1/4/2003 05:15:03.84 -20.57 -177.661 6.5 Mw 378 East of Tonga Islands, South Pacific Ocean Q. C a 2003.007.00.54 1/7/2003 00:54:51.56 -33.765 -70.054 5.7 Mw 110.8 Chile/Argentina Border O 3 ■D 2003.008.00.28 1/8/2003 00:28:35.42 -174.682 5.7 Mw 70.7 Tonga Islands, South Pacific Ocean O -20.577 2003.009.02.50 1/9/2003 02:50:45.79 -19.664 -176.295 6.0 Mw 10 West of Tonga Islands, South Pacific Ocean CD Q. 2003.020.19.04 1/20/2003 19:04:50.93 -15.598 -173.536 5.8 Mw 100 East of Tonga Islands, South Pacific Ocean

2003.021.02.46 1/21/2003 02:46:47.74 13.626 -90.774 6.5 Mw 24 Offshore Guatemala, Pacific Ocean ■D CD 2003.022.02.06 1/22/2003 02:06:34.61 18.77 -104.104 7.6 Mw 24 Offshore Mexico, Pacific Ocean C/) C/) 2003.022.19.41 1/22/2003 19:41:38.51 18.822 -104.374 6.2 Mw 10 Offshore Mexico, Pacific Ocean

Mw - estimates of the seismic moment mb - maximum amplitudes of teleseismic body waves Figure 9. Global map showing all the teleseismic events (white stars) used for the travel time delay calculations. The approximate location of the LVVBBl is show as the black triangle. White lines delineate approximate source-receiver paths. Notice that there are no events available from the north and east directions with respect to the LVVBBl. The lack o f data in this region is attributed to low seismicity.

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Table 4a; List of regional earthquakes recorded by the LVVBBl and examined for the interstation group velocity methods from the NEIC circular area earthquake search centered at Las Vegas, NV (http://neic.usgs.gov/neis/epic/epic_circ.html).

3. 3 " CD Distance CD ■D from Las O Q. Date Origin Time Depth Vegas C a w Event ID (GMT) Latitude Longitude Magnitude(km) (km) Approximate Location o w 3 2002.291.08.04 10/18/2002 08:04:55.095 34.710 -116.290 ML 3.0 2.0 190 Lavic, California "O o 2002.295.07.15 10/22/2002 07:15:26.007 34.300 -116.880 ML 3.6 7.0 258 Big Bear Lake, California 2002.302.14.16 10/29/2002 14:16:54.008 34.800 -116.270 ML 4.8 4.0 180 Lavic, California

CD 2002.311.11.02 11/7/2002 11:02:11.009 34.800 -116.280 ML 3.1 3.0 180 Lavic, California Q. 2002.313.21.06 11/9/2002 21:06:31.024 35.950 -117.290 ML 3.1 5.0 194 Valley Wells, California 2002.329.00.03 11/25/2002 00:03:10.052 37.380 -117.190 ML 3.9 7.0 228 Scotty's Junction, Nevada 2002.329.07.51 11/25/2002 07:51:17.031 37.390 -117.180 ML 3.2 5.0 228 Scotty's Junction, Nevada ■D 2002.332.10.05 11/28/2002 10:05:58.045 34.810 -116.270 ML 3.0 6.0 180 Lavic, California CD 2002.348.13.07 12/14/2002 13:07:09.087 37.970 -117.110 ML 3.6 10.0 266 Tonopah, Nevada 2002.356.12.55 12/22/2002 12:55:32.086 33.880 -116.260 ML 3.5 10.0 270 Palm Snrinas. California C/) C/) CD ■ D O Q. C g Q.

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8 Table 4b: List of regional earthquakes recorded by the LVVBB2 and examined for the interstation group velocity methods ci' from the NSL rectangular area earthquake search centered at Las Vegas, NV (http://neic.usgs.gov/neis/epic/epic circ.html).

Distance 3. from Las 3 " CD Date Origin Time Depth Vegas

CD Event ID mm/dd/yy (GMT) Latitude Longitude Magnitude(km) (km) Approximate Location ■D O 2003.297.18.18 10/24/2003 18:18:53.92 35.9393 -114.7266 ML 2.90 0.0 44 Boulder City, Nevada Q. C 2003.327.07.16 11/23/2003 07:16:54.12 36.6224 -116.4759 ML 2.59 5.8 130 Amargosa Valley, Nevada a o 2004.032.06.43 2/1/2004 06:43:29.97 37.1096 -115.1282 ML 3.43 12.7 107 Alamo, Nevada 3 ■4^ 2004.055.14.38 2/24/2004 37.8817 -115.8986 ML 2.81 0.0 204 Rachel, Nevada "O 14:38:56.53 o 2004.083.21.30 3/23/2004 21:30:56.52 36.5757 -114.9648 ML 2.58 14.9 51 Garnet, Nevada 2004.111.23.30 4/20/2004 23:30:16.33 35.9675 -116.7627 ML 2.81 9.0 147 Ashford Junction, California

CD 2004.117.11.19 4/26/2004 11:19:09.59 35.9655 -116.7611 ML 2.58 8.0 147 Ashford Junction, California Q. 2004.117.23.26 4/26/2004 23:26:59.38 36.4935 -116.5893 ML 2.81 6.4 135 Death Valley, California 2004.131.04.35 5/10/2004 04:35:56.26 37.4248 -115.9678 ML 2.64 6.3 160 Nevada Test Site, Nevada 2004.135.10.58 5/14/2004 10:58:01.67 36.0498 -114.1229 ML 2.78 0.0 93 South Cove, Nevada ■D 2004.137.01.29 5/16/2004 01:29:39.28 37.2798 -114.8400 ML 4.53 0.0 129 Alamo, Nevada CD 2004.141.17.00 5/20/2004 17:00:20.84 37.3064 -114.8222 ML 3.86 0.3 133 Alamo, Nevada 2004.185.23.24 7/3/2004 23:24:10.74 37.2132 -115.0391 ML 3.23 0.0 119 Alamo, Nevada C/) C/) 2004.185.23.28 7/3/2004 23:28:20.15 37.2294 -114.9043 ML 2.70 9.3 123 Alamo. Nevada

ML - local magnitude commonly referred to as "Richter magnitude" ★ Earthquake within 300 km of Las Vegas Earthquake used in the analyses A IW BRI Station

Figure 10. a. Regional map of the southwestern US showing all regional earthquakes (purple stars) within 300 km from the L W and recorded by the LVVBBl (blue triangles). These ten events were examined in closer detail for the interstation group velocity calculations. Of the ten events, two earthquakes (yellow stars) were used in the analyses. Notice all the available events during this epoch are located west of the array. DEM tiles for the basemap are from Sterner (1995). The final basemap mosaic was created by Birrell (1994). 35

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

★ Earthquake within r 200 km of Las \egas Earthquake used in the analyses i LVVBB2 Station i . I

Figure 10. b. Regional map of the southwestern US showing all regional earthquakes (red stars) within 200 km from the LVV and recorded by the LW BB2 (yellow triangles). These fourteen events were exaiuined in closer detail for the interstation group velocity calculations. Of the fourteen events, one earthquake (yellow star) was used in the analyses. DEM tiles for the basemap are from Sterner (1995). The final basemap mosaic was created by Birrell (1994).

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8 Table 5a: List of regional earthquakes recorded by the LVVBBl and the interstation paths ci' 3" examined in the interstation group velocity methods. i 3 Event ID Date Available Stations Potential Interstation#o f Interstation CD mm/dd/yy Paths Paths "n c 2002.291.08.04 10/18/2002 CHY, F02, F04, F20, LVM, F20-F04, VAH-LVM 2 3. 3 " VAH CD 2002.295.07.15 10/22/2002 CHY, F02, F04, F20, F23 F20-F04, VAH-LVM 2 ■DCD LVM. SOP. VAH O 2002.302.14.16 10/29/2002 CHY, F02, F04, F20, F23 F20-F04, VAH-LVM 2 Q. C LVM. SOP. VAH a O U) 2002.311.11.02 11/7/2002 CHY, F02, F04, F20, F23 F20-F04, SGS-F02, VAH- 3 3 <1 ■D LVM. SGS. VAH LVM O 2002.313.21.06 11/9/2002 CHY, F02, F04, F20, SGS, F04-F02 1 VAH

CD 2002.329.00.03 11/25/2002 CHY, F02, F04, F20, F23 CHY-F20, VAH-F04 2 Q. GPS. LVW. VAH 2002.329.07.51 11/25/2002 CHY, F02, F04, F20, F23 CHY-F20, CHY-SGS, F20- 4 GPS. LVW. SGS. SOP. VAH SGS. VAH-F04 ■D 2002.332.10.05 11/28/2002 CHY, F02, F04, F20, F23, F20-F04 1 CD GPS. SGS. VAH 2002.348.13.07 12/14/2002 CHY, F02, F04, F20, F23, F20-SGS, LVM-F04 2 C/) C/) GPS. LVM. LVW. SGS. SOP. 2002.356.12.55 12/22/2002 CHY, F02, F20, F23, GPS, F20-LVM 1 LVM. LVW. SOP. ULV. VAH CD ■ D O Q. C 8 Q.

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WC/) 3o" O Table 5b: List o f regional earthquakes recorded by the L W B B 2 and the interstation paths examined in the interstation group velocity methods.

8 Event ID Date Available Stations Potential Interstation #o f Interstation

(O' mm/dd/yy Paths Paths 3 " 2003.297.18.18 10/24/2003 F02, F04, HMS, MHS, N06, none none i OMS 3 CD 2003.327.07.16 11/23/2003 F02, F04, MHS, N06 MHS-N06 1 "n c 1 3. 2004.032.06.43 2/1/2004 F04,N06 F04-N06 3 " CD 2004.055.14.38 2/24/2004 F04, MHS, N06 none none ■DCD O 2004.083.21.30 3/23/2004 HMS, N06 none none Q. C a O U) 2004.111.23.30 4/20/2004 F02, HMS, MHS, N06, OMS HMS-OMS 1 3 00 ■D O 2004.117.11.19 4/26/2004 F02, HMS, MHS, OMS HMS-OMS 1

CD 2004.117.23.26 4/26/2004 F02, HMS, MHS, OMS none none Q.

2004.131.04.35 5/10/2004 F02, F04, HMS, N06 none none

■D 2004.135.10.58 5/14/2004 F02, F04, HMS, N06 none none CD

C/) 2004.137.01.29 5/16/2004 F02, F04, HMS, N06 F04-N06 1 C/) 2004.141.17.00 5/20/2004 HMS none none

2004.185.23.24 7/3/2004 F02, F04, HMS, MHS, N06, F04-N06, F04-OMS 2 OMS 2004.185.23.28 7/3/2004 F02, F04, HMS, MHS, N06, F04-N06, F04-OMS 2 OMS CHAPTER SIX

METHODS

This study inspects data recorded by the LVVBB using two methods. The first

method examines the delays in teleseismie P-wave arrivals times observed between a

basin site and a hard-rock site. Calculated delays are used to obtain estimates for basin

depth. The second method calculates the group velocity dispersion of surfaee waves

along interstation paths of the LVVBB. Interstation group velocities are used to create 1-

D vertical profiles of shear velocity (Vs) structure within the basin through inversion.

P-wave Travel Time Delay Methods

This method examines the minute differences in teleseismic P-wave arrival times

observed between the basin sites and the hard-rock site (SGS) of the LVVBBl. Because

the source-reeeiver distances are great and P-wave arrivals are predicted using the 1-D

IASP91 earth model (Figure 11), differences in the calculated residuals reflect changes in

upper erustal structure, in this case the sediment thickness beneath the Las Vegas basin

(Tkalcic et al., 2003). Residuals are used to estimate basin depth beneath each station

using a 2-D model of P-wave velocity beneath LVV created from refraetion analyses

(Zaragoza et al., 2004).

Differential travel-time calculations of P-wave arrivals for teleseismic events are

determined through cross-correlation. Two seismograms, one from a basin site and the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. other from the hard-rock site, were compared in Seismic Handler (Stammler, 1992), a

seismic processing software, to determine residual times of P-wave arrivals. P-wave

arrivals were manually picked and cross-correlated using the initial quarter cycle (Figure

12). Travel-time residuals for the arrivals are calculated by computing the difference in

the predicted and observed times for the two stations, using the equation

where Tbasin refers to the P-wave arrival time at a basin site and Tref is the P-wave arrival

time observed at the reference or hard-rock site (e.g., Tkalcic et al., 2003).

For some events, seismograms for SGS are unavailable or missing due to operational

errors. In cases where data from SGS are unavailable, events recorded at F02 have been

substituted as the hard-rock site. Seismograms recorded at F02 have high signal-to-noise

ratios and data are available for a majority of events. In addition, F02 is located near

bedrock exposed in Frenchman Mountain and site response curves calculated from

teleseismic events exhibit “hard-rock” site behavior (Rodgers et al., 2004; Tkalcic et al.,

2003). Some events are recorded at both F02 and SGS, allowing for an average delay

between these two stations to be determined. This average delay allows calibrated

residuals for basin sites to be determined with respect to SGS using delays calculated

with respect to F02. This calibration is important because it allows all sites to be

compared to the hard-rock site, SGS. In addition, seismograms from ULV were not

available for the same earthquakes recorded at SGS; the calibration allows for residuals at

ULV to be determined with respect to SGS.

Plots of the back-azimuths from the LVVBB1 and the residual pattern across the

LVV were created in GMT, Generic Mapping Tools, (Wessel and Smith, 1991) to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. visually determine the relationship between back-azimuth and observed residuals. A

continuous curvature gridding algorithm was used to produce the color contours of the

residual values across the array (Wessel and Smith, 1990).

Residual times are used to calculate basin depth beneath each station using a 2-D

model of P-wave velocity of the upper crust and mantle beneath LVV (Zaragoza et al.,

2003) (Figure 13). A regional model characterizing the basin velocity is required to

calculate basin depth because global models such as the 1-D 1ASP91 model do not take

into account regional upper crustal structures like the Las Vegas basin. An average P-

wave velocity of 4.37 km/s (Zaragoza et al., 2003) was used to calculate basin depth.

Basin depth was calculated by modifying the elementary physics equation:

Velocity (km/s) — Distance (km) / Time (s)

to solve for distance or in our case depth:

Basin depth (km) = Residual (s) * Basin velocity (km/s)

(e.g., Stein and Wysession, 2003; Shearer, 1999). This equation assumes that the residual

time observed relative to SGS solely represents the delay due to the slower velocity of the

basin sediments.

Interstation Group Velocity and Vs Inversion Methods

Rayleigh waves are a type of surface wave generated by the constructive interaction

of body waves at the Earth’s free surface (e.g.. Lay and Wallace, 1995). All surface

waves (except for Rayleigh waves in an isotropic half-space) experience a phenomenon

known as dispersion (e.g.. Lay and Wallace, 1995). Constmctively interfering surface

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. waves behave as packets or envelopes which travel at a well-defined velocity known as

the group velocity, U, where

TT &Ü X cok T

CO = angular frequency and k = wave number (e.g., Stein and Wysession, 2003; Shearer,

1999) (Figure 14). Individual phases or each harmonic composing the wave packet travel

at a velocity known as the phase velocity, c, where

1 c = — P

and p = ray parameter (e.g.. Lay and Wallace, 1995) (Figure 14). This thesis examines the

fundamental mode Rayleigh wave, which is the lowest-order harmonic mode and

calculates its group velocity.

This method analyzes the dispersion of surface waves generated by regional

earthquakes to constrain the S-wave (shear wave) velocity stmcture of the upper crust

and basin beneath the LVV. Group velocities of fundamental mode Rayleigh waves are

calculated along an interstation path. Group velocities were determined using a Multiple

Filter Analysis (MFA) technique (Dziewonski et al., 1969) and refined with Phase

Matched Filtering (PMF) (Herrin and Goforth, 1977). Since Rayleigh wave group

velocities are sensitive to changes in Vs (Lay and Wallace, 1995), a least square inversion

technique was used to invert group velocity values for 1-D S-wave velocity profiles of

basin and upper crustal sediments.

Group velocities of fundamental mode Rayleigh waves generated from regional

earthquakes were determined through MF A (Dziewonski et al., 1969) and refined with

PMF (Herrin and Goforth, 1977). MF A is an analytical method used to study dispersed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. signals with multiple modes like those examined in this study (Dziewonski et al., 1969).

MF A can resolve signals composed of several modes that arrive at a seismic station at the

same time using a set of narrow-hand filters (Dziewonski et al., 1969). This method

graphically displays a seismic signal by its spectral amplitude as a function of period and

group velocity and allows multiple mode group velocities to be interpreted (Dziewonski

et al., 1969) (Figure 15). The MF A technique was implemented in this thesis via the

d o jn ft program, part of a suite of programs included in the Computer Programs in

Seismology package of Herrmann and Ammon (2002) (Figure 15). PMF is an application

which allows multipath interference to be identified and removed in order to isolate a

particular wavetrain, in this case the fundamental mode Rayleigh wave (Herrin and

Goforth, 1977). This technique is implemented through the match option in the d o jn ft

program, where the fundamental mode Rayleigh wave is interactively picked (Herrmann

and Ammon, 2002). In this study, the PMF process was repeated until the phase of the

filter identically matches the fundamental mode Rayleigh wave (Herrin and Goforth,

1977) (Figure 16).

Interstation group velocities are determined by calculating “the difference in arrival

times of the filtered wave packets” between two stations lying along the same great cirele

path using the equation

(X2-Xi)/(t2-ti)

(e.g.. Lay and Wallace, 1995) where X 2 and x, are the distanees from the earthquake

source to the two stations and t 2 and t] are the arrival times of the dispersed signals as a

function of period. In this study, interstation paths were chosen if two stations lie within

an azimuth interval of less than 1°. Interstation paths calculated between two stations

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. using the same seismic sensor did not need instrument correction. Paths located between

two stations with different seismic sensors were instrument corrected in SAC2000,

Seismic Analysis Code 2000 (Goldstein and Minner, 1996), prior to MF A and PMF.

After interstation group velocities were computed. Vs profiles were determined

through inversion. Surface wave dispersion is sensitive to changes in crustal and upper

mantle velocity structure, where strong velocity gradients produced more prominent

dispersion (e.g.. Lay and Wallace, 1995). This sensitivity to changes in velocity gradients

with depth makes inversion of group velocity dispersion an effective method for

determining velocity structure along a surface wave path (e.g., Zhou and Stump, 2004).

Longer period surface wave group velocities sample deeper velocity structure and the

capturing of long period waves is limited by interstation distance.

Determination of Shear Velocitv Stmcture

Estimates of interstation group velocities were used for the inversion of shear velocity

stmcture as a function of depth using the surf96 program (Herrmann and Ammon, 2002).

The inversion implemented in surf96 (Herrmann and Ammon, 2002) uses a stochastic

dampened least squares inversion technique that minimizes the misfit to the calculated

interstation group veloeities (Herrmann and Ammon, 2002). This inversion technique

produces non-unique results; therefore a reasonable starting model is needed. Since Vs

profiles were virtually unavailable for the basin (up to 5 km depth), a reasonable starting

model was determined through forward modeling.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

12 ICB

-Î2 8 I CMB

’o JB m odel o 6 1ASP91 model Low er m antle 4

Transition Inner 2 z o n e O u ter core co re — Upper mantle 0 0 1000 2000 3000 4000 5000 6000 D epth (km)

Figure 11. Comparison of the IASP91 earth model shown as the solid line and developed by Kennett and Engdahl (1991) with the classic 1-D earth model shown as the dashed line and developed by Jeffreys and Bullen (1940). P is the P-wave velocity curve; S is the S-wave velocity curve. CMB stands for the core-mantle boundary and ICB stands for the inner core boundary (after Stein and Wysession, 2003).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID = 2002.289.10.12 Depth (km) = 102.40 I I Azimuth (deg ) = 67.24

I— I— I— I 1— I— I— I— r |— I— I— [i— I— I— I— I— I— I— I— I— I 60.0 162.5 I 65.0 67.5 Time (s)

F02 VAH Event ID = 2002.289.10.12 Depth (km) = 102.40 Azimuth (deg ) = 67.24

f "|‘ "T“ I I I I I I I I I I I I I I I I " r'i'T I I r I I I I' I I I I I I I 62.4 62.6 62.8 63.0 Time (s)

Event ID = 2002.289.10.12 Depth (km) = 102.40 Azimuth (deg ) = 67.24 Residual = 0.19 s

I I I I I I I I I I" I I I I I I I I I I I I I I I I I I I I I I I I I I I 62.4 62.6 62.8 63.0 Time (s)

Figure 12. a. P-wave arrivals for stations F02 in black ("hard-rock" site) and VAH in blue (basin site) for a M6.2 earthquake on the Kamchatka Peninsula, Russia, b. P-wave arrivals after isolating the initial P-wave motion, c. P-wave arrivals after basin site VAH is shifted to match the F02 arrival through cross-correlation. Note the residual time (seconds) calculated between the two arrivals. 46

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C/) C/)

Las Vegas Basin Kingman 8 Eldorado Mountains Indian Springs

(O'

5.00 km/s

3"3. CD 6.00 km/s

CD T3 O Q. 7.00 km/s aC 3o T3 O 8.00 km/s

CD Q. 100 120 140 160 200 220 240 260 2x vertical exaggeration Distance (km)

T3 CD

C/) C/)

Figure 13. 2-D model of upper mantle and crustal structure beneath the Las Vegas basin, determined from the modeling of crustal refraction data by Zaragoza et al. (2004). The average basin velocity was determined to be 4.37 km/s. ^ Time

BCD

Figure 14. Example of increasing wave dispersion with increasing distance. Solid lines indicate group velocities of a particular frequency and dashed lines indicate phase velocities of individual harmonic components. A, B, C and D are the phase velocities of initial four individual harmonic components (after Lay and Wallace, 1995).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a . Amplitude (count-sec) U (km/s)

I I M I i| I I I I I I I I 1—rp 3 .0 1

2.8 H

2.6 ...... a

2.4

10 ' l I Mill ^ I I II I I III "T T r TII I I I QI I I III I I I lOperiocl (s)U Period (s)

b. Amplitude (count-sec) U (km/s)

I II I M l| 1---- 1 M I I M |------1

s * « ^ î î ^ S î »

I ll| A I I I I IMI| 1 Period (s)lO Period (s)

Figure 15. a. Initial display for MFT with a plot of period vs amplitude (left), period vs group velocity with color contours indicating amplitude (center) and time series waveform (right) for station F23. From this display the fundamental mode Rayleigh wave is manually picked (white dots), b. Display for MFT after seismogram has been refined using PMF. Notice the ftmdamental mode Rayleigh wave is very prominent and the time series (right) is free of noise. 49

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Figure 16. Two seismograms showing a PMF trace (red) and raw trace (black). The PMF seismogram (red) shows the isolated fundamental mode Rayleigh wave for station F23 determined through MFT (Figure 15). CHAPTER SEVEN

RESULTS

P-wave Travel Time Delay Results

The P-wave travel time calculation examined 27 events of which four earthquakes

were used to calculate travel time residuals through the cross-correlation of P-wave

arrivals with respect to SGS. All 27 earthquakes were cross-correlated with respect to

F02 and used to calibrate delays to SGS for events where data from SGS are unavailable.

Residuals calculated with respect to SGS

Residuals for the SGS events range from a maximum residual of 0.450 s recorded at

VAH for a Sea of Okhotsk event to no observed delay at F02 for the Mb 5.6 earthquake

in the Aleutian Islands (Figure 17a; Appendix F). The greatest average residuals are

observed at stations SQP, VAH, and CHY with residuals of 0.350 s, 0.343 s, and 0.340 s

respectively (Figure 18a; Table 6a). Standard deviations of these residuals are greatest at

VAH with a deviation of ± 0.085 s and least at LVM with a deviation of ± 0.014 s

(Figure 18a; Table 6a). The smallest average residuals range from 0.095 s at F02 to 0.280

s at LVM (Figure 18a; Table 6a). Standard deviations of residuals observed at stations

F02, F20, F04 and LVM are ± 0.083 s, ± 0.050 s, ± 0.066 s and ± 0.014 s respectively

(Figure 18a; Table 6a).

Residual patterns for the events cross-correlated to SGS are consistent across the

LVVBB 1 despite differences in back-azimuth (Figures 19 - 22; Appendix F). Positive

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. delays trend northwest across the array; the smallest delays are observed at stations

located along the periphery of the basin near Frenchman Mountain. Increased delays

greater than 0.300 s are observed at stations located closer to the center of the basin.

Residuals calculated with respect to F02

The maximum residual determined with respect to F02 was observed at SQP with a

residual of 0.410 s (Figure 17b; Appendix G). The minimum residual was calculated at

SGS with a negative arrival of -0.210 s (Figure 17b, Appendix G). The greatest average

residuals are 0.231 s, 0.231 s, 0.195 s and 0.192 at stations CHY, SQP, LVM and VAH

respectively (Figure 18b; Table 6b). Standard deviations for these residuals range from ±

0.078 s at SQP and ± 0.027 s at VAH (Figure 18b; Table 6b). The smallest average

residuals were 0.116 s, 0.116 s, 0.043 s and -0.115 s at stations F04, ULV, F20 and SGS

respectively (Figure 18b; Table 6b). Standard deviations ranging from ±0.111 s at ULV

and ± 0.035 s at F20 are observed for these averages (Figure 18b; Table 6b).

Residual patterns for the arrivals cross-correlated to F02 are consistent across the

LVVBB 1 despite differences in back-azimuth (Figures 23 - 25; Appendix F). Similarly

to SGS, positive delays increase from the southeast to the northwest across the array.

Increased delays up to 0.300 s are observed within the basin, while a neutral or negative

delay is observed at SGS.

Residuals calibrated to SGS

Calibrated residuals denote the travel-time delays initially calculated with respect to

F02, but converted to reflect delays observed with respect to SGS. An average delay of

0.105 s was used as the conversion factor to calibrate all residuals to SGS. This

conversion factor was determined by taking the average of the difference in the delay

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. times recorded at the same station for the four earthquakes containing data at both SGS

and F02. Calibrated residuals were only determined for the average residuals calculated

with respeet to F02.

The maximum ealibrated residual was observed at SQP and CHY with a value of

0.336 s; the minimum calibrated residual was observed at SGS. Large calibrated residuals

up to 0.336 s were observed at stations CHY, SQP, LVM and VAH (Figure 26; Table

6b). Median residuals were observed at stations F04, ULV and F20 with values of 0.228

s, 0.215 s, and 0.154 s respectively (Figure 26; Table 6b). The average residual observed

at hard-rock site SGS is -0.003 s.

Residual Errors

All standard deviations for residuals calculated with respect to SGS are less than ±

0.085 s. Although the errors associated with residual calculations determined with respect

to SGS are minimal, a greatest percent error of 87.4% was observed at F02 (Table 6a).

All other percent errors determined with respect to SGS are less than 30% (Table 6a).

Standard deviations for residuals caleulated with respect to F02 are less than ±0.111

s. Errors determined from events cross-correlated to F02 have varied percent errors. The

largest pereent error was observed at ULV with an error of 95.7% due to its short delay

times and lack of data availability for the majority of events. Other large errors include

F20 with a percent error of 81.4% and SGS with a percent error of 67.8%. All remaining

errors calculated with reference to F02 are less than the 42.2% error observed at F04.

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

Basin depths were determined based on the average P-wave velocity of the basin fill

(Zaragoza et al, 2003) and the average residuals determined with respect to SGS as well

as the calibrated residuals. The largest average basin depth was observed beneath the

station with the largest average delay while the smallest average basin depth was

observed beneath the station with the shortest average delay.

For basin depths calculated using average residuals determined with respect to SGS,

SQP had the largest average depth of 1.52 km (Table 6a). The next largest basin depths

were observed beneath VAH, CHY and LVM with depths of 1.50 km, 1.49 km and 1.22

km respectively (Table 6a). The smallest average basin depth was 0.42 km observed at

F02 (Table 6a).

For the calibrated residuals the largest basin depth was 1.47 km, calculated at both

SQP and CHY (Table 6b). Other large basin depths were 1.31 km and 1.30 km at

stations LVM and VAH respectively (Table 6b). Basin depths ealculated for ULV, F20,

F04, and SGS range from 0.97 km at stations ULV and F04 to -0.04 km at SGS (Table

6b).

Interstation Group Velocity and Vs Inversion Results

Only three earthquakes and five interstation paths were successfully completed in the

interstation group velocity analyses of the 24 events examined in closer detail (Figures

10a, 10b, and 27; Appendix H). A variety of factors contributed to the exclusion of the

majority of the data sets, namely low signal-to-noise ratios and lack of interstation paths.

However, the available data do provide a preliminary assessment of group veloeities and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shear velocities within the basin sediments of the LVV. 1-D shear velocity profiles

presented here represent the average shear velocity beneath the full length of the

interstation paths, which are confined completely within the basin.

Group velocities for LVVBB 1 events

Ten events were examined in closer detail from the LVVBB 1. Only two events had

good signal-to-noise ratios, a ML 4.8 near Lavic, California and a ML 3.9 near Scotty’s

Junction, Nevada (Figure 10a; Appendix H). For the two events, four sets of interstation

group velocities were determined (Figure 27).

Group velocities measured along the full source-receiver paths for the Lavic event

range from approximately 2.50 to 2.81 km/s recorded for periods ranging from 1.2 s to

4.0 s at stations CHY, F23 and SQP (Table 7). For stations F04 and F20, group velocities

range from 2.37 to 2.48 km/s over periods 1.5 s to 4.0 s. Source-receiver group velocities

for the event located near Scotty’s Junction, Nevada range from 2.87 to 2.93 km/s for

periods ranging from 1.3 s to 2.4 s at stations F04 and VAH respectively (Table 7).

Interstation group velocities for the two events vary significantly along each

interstation path. Interstation group velocities for the event near Lavic, California are

highest along the SQP-F23 path ranging fi-om 0.92 to 1.14 km/s, and lowest along the

SQP-CHY path ranging from 0.65 to 0.73 km/s (Table 8). Group velocities for these

interstation paths are for periods ranging from 1.5 s to 4.0 s; wavelengths range from 1.1

km to 3.7 km (Table 8). Based on 1/4 of the wavelength for vertical resolution (e.g.,

Mussett and Khan, 2000) layers as thin as 300 m can be resolved (Table 8). For the event

located near Scotty’s Junction, Nevada, the interstation group velocities along the VAH-

F04 path range from 1.24 to 2.14 km/s over periods of 1.3 s to 2.4 s. Wavelengths for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. these periods range from 2.8 km to 3.0 km and are capable of resolving layers as thin as

700 m based on 1/4 wavelength (Table 8).

Group velocities for LVVBB2 events

Fourteen events were examined in closer detail from the LW BB2. Group velocities

for only one event and one interstation path were calculated (Figures 10b and 27;

Appendix H). The majority of these data were excluded due to the lack of interstation

paths associated with the geometry of the array as well as low signal-to-noise ratios

attributed to low magnitude regional earthquakes and cultural noise.

Group velocities for the source-receiver path between F04 and N06 of the LVVBB2

range from 1.81 to 2.06 km/s over periods from 1.3 s to 2.4 s (Table 7). Interstation group

velocities range from 0.25 to 0.39 km/s over these same periods (Table 8). Interstation

wavelengths are approximately 0.6 km to 0.7 km; the vertical resolution is on the order of

100 m to 200 m thick.

Group velocitv errors

Errors associated with the picking of group velocity along the source-receiver paths

are less than 0.17 km/s and as low as 0.05 km/s for all events. Larger picking errors were

observed at higher periods (up to 4.0 s) and low picking errors were observed at lower

periods (-1.3 s). In addition, earthquake location errors for these low magnitude events

could be large, resulting in errors in back-azimuth.

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

A variety of limitations were found during the analyses of the 29 available earthquake

events. Some limitations include the lack of interstation paths, the distribution of the

earthquakes, low signal-to-noise ratios, and data availability.

First and foremost, the main reason many of the earthquake data could not be used for

the interstation group velocity analyses is the lack of interstation paths. Interstation paths

were chosen only if two stations have the same back-azimuth within 1° of difference. The

small increment of 1° was used because source-receiver distances as well as interstation

distances were relatively small, less than 300 km and 10 km respectively. A larger

window of 2“ or 3° would increase errors in interstation measurements.

The lack of interstation paths can be attributed to two factors: the geometry of the

stations of both the LVVBB 1 and the LVVBB2 and the distribution of earthquakes

within the region. Although the stations of the LVVBB 1 were roughly oriented southwest

to northeast and were capable of capturing interstation paths from earthquakes originating

in southern California, few stations were located on the southwest side of the Valley to

capture interstation paths that cross the basin. The majority of the stations of the

LVVBB 1 were concentrated in the northeastern portion of the LVV, which lead to

interstation distances too small to capture wavelengths longer than 2 or 3 km (Figure 27).

Unlike LVVBB 1, stations of the LVVBB2 did not have a specific geometry. Four

stations were located on the eastern side of the basin while only two were located in the

central and southwestern part of the LVV. The geometry and number of stations severely

limited the possibility of capturing potential interstation paths from earthquakes deriving

from a majority of backazimuths. The locations of stations HMS and MHS allowed for

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. potential paths to exist for a narrow margin of back-azimuths. MHS was able to capture

interstation paths originating northwest of the array while HMS was capable of capturing

paths from the southwest. Although interstation paths were captured for earthquakes

originating northeast and southeast of the array, interstation distances from these events

were extremely small, usually less than 3 km and therefore incapable of recording

wavelengths longer than 3 km (Figure 27). Some events recorded by the LVVBB2 were

so close to the array that no interstation paths were generated.

Another limitation of the data set was low signal-to-noise ratios for the regional

earthquake records. Many of the regional earthquakes extraeted from the LVVBB 1

continuous data set were overrun with low period, higher mode, Rayleigh wave signals

recorded from earlier earthquakes. These long period waves are attributed to an earlier

larger magnitude event (M 5.0 or larger); however, determining the earlier earthquake is

difficult because large earthquakes ean excite Rayleigh waves that encircle the Earth

numerous times over several hours (e.g., Stein and Wysession, 2003). Although these

signals are long period, they spread over a range of low frequencies and filtering caused

portions of the regional earthquake data to be removed. Data recorded by LVVBB2 also

have low signal-to-noise ratios, but are attributed to short period or high frequency noise

from local site conditions as well as weak (< ML 2.0) earthquake magnitudes. Records

examined for many events were virtually unavailable and attributed to weak event signals

and high cultural noise. In addition, some earthquake arrivals obtained for regional

earthquakes contained complex Rayleigh waves. For these events, fundamental mode

Rayleigh waves could not be isolated for some or all stations and are attributed to

interference from higher mode arrivals as well as lateral refractions within the basin.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Although the fundamental mode Rayleigh waves were isolated for some events using

MFT and PMF techniques, others were so overrun with mixed arrivals that the

fundamental mode Rayleigh wave could not be retrieved.

Station availability also played a role in limiting the number of interstation paths.

Because a good majority of earthquakes occurred southwest of the arrays in southern

California, stations located on the west side of the Valley were crucial in creating

interstation paths. ULV, located in the southwest portion of the LVV was unavailable for

the majority of the LVVBB 1 epoch due to mechanical problems. LVVBB2 records from

MHS, located on the northwest side of the LVV, were available but of poor quality due to

electrical problems.

Finally of the events available for the analyses, fundamental mode Rayleigh

waves could not be isolated for some stations and are attributed to interference from

higher mode arrivals as well as scattering of waves within the basin.

1-D Shear Velocitv Profiles

Shear velocities (Vs) along five interstation paths were determined through inversion.

The five interstation paths inverted were F20-F04, SQP-CHY and SQP-F23 for the Lavic,

California event, VAH-F04 for the Scotty’s Junction, Nevada event, and F04-N06 for the

Alamo, Nevada event (Figure 27). Due to the differences in the periods and wavelengths

sampled for each path, depths and layer thicknesses in each model vary. Initial models

consisted of numerous layers over a single homogeneous half-space. Parameters used in

the initial starting models consisted of layer thicknesses and depths, P-wave velocities, S-

wave velocities and density. Sensitivity tests were conducted to determine the effects of

each parameter in the inversion. Results from these tests suggest that the inversions were

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. most sensitive to layer thicknesses, depths and S-wave velocities. In addition, a damping

factor of 1.0 was used, causing the inversions to converge to a final model with a minimal

number of iterations. Higher damping factors were tested and typically converged to the

same final model with more iterations.

Path F20-F04

The F20-F04 path is a roughly north-south path that lies along the eastern edge of the

deepest portion of the basin (Langenheim et al., 2001a and 2001b) (Figure 27). The initial

model for the Vs inversion consisted of seven layers over a half-space (Figure 28). The

Vs profile created through inversion reaches a depth of 1.5 km with individual layer

thicknesses of 200 m (Figure 28). Shear velocities range from 0.96 km/s (960 m/s) in the

upper 600 m to 1.39 km/s at depths greater than 1.5 km (Figure 28). A slight low-velocity

zone (LVZ) is observed at depths between roughly 500 m to 700 m having a shear

velocity of 0.96 km/s (Figure 28). The theoretical dispersion curve and observed data

match well (Figure 28). The resolution matrix (Figure 29) indicates that Vs values at

depths of 0.85 km, 1.15 km, and 2.00 km are well resolved based on a damping factor of

I.O.

Path SQP-CHY

The SQP-CHY path is oriented southwest-northeast and roughly in the center of the Las

Vegas basin (Figure 27). The path captures the transition between the shallower and

deeper portions of the basin (Langenheim et al., 2001a and 2001b). The initial model for

the Vs inversion consisted of six layers over a half-space (Figure 30). The Vs profile

created through inversion reaches a depth of just under 3 km with individual layer

thicknesses of 400 m. Shear velocities range from 0.80 to 0.56 km/s in the upper 1.8 km

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the profile creating a large LVZ (Figure 30). Shear velocities increase from 0.55 to 1.1

km/s in the next 800 m until it reaches another LVZ at 2.6 km depth with a velocity of

0.60 km/s (Figure 30). The theoretical dispersion curve and observed data match well

(Figure 30). For the SQP-CHY path, the resolution matrix indicates good resolution at

depths of 2.00 km, 2.70 km and 3.30 km based on a damping factor of 1.0 (Figure 31).

Path SOP-F23

The SQP-F23 path is oriented southwest-northeast extending slightly further north of the

SQP-CHY path (Figure 27). The initial model for the Vs inversion consisted of nine

layers over a half-space (Figure 32). The Vs profile created through inversion reaches a

depth of just under 4 km with individual layer thicknesses of about 400 m. Shear

velocities range Irom 1.22 to 1.18 km/s in the upper 2.6 km of the profile with a LVZ

observed between 2.2 km and 2.6 km (Figure 32). At depths greater than 2.6 km shear

velocities increase from 1.73 to 2.35 km/s (Figure 32). The theoretical dispersion curve

and observed data match well (Figure 32). Depths of 0.90 km, 1.30 km, 1.80 km, 2.20

km, 3.10 km, and 4.00 km are well resolved based on a damping factor of 1.0 (Figure

33^

Path VAH-F04

The VAH-F04 path is the only path oriented east-west. It crosses the basin just north of

the area estimated to overlie the deepest portion of the basin (Langenheim et al., 2001a

and 2001b) (Figure 27). The initial model for the Vs inversion consists of seven layers

over a half-space (Figure 34). The Vs profile created through inversion reaches a depth of

3 km with individual layer thicknesses of 400 m. Shear velocities for this path are the

highest observed from the data set. Shear velocities range from 2.86 to 2.17 km/s in the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. upper 2.6 km of the whole profile with two LVZs. The first LVZ is a decrease in velocity

from 2.85 to 1.82 km/s observed over the first 1 km (Figure 34). At depths greater than 1

km shear velocities increase from 1.91 to 2.67 km/s until another LVZ of 2.17 km/s

(Figure 34). The theoretical dispersion curve fits the general trend of the observed data

(Figure 34). There is good resolution along the VAH-F04 path at depths of 1.10 km, 2.30

km, 2.90 km, and 4.00 km based on a damping factor of 1.0 (Figure 35).

Path F04-N06

The VAH-F04 path is shortest of the five interstation paths and the only path calculated

from data of the LVVBB2. It lies roughly northeast-southwest across the northeastern

portion of the basin just west of Frenchman Mountain (Figure 27). The initial model for

the Vs inversion consists of seven layers over a half-space (Figure 36). The Vs profile

created through inversion reaches a shallow depth of 0.75 km with individual layer

thicknesses of 100 m. Shear velocities for this path decrease from 0.47 to 0.28 km/s in the

upper 0.35 km (Figure 36) before gradually increasing in velocity to 0.63 km/s at a depth

of 0.75 km (Figure 36). The half-space depicts a LVZ with a shear velocity of 0.56 km/s

(Figure 36). The theoretical dispersion curve and observed data match well (Figure 36).

For path F04-N06, depths of 0.28 km, 0.43 km, 0.57 km, and 0.86 km are well resolved

based on a damping factor of 1.0 (Figure 37).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a . Travel-time Residuals to SGS

0.5 X 0.4 t ... ♦ X « 0.3 X » + I X + S ♦ PÜ ^ * u 0.2 # X

« 0.1 o g o

- 0.1 Station Name SQP *CHY XVAH XF20 #F04 +LVM OF02

b. Travel-time Residuals to F02

0.4

0.3

^ 0.2 *co

H -0.1

- 0.2

-0.3 Station Name *SQP "ULV ACHY XVAH XF20 #F04 +LVM ■SGS

Figure 17. a. Travel-time residuals calculated for each station for all available events with respect to SGS. b. Travel-time residuals calculated for each station for all available events with respect to F02.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a . Average Travel-time Residuals to SGS

SQP CHY VAH F20 F04 LVM F02

- 0.2 Station Name

Average Travel-time Residuals to F02

I C /3

(D S

til LI..... I ULV CHY VAH F20 F04 LVM 1

Station Name Figure 18. a. Average travel-time residuals (bars) calculated for each station for all available events with respect to SGS witli error estimates for each station, b. Average travel-time residuals (bars) calculated for each station for all available events with respect to F02 with error estimates for each station.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ D O Q. C g Q.

■D CD

C/) 3o' O Table 6a. Average residuals and basin depths detenuined with respect to SGS

o Average Residuals Recorded at SGS

CQ Average Standard Estimated Station Residual (s) Deviation (±s) Percent Error Basin Depth (km) SQP 0348 0.068 193 132 CHY 0.340 0.033 9.6 1.49 VAH 0343 0.085 2A9 130 3. 3 " F20 0.168 0.050 29.7 033 CD F04 0.250 0.066 263 1.09 "OCD O LVM 0380 0.014 5.1 132 Q. C F02 0t095 0.083 879 0W2 a C\ o LA ■D Table 6b. Average residuals and basin depths determined with respect to F02, and data calibrated to SC O Average Residuals Recorded at F02 CD Q. Average Standard Average Residual (s) Estimated Station Residual (s) Deviation (± s) Percent Error Calibrated to SGS Basin Depth (km) ■D SQP 0.231 0.078 319 0336 1.47 CD ULV 0.116 0.111 95.5 0321 037 C/) CHY C/) 0.231 0.050 21.7 0336 1.47 VAH 0.192 0.027 14.0 0397 1.30 F20 0.043 0.035 813 0348 035 F04 0316 0.049 41.9 0321 0.97 LVM 0.195 0XM3 223 0300 1.31 SGS -0.115 &078 673 -0.010 -0.04 Event ID 2002.290.04.23 M6.4 Fiji Islands, South Pacific Ocean

36 24

36 12'

36 00 24 -115 12’ -115 00 114 48115

Figure 19. a. Map of the LVV showing basin depth contours in black (Langenheim et al., 2001a and 2001b) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, h. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 66

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID 2002.311.15.14 M6.6 Aleutian Islands, Alaska

a .

TT 0.0 0.1 0.2 0.3 0.4 0.5 36 24' 1

36 12'

36 00' -115 24' -115 12' -115 00' -114 48'

Figure 20. a. Map of the L W showing basin depth contours in black (Langenheim et al., 2001a an5 200Ih) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of hack-azimuth shown as the red lines, b. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 67

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID 2002.321.04.53 M7.3 Russia, East of Sakhalin Island, Sea of Okhotsk

a .

o 0.0 0.1 0.2 0.3 0.4 0.5 36 24'

36 12'

36 00' -115 24 -115 12' -115 00 -114 48'

Figure 21. a. Map of the L W showing basin depth contours in black (Langenheim et al., 2001a and 200 Ih) and station of the L W B B l as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, b. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 68

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID 2002.331.01.35 M5.3 Alaska, Aleutian Islands, North Pacific Ocean

a .

36 24'

36 12

36 00' -115 24' 115 12’-115 00' -114 48'

Figure 22. a. Map of the LVV showing basin depth contours in black (Langenheim et al., 2001a and 2001b) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, b. Map of the L W showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 69

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID: 2002.285.20.09 M6.9 Brazil/Peru border

3624

36 2'

36 00' -115 24 115 12' -115 00' 114 48'

Figure 23. a. Map of the LVV showing basin depth contours in black (Langenheim et al., 2001a and 2001b) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, b. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 70

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID: 2002.286.20.55 M 6.1 Fiji Islands, South Pacific Ocean

36 24' sec

36 12'

36 00' -115 24' -115 12 -115 00 -114 48'

Figure 24. a. Map of the LVV showing hasin depth contours in black (Langenheim et al., 2001a and 2001b) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, b. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 71

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Event ID 2002.297.03.34 M5.6 Kamchatka Peninsula, Russia, North Pacific Ocean

a . ” m

36 24'

36 12

36 00' -115 24 115 12' -115 00' -114 48

Figure 25. a. Map of the LVV showing basin depth contours in black (Langenheim et al., 2001a and 2001b) and station of the LVVBBl as yellow triangles. The reference station, SGS is shown as a red triangle with the direction of back-azimuth shown as the red lines, b. Map of the LVV showing the residual pattern across the array with the reference station, SGS in red. Positive delays are denoted by the black circles with larger circles indicating longer delays. Negative delays (not shown here) are denoted by black triangles. 72

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Comparison of the Average Residuals Calibrated to SGS and the Calculated Residual to SGS

375 350 325 THT SOP 300 275 -VAfL LVM 250 225 Itn 200 LLV (L> 175 s 150 125 100 75 I 50 25 0 JSGS_ -25 Station Name ■ Calibrated F02 Residual to SGS * Calculated Residual to SGS

Figure 26. A comparison of the average residuals calibrated to SGS, shown as squares, to the average calculated residual to SGS, show as the triangles. No residual was calculated for ULV with respect to SGS due to data availability; however, a calibrated delay was calculated using the average delay observed with respect to F02.

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

-11530' -Il5'00' . I

Figure 27. Close-up map of the LVV with the LVVBBl (blue triangles) and the LVVBB2 (yellow triangles). Black lines indicate interstation paths inverted for Vs with participating stations labelled. Shown in gray are the basin contours in km determined by Langenheim et al. (2001a and 2001b).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ D O Q. C g Q.

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C/) C/)

8 Table 7: Group velocity, U (km/s), versus Period (s) for source-receiver paths of the LVVBBl and LVVBB2 ci' Event 2002.302.14.16 ML 4.8 Lavic, California F04 F20 Period (s) U (km/s) Period (s) U (km/s) Period (s) U (km/s) Period (s) u (km/s) Period (s) U (km/s) 4 2.5357 4 2.5811 4.0 2.8111 3 3 " 3.8 2.5298 3.8 2.5781 3.8 2.8028 CD 3.6 2.5206 3.6 2.5752 3.6 2.7897 CD 3.4 2.5060 3.4 2.5730 3.4 2.7716 ■D O 3.2 2.4986 3.2 2.5712 3.2 2.7503 Q. C 3 2.4974 3 2.5691 3.0 2.7328 a 2.9 2.5682 2.9 2.7274 O 2.9 2.4972 3 LA 2.8 2.4972 2.8 2.5674 2.8 2J244 "O O 2.7 2.4974 2.7 2.5667 2.7 2.7235 2.6 2.4977 2.6 2.5660 2.6 2.7228 2.5 2.4983 2.5 2.5654 2.5 2.7227 CD Q. 2.4 2.4989 2.4 2.5649 2.4 2.7231 2.3 2.4996 2.3 2.5643 2.3 2.7239 2.2 2.5003 2 j 2.5639 2.2 2.7250 2.1 2.5011 2.1 2.3745 2.1 2.4826 2.1 2.5635 2.1 2.7262 ■D CD 2 2.5018 2 2.3727 2 2.4825 2 2.5631 2.0 2.7276 1.9 2.5026 1.9 2.3713 1.9 2.4823 1.9 2.5628 1.9 2.7291 C/) C/) 1.8 2.5033 1.8 2.3702 1.8 2.4821 1.8 2.5625 1.8 2.7305 1.7 2.5040 1.7 2.3694 1.7 2.4818 1.7 2.5622 1.7 2.7319 1.6 2.5046 1.6 2.3686 1.6 2.4815 1.6 2.5621 1.6 2.7333 1.5 2.5051 1.5 2.5619 1.5 2.7345 CD ■ D O Q. C g Q.

■D CD

C/) C/)

8 ci'

.Event 2002329.00.03 A& 3.9 Event 2004.032.06.43 ML 3.4 Scotty's Junction, Nevada Alamo, Nevada 3 3 " FO'/ FO'/ N06 CD Period (s) U (km/s) Period (s) U (km/s) Period (s) U (km/s) Period (s) U (km/s) CD ■D 2.4 2.8840 2.4 2.9272 2.7 2.0614 2.7 1.8103 O Q. 2.3 2.8823 2.3 2.9222 2.6 2.0548 2.6 1.8164 C a 2.2 2.8807 2.2 2.9177 2.5 2.0485 2.5 1.8221 O 2.1 2.8793 2.1 2.9137 2.4 2.0425 2.4 1.8279 3 G\ "O 2 2.8781 2 2.9101 2.3 2.0368 2.3 1.8333 O 1.9 2.8770 1.9 2.9066 2.2 2.0314 2.2 1.8379 1.8 2.8761 1.8 2.9028 2.1 2.0263 2.1 1.8414 CD 1.7 2.8753 1.7 2.8984 2 2.0216 2 1.8442 Q. 1.6 2.8746 1.6 2.8942 1.9 2.0173 1.9 1.8467 1.5 2.8741 1.5 2.8903 1.8 2.0133 1.8 1.8490 1.4 2.8736 1.4 2.8867 1.7 2.0098 1.7 1.8511 ■D 1.3 2.8732 1.3 2.8842 1.6 2.0066 1.6 1.8531 CD

C/) C/) CD ■ D O Q. C g Û.

■D CD

C/) C/)

Table 8: Interstation group velocity (U), versus period, wavelength (k), and vertical resolution (VR) for the interstation paths of the LVVBB1 and LVVBB2 8

ci' Event 2002.302.14.16 ML 4.8 Lavic, California Interstation Paths W F - c /f y Period (s) U (km/s) À. (km) KR W Period (s) U (km/s) X (km) VR (km ) Period (s) U (km/s) k (lan) 3 4 0.6531 2.6 0.7 4 0.9253 3.7 0.9 3 " 0.6 3.6 CD 3^ 0.6543 2.5 3.8 0.9363 0.9 3.6 0.6566 2.4 0.6 3.6 0.9604 3.5 0.9 CD ■D 3.4 0.6561 2.2 0.6 3.4 LM%5 3.4 0.9 O Q. 3.2 0.6766 2.2 0.5 3.2 1.0605 3.4 0.8 C a 3 0.7065 2.1 0.5 3 1.1119 3.3 0.8 O O 2.1 0.5 2.9 1.1280 3.3 0.8 3 2.9 0.7169 "O 2.8 0.7231 2.0 0.5 2.8 1.1361 3.2 0.8 O 2.7 0.7253 2.0 0.5 2.7 1.1363 3.1 0.8 2.6 0.7277 1.9 0.5 2.6 1.1359 3.0 0.7 CD 2.5 0.7294 1.8 0.5 2.5 1.1338 2.8 0.7 Q. 2.4 0.7301 1.8 0.4 2.4 1.1297 2.7 0.7 2.3 0.7302 1.7 0.4 2.3 1.1244 2.6 0.6 2.2 0.7298 1.6 0.4 2.2 1.1183 2.5 0.6 ■D 2.1 0.8649 1.8 0.5 2.1 0.7291 1.5 0.4 2.1 1.1118 2.3 0.6 CD 2 0.8557 1.7 0.4 2 0.7282 1.5 0.4 2 1.1053 2.2 0.6

C/) 1.9 0.8491 1.6 0.4 1.9 0.7271 1.4 0.3 1.9 1.0988 2.1 0.5 C/) 1.8 0.8448 1.5 0.4 1.8 0.7259 1.3 0.3 1.8 1.0925 2.0 0.5 1.7 0.8418 1.4 0.4 1.7 0.7247 1.2 0.3 1.7 1.0866 1.8 0.5 1.6 0.8394 1.3 0.3 1.6 0.7235 1.2 0.3 1.6 1.0812 1.7 0.4 1.5 0.7222 1.1 0.3 1.5 1.0763 1.6 0.4 CD ■ D O Q. C g Q.

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C/) C/)

8 ci'

Event Event 2004.032.06.43 ML 3.4 Scottv's Junction, Nevada Alamo, Nevada 3 3 " CD Period (s) U (km/s) k (km) VR (km) Period (s) 1 (km) XRf%7n) ■DCD 2.4 1.2392 3.0 0.7 2.7 0.2503 0.7 0.2 O 3.0 0.7 2.6 0.2619 0.7 0.2 Q. 2.3 1.2939 C 2.2 1.3472 3.0 0.7 2.5 0.2739 0.7 0.2 a O 2.1 1.3984 2.9 0.7 2.4 0.2867 0.7 0.2 3 00 "O 2 1.4470 2.9 0.7 2.3 0.3000 0.7 0.2 O 1.9 1.5018 2.9 0.7 2.2 0.3129 0.7 0.2 1.8 1.5757 2.8 0.7 2.1 0.3247 0.7 0.2 CD 1.7 1.6742 2.8 0.7 2 0.3358 0.7 0.2 Q. 1.6 1.7878 2.9 0.7 1.9 0.3466 0.7 0.2 1.5 1.9106 2.9 0.7 1.8 0.3571 0.6 0.2 1.4 2.0399 2.9 0.7 1.7 0.3674 0.6 0.2 ■D 1.3 2.1433 2.8 0.7 1.6 0.3772 0.6 0.2 CD 1.5 0.3863 0.6 0.1

C/) C/) CD ■ D O Q. C g Q.

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C/) o" Event 2002.302.14.16 Lavic, California Interstation path F20-F04 3 O Vs (km/s) 0.90 0.90 1.20 1.50 1.80 8 3 ci' 0.160 - 0.88

0.320

3. 0.87 3" 0.480 (D

(D T3 O 0.640 Q. î aC =) 0.85 o « 0.800 3 'O T3 O 0.960 0.83 (D Q. 1.120

1.360 0.82 T3 (D

(/) 1.440 - (/)

0.80 Initial 1.67 1.83 2.00 2.17 2.33 2.501.50 Final Period (s)

Figure 28. Vs profile (left) produced through inversion. The initial model is the dashed line and the final model is the solid line. Dispersion curve (right) fit to the observed values of interstation group velocity for the F20-F04 path for the Lavic, CA event. CD ■ D O Q. C 8 Q.

■D CD

C/) C/) Event 2002.302.14.16 Lavic, California Interstation path F20-F04

Vs (km/s) Normalized Resolution Matrix 0.60 0.80 1.00 1.20 1.40 I - - ' - _ I - I I 0.171, 0.407, 0.429, 8 3 ci' 0.200

0.400

3. 0.600 - 3 " CD

■DCD e 0.800 O 0.230 o Q. C a 0) O oc û 3 O ■D O 1.200

CD Q. 1.400

■D CD 1.800 Next C/) C/) Current 00^L0.03sl

Figure 29. Vs profile (left) created through inversion. Dashed line indicates the next model and the solid line indicates the current model. Normalized resolution matrix (right) indicates the resolution with depth. Peaks to the right indicate depths that are well resolved. For path F20-F04, depths of 0.85 km, 1.15 km, and 2.00 km are well resolved. Values associated with resolution peak are velocity/attenuation (Q). Note the damping factor is 1.0. CD ■ D O Q. C g Q.

■D CD

C/) Event 2002.302.14.16 Lavic, California Interstation path SQP-CHY 3o" O Vs (km/s) 0.75 0.30 0.60 0.90 1.20 8 ,j.I II I I I II 1,1.11 ci' 0.290 0.73

0.560

0.72 3 0.840 3 " CD

CD ■D 1.120 O Q. C 3 0.70 a ^ 1.400 O 00 3 ■D Q O 1.680 0.68 CD Q. 1.960

2.240 0.67 ■D CD

C/) 2.520 C/) 0.65 Initial 1.50 2.00 2.50 3.50 4.00 4.503.00 Final Period (s)

Figure 30. Vs profile (left) produced through inversion. The initial model is the dashed line and the final model is the solid line. Dispersion curve (right) fit to the observed values of interstation group velocity for the SQP-CHY path for the Lavic, CA event. CD ■ D O Q. C 8 Q.

■D CD

C/) C/) Event 2002.302.14.16 Lavic, California Interstation path SQP-CHY

Vs (km/s) Normalized Resolution Matrix 0.60 0.80 1.00 1.20 1.40 0.525_ 0.504- 8 ci' 0.400

0.800

3"3 1.200 (D

(D T3 E 1.600 O Q. C a g- 2.000 o 0.406 o 0.392 3 00 O T3 w O 2.400

(D 0.040 Q. 2.800

3.200 T3 0.028f> 0.01 Oé (D Next (/) (/) Current

Figure 31. Vs profile (left) created through inversion. Dashed line indicates the next model and the solid line indicates the current model. Normalized resolution matrix (right) indicates the resolution with depth. Peaks to the right indicate depths that are well resolved. For path SQP-CHY, depths of 2.00 km, 2.70 km and 3.30 km are well resolved. Values associated with resolution peak are velocity/attenuation (Q). Note the damping factor is 1.0. CD ■ D O Q. C S Q.

■D CD

C/) Event 2002.302.14.16 Lavic, California interstation path SQP-F23 3o" O Vs (km/s) 1.50 1.20 1.50 1.80 2.10 2.40 8 ci' 0.400 - 1.41 3 CD 0.500

3. .32 3 " 1.200 CD

■DCD O 1.600 I Q. C t 3 1.23 a O 00 t 2.000 3 W ■D Q O 2.400 1.13 CD Q. 2.800 i k

■D 3.200 1.04 CD

C/) C/) 3.600 0.95 — Initial 1.50 2.00 2.50 3.00 3.50 4.00 4.50 Period (s) - Final

Figure 32. Vs profile (left) produced through inversion. The initial model is the dashed line and the final model is the solid line. Dispersion curve (right) fit to the observed values of interstation group velocity for the SQP-F23 path for the Lavic, CA event. CD ■ D O Q. C 8 Q.

■D CD

C/) C/) Event 2002.302.14.16 Lavic, California Interstation path SQP-F23

Vs (km/s) Normalized Resolution Matrix 1.20 1.50 1.80 2.10 2.40 0.378.0.580 8 ci' 0.400

0.800 0.409?>

1.200 3 3 " 0.278 CD

CD E 1.600 ■D & O 0.174 Q. £ C § ■ 2.000 a Q O 3 oc ■D 2.400 O

CD 2.800 Q.

3.200 ■D CD

C/) Next C/) Current 0.001^.00^0.00%

Figure 33. Vs profile (left) created through inversion. Dashed line indicates the next model and the solid line indicates the current model. Normalized resolution matrix (right) indicates the resolution with depth. Peaks to the right indicate depths that are well resolved. For path SQP-F23, depths of 0.90 km, 1.30 km, 1.80 km, 2.20 km, 3.10 km, and 4.00 km are well resolved. Values associated with resolution peak are velocity/attenuation (Q). Note the damping factor is 1.0. CD ■ D O Q. C g Q.

■D CD

C/) 3o" Event 2002.329.00.03 Scotty's Junction, Nevada Interstation path VAH-F04 O Vs (km/s) 2.50 1.00 1.50 2.00 2.50 8 11111111111 ; I il 1111111 ci' 0.320 2.25

0.640

00 3 0.960 3 " CD

■DCD 1.280 O I Q. I .75 C a O 1.600 3 00 ■D LA È ▲ \ O 1.920 .50 CD Q. 2.240

2.560 .25 ■D CD

C/) 2.880 C/) .00 nitial 1.00 1.25 1.50 1.75 2.00 2.25 2.50 Final Period (s) Figure 34. Vs profile (left) produced through inversion. The initial model is the dashed line and the final model is the solid line. Dispersion curve (right) fit to the observed values of interstation group velocity for the VAH-F04 path for the Scotty's Junction, NY event. CD ■ D O Q. C 8 Q.

■D CD

C/) C/) Event 2002.329.00.03 Scotty's Junction, Nevada Interstation path VAH-F04

Vs (km/s) 1.20 1.60 2.00 2.40 2.80 0.545 8 ci' 0.400

0.800

3. 1.200 3 " CD

■DCD E 1.600 O Q. S C g- 2.000 a Q O oc 3 0\ ■D O 2.400

CD Q. 2.800 -

3.200 ■D CD 3.600 Next C/) C/) Current

Figure 35. Vs profile (left) created through inversion. Dashed line indicates the next model and the solid line indicates the current model. Normalized resolution matrix (right) indicates the resolution with depth. Peaks to the right indicate depths that are well resolved. For path VAH-F04, depths of 1.10 km, 2.30 km, 2.90 km, and 4.00 km are well resolved. Values associated with resolution peak are velocity/attenuation (Q). Note the damping factor is 1.0. CD ■ D O Q. C g Q.

■D CD

WC/) 3o" Event 2004.032.06.43 Alamo, Nevada Interstation path F04-N06 0 Vs (km/s) 3 0.40 CD 0.30 0.50 0.70 0.90 8 I II 11111111111111 II 111111 n 11111 h 111111 ci' 3" 0.080 0.38 1 3 CD 0.160 "n c 3. 0.35 3 " 0.240 CD

■DCD O 0.320 I Q. C 3 0.33 a O 0.400 3 00 ■D O 0.480 0.30 CD Q. 0.560

■D 0.640 0.28 CD

C/) C/) 0.720 0.25 Initial 1.00 1.25 1.50 1.75 2.00 2.25 2.50 Final Period (s)

Figure 36. Vs profile (left) produced through inversion. The initial model is the dashed line and the final model is the solid line. Dispersion curve (right) fit to the observed values of interstation group velocity for the F04-N06 path for the Alamo, NV event. CD ■ D O Q. C 8 Q.

■D CD

C/) C/) Event 2004.032.06.43 Alamo, Nevada Interstation path F04-N06

Vs (km/s) Normalized Resolution Matrix 0.32 0.40 0.48 0.56 0.640. 0.566 8 ci' 0.100

0.200

3 3 " CD

■DCD E 0.400 O 0.362 6 \ Q. C a O 3 00 ■D oo 0.287* O 0.600

CD Q. 0.700 0.169# 0.089

0.800 ■D CD 0.108,) 0.900 Next C/) C/) Current

Figure 37. Vs profile (left) created through inversion. Dashed line indicates the next model and the solid line indicates the current model. Normalized resolution matrix (right) indicates the resolution with depth. Peaks to the right indicate depths that are well resolved. For path F04-N06, depths of 0.28 km, 0.43 km, 0.57 km, and 0.86 km are well resolved. Values associated with resolution peak are velocity/attenuation (Q). Note the damping factor is 1.0. CHAPTER EIGHT

INTERPRETATIONS

P-wave Travel Time Delay Interpretations

Calculated P-wave travel time residuals and estimated basin depths at stations located

within the basin and along the basin’s periphery are fairly consistent across the array.

Residuals observed with respect to SGS typically show longer delays up to 0.45 s than

those observed with respect to F02, which show delays up to 0.41 s. Calibrated residuals

for each station compare well with residuals calculated for events cross-correlated to

SGS, indicating good coherency among the different events.

Travel Time Residuals

The smallest delays are observed along the eastern edge of the basin with increasing

delays toward the center of the basin (Figure 38). A negative residual was observed at

station SGS for residual times calculated with respect to F02. This negative residual

indicates that initial P-wave arrivals arrived earlier at SGS than at F02. SGS is located at

the base of Frenchman Mountain on less than 100 m of alluvial fan deposits while F02,

although close to Frenchman Mountain, was located within the basin above a thin

accumulation (< I km) of basin sediments. Therefore, earlier arrivals are expected at SGS

because P-waves recorded at SGS travel primarily through hard rock (< 100 m of basin

sediments) where P-waves recorded at F02 must travel through a thin veneer of basin

sediments (< 1 km of basin sediments) (Figure 38).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The longer delays observed at sites located within the basin (Figure 38) are attributed

to greater accumulations of basin sediments or basin depth. As modeled by Zaragoza et

al. (2004) and Snelson et al. (2004), sediments of the Las Vegas basin have slower P-

wave velocities than the surrounding hard-rock geology. A delay in P-wave arrivals is

observed when the initial P-wave arrival at basin site arrives later in time than the P-wave

arrival at the hard-rock site. Positive residuals, up to 0.450 s, indicate that the basin is

detectable with this technique and P-wave arrivals are affected by the slow basin

sediments.

Typically longer travel time delays are observed at stations calculated with respect to

SGS than delays observed at the same stations calculated with respect to F02 for the same

events. Greater delays calculated with respect to SGS are expected, because SGS is the

hard-rock site and P-wave arrivals at SGS travel through minimal (< 100 m) of the slow

sediments of the basin. Although F02 exhibits hard-rock site qualities, it is still located

above a thin portion of basin sediments (< 1 km), thereby decreasing the observed delay.

Residuals calibrated to SGS were based on only four events in which data for both

reference stations, SGS and F02, were available. Nevertheless, the calibrated residuals

compare well with residuals determined with respect to SGS (Table 4). The uniformity

among calibrated and residuals calculated with respect to SGS indicates that the

calibration factor is reasonable. In addition, for events in which data from ULV were

available, data for SGS were unavailable. The conversion factor allowed for residuals to

be computed at ULV with respect to the true hard-rock site, SGS, therefore extending our

data set of residuals determined with respect to SGS.

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

Delay times vary for each station, for each event, causing minute differences in the

observed delay times. All standard deviations for travel time delays determined with

respect to both F02 and SGS are less than ± 0.085 s with one exception; a standard

deviation of ± 0.111 s was observed at station ULV. Because delay times are all less than

0.500 s, small deviations can cause large percent errors as were observed at stations F02,

ULV, F20, and SGS (Table 4). In addition, picking errors are estimated to be less than

0.1 s and the sample rate (40 sps) also forms a minimal error.

Another factor contributing to the uncertainty of the travel time delay measurements

is the location of SGS on less than 100 m of basin sediments. The thin section of basin

sediments causes a minimal decrease in the ealeulated travel time delays.

Residual Gradient Maps

The residual gradient produced by the contour gridding of the residual values at each

station is consistent across the Valley for all 27 events (Appendix G). The pattern shows

a general increase in residual time with the earliest arrivals in the southeast and the latest

arrivals toward the northwest. In addition, the residual gradient stays comparatively

consistent despite changes in baek-azimuth, the number of available stations, and the

location of available stations for the events analyzed.

The residual gradient appears to match the basin depth contours determined by

Langenheim et al. (2001a and 2001b) along the western edge of Frenchman Mountain

(Tkalcic et al., 2003). The residual pattern is primarily determined by the number and

location of available stations, however as stated above, it typically remains eonstant

despite changes in back-azimuth (Tkalcic et al., 2003). The eonsistency of the residual

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. gradient matching the basin depth contours, despite changes in back-azimuth, suggests

that the travel time delays are correlative with depth to basement (Tkalcic et al., 2003)

(Appendix F). This eorrelation indicates that stations located over thicker aceumulations

of basin sediments have significantly larger residuals than those located over thin

accumulations of sediments when compared to the hard-rock site.

Basin Depth

Estimates for basin depth computed using the 2-D model of the LVV by Zaragoza et

al. (2004) are shallower than predicted by the Langenheim et al. (2001a and 2001b)

model (Figure 39). Basin depths were calculated based on the average residuals

calculated at SGS and the calibrated residuals.

The deepest value observed was beneath SQP with a depth of 1.52 km determined

with respect to SGS. Basin depths calculated using both average residuals calculated at

SGS and the calibrated residuals have comparable results (Table 4). Basin depths

determined for stations SQP, CHY, and VAH have depths ranging from 1.30 km to 1.52

km while stations F20 and F04 have depths ranging from 1.09 km to 0.65 km (Table 4).

A negative depth of -0.04 km was calculated at SGS indicating that SGS was located on

hard-rock, or minimal/undetectable basin sediments.

As mentioned above, the calculated basin depths are shallower than predicted by the

Langenheim et al. (2001a and 2001b) model (Figure 39). This discrepancy may be

attributed to a variety of factors, mainly the shape of the basin and the small variations in

the direction of the incoming teleseismic P-waves. Flowever, these values do follow the

trend in depth to basement if not the actual basin depths determined by Langenheim et al.

(2001a and 2001b). In addition, an average P-wave velocity of 4.37 km/s (Zaragoza et al.,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2004) was used to calculate the basin depth. This average velocity may not accurately

represent known heterogeneities in the basin sediments and therefore affect calculated

depths. In addition, there are some uncertainties in the average P-wave velocity model

(Zaragoza et al., 2004) which contribute to the uncertainties in ealeulated basin depths.

Other factors that may affect calculated depths include accuracy of the travel time delay

calculations, differences in near source structure, differences in near receiver structure

(Tkalcic et al., 2003) and the location of SGS on less than 100 m of alluvial deposits.

Depth estimates may correspond to the unconsolidated late Neogene alluvial fill

overlying the more consolidated late Miocene and Pliocene sediments within the basin as

modeled by Taylor et al. (2004) (Figure 40). If these sediment thicknesses are attributed

to unconsolidated materials, it may represent a shallower sub-basin of low velocity

sediments suggested by Langenheim et al., (2001a; 2001b) and Snelson et al. (2004).

Interstation Group Velocity and Vs Inversion Interpretations

Interstation group velocities and Vs vary for each of the five interstation paths

calculated in this study. Interstation group velocities were determined along five separate

paths and with respect to different periods and wavelengths. As a result. Vs values and

depths of penetration are dissimilar. In addition to different path locations, a number of

factors may contribute to the variety in calculated values for interstation group velocity

and Vs including location errors, vertical and lateral heterogeneities within the basin fill,

depth to the water table, and depth to the more consolidated late Miocene and Pliocene

sediments within the basin.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Group Velocities for the LVVBBl and LVVBB2

Group velocities for source-receiver paths are fairly consistent across the array,

especially for group velocities determined with respect to the same earthquake source and

having similar source-receiver distances. Source-receiver group velocities capture the

average group velocity along the full source-receiver path; therefore, travel paths from

the same source are similar and typically exhibit similar group velocities. For the Lavic,

California event, group velocities are fairly consistent ranging from about 2.3 to 2.8 km/s

(Table 7). Group velocities observed at SQP, the station located elosest to the earthquake

source, are highest with values ranging from 2.7 to 2.8 km/s over periods of 1.5 s to 4 s

(Table 7). At stations CHY and F23, located at roughly the same distance from the

earthquake source, group velocities are most similar with values of about 2.5 km/s over

the same range in period (Table7). For this same event, group velocities observed at

stations F04 and F20 are the slowest with values of approximately 2.3 and 2.4 km/s over

periods of 1.6 s to 2.1 s (Table 7). As illustrated above, group velocities observed at

stations located closer to the earthquake source typically have higher group velocities

than those located further away from the source for the same event (Table 7a and 7b).

Higher group veloeities are expected for stations closer to the source, as surface waves

arrive and are recorded at these stations before reaching stations located at farther

distances within the basin. This trend is also apparent for the Scotty’s Junction event as

well as the Alamo event. In addition, stations loeated further away from the earthquake

source have paths that extend through more of the basin sediments; slower basin

velocities contribute to the slowing of these waves. Other factors, in addition to distance

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that may contribute to small differences in group velocity observed for the different

events, include variations in azimuth, different magnitudes, and different focal depths.

Interstation Group Velocities of the LVYBBl and LVVBB2

Interstation group velocities exhibit slower speeds than the group velocities calculated

along source-receiver paths. In addition, group velocities along each interstation path

vary throughout the basin. For example, interstation group velocities observed for the

Lavic, California event are slowest for the SQP-CHY path with group velocities between

0.6 and 0.7 km/s and fastest for the SQP-F23 path with group velocities between 0.9 and

1.1 km/s (Table 8) over the same periods despite similar path locations. Interstation group

velocities are much slower than those observed for the source-receiver paths (Table 7).

Slower velocities may be attributed to the location of each path within the confines of the

basin sediments, which have lower velocities than the surrounding eountry rock. In

addition, the paths are isolated within the heterogenous basin deposits (Taylor et al.,

2004) therefore group velocities vary along each path.

Vs Inversions

Shear velocities were calculated along interstation paths because they provide more

detailed information of the basin fill compared to group velocities. Since there are a

limited number of paths, this interpretation provides a preliminary assessment of the

shear velocities within the basin sediments of the LVV. It should be noted that the Vs

profiles created through inversion are a non-unique solution. In some cases, the

interstation group velocity calculated may only represent a portion of the full dispersion

curve. As a result, it is possible that a more general Vs model may fit and explain the

segment of the dispersion curve. In addition, these inversions are sensitive to the initial

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. starting model. As mentioned previously, Vs profiles of the basin at this scale do not

exist, so starting models were created through forward modeling and are assumed to be

satisfactory.

Paths SQP-F23 and SQP-CHY have the same source originating in Lavic, California

and the travel paths are roughly the same length and traverse similar parts of the basin

(Figure 27). Although average Vs values are slightly higher for the SQP-F23, both

profiles record a LVZ at a depth between roughly 2.0 km to 2.5 km suggesting this

feature is legitimate (Figures 30 and 32). The LVZ may represent the sub-basin contact

between the unconsolidated late Neogene alluvial fill overlying the more consolidated

late Miocene and Pliocene sediments within the basin as modeled by Taylor et al. (2004)

(Figure 40) indicating that contact may be lined by a low velocity clay layer or other low

velocity material. In addition, the LVZ is well below the depth to the water table at this

location (Zikmund, 1996) (Figure 41), however the LVZ may indieate that water-

saturated fluids extend to these depths.

Paths F20-F04 and F04-N06 have the same location and orientation although path

F04-N06 is roughly half the length of F20-F04 (Figure 27). These paths were calculated

for two different earthquakes; path F20-F04 captures waves derived from the Lavic,

California event located southwest of the LVVBBl while path F04-N06 records waves

generated by the Alamo, Nevada event located northeast of the LVVBB2 (Figure 10a and

I Ob). Average values of Vs are higher for the F20-F04 event with a general increase in

Vs with depth (Figure 28). Vs for the F04-N06 path are slower with a gradual LVZ

extending from 0.1 km to 0.4 km depth (Figure 36). The gradual LVZ of path F04-N06 at

shallow depths may be attributed to low velocity clays found on the east side of the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Valley (Taylor et al., 2004) (Figure 7). An interesting feature of both Vs profiles is the

change in velocity observed at roughly 0.6 km depth; path F04-N06 depicts a LVZ while

path F20-F04 indicates an increase in velocity. Although the paths are different lengths

and derive from different back-azimuths, the reverse change in shear velocity indicates a

lateral heterogeneity in the basin sediments where incoming waves from the southwest

perceive a gradual increase in velocity while waves originating from the northeast

experience a gradual decrease in velocity.

Path VAFI-F04 is the only path oriented east-west across the northeastern part of the

basin. Shear velocities estimated from inversion and captured for this interstation path are

the highest for the Valley with Vs ranging from 1.82 to 2.86 km/s. These values for Vs

appear to be too high especially for the shallow depths imaged and when compared to the

other Vs profiles. More Vs profiles in this region need to be acquired to support or refute

this model. An interesting feature of the Vs profile is a LVZ between 0.8 km and 1.2 km

depth. This feature seems to correspond well with the depth to the shallow aquifer in the

eastern portion of the LVV (Zikmund, 1996) (Figure 41).

Resolution kernels (Figures 29, 31, 33, 35, and 37) indicate the resolution of Vs with

depth. Based on the resolution matrices for the five interstation paths, paths SQP-CHY,

SQP-F23, VAH-F04 and F04-N06 are well resolved for the majority of the Vs profile

(Figures 31,33, 35, and 37). Path F20-F04 is well resolved at depths greater than 0.85

km, indicating that the shallow portion of the Vs profile may not be well constrained

(Figures 28 and 29). Path SQP-CHY is well resolved at depths greater than 2 km,

questioning the validity of the LVZ imaged between 1.2 and 1.6 km depth (Figures 30

and 31). The interstation path between SQP and F23 is well resolved throughout the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. entire Vs profile imaging the LVZ at about 2 km depth (Figures 32 and 33). For the

roughly east-west path, VAH-F04, Vs values at depths greater than 1.1 km are well

resolved suggesting that the LVZ between 0.8 and 1.2 km depth is valid (Figures 34 and

35). The resolution matrix for the shortest interstation path, F04-N06, indicates that Vs

values are well resolved throughout the profile (Figures 36 and 37).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ D O Q. C g Q.

■D CD

C/) C/)

8 Frenchman untains ountain CD' F04F20

3 3 " CD basi

CD ■ D O Q. 5 km C a residual 3o "O o

CD Q.

■D CD

C/) C/) Figure 38. A cartoon depiction of a cross-section of the Las Vegas basin. The LVVBBl stations are labelled at the surface, with the rough location of the Las Vegas "strip" indicated by buildings. Incident P-waves are indicated by the white arrows. The change in sign of the observed travel time delay is illustrated by black arrow. The travel time delay calculation is shown at the bottom of the figure (from Tkalcic et al., 2003). CD ■ D O Q. C 8 Q.

■D CD

C/) C/)

5.00

8 c i'

3 3.00 3 " CD

■DCD û 2.50 O Q. C a O O 3 O ■D O

CD Q.

■D CD 0.00 C/) F20 F04 LVM F02 C/) SQP CHY VAH Station Name 1 Calculated Basin Depth B Langenheim et al. 2001 Mode!

Figure 39. Comparison of the calculated basin depths beneath each station determined through the travel time delay analyses with the Langenheim et al. (2001a; 2001b) depth to basement model beneath each station. ■o & I

■o CD

C/î(g o'3 Sand CD 8 Clay a- 3 " Gravel and alluvial

3 fan deposits CD Cement/Caliche C 3. 3 " CD Oligocene - Miocene CD Rocks "O Pre-Oligocene ac Rocks ao 3 O 650000 3 " 655000 a 670000 3o " 675000 5- -g

C(g /i o" 3 Figure 40. A 3-D slice across the Las Vegas basin from Taylor et al. (2004) depicting the has in-bedrock contact (pink to blue contact) and the basin-subbasin contact (olive to pink contact). The vertical axis has 4x vertical exaggeration. May 2002 Depth to Water Contours Saturated Zone of the Las Vegas Wash Aquitard (Shallow System)- Blue Depth to Potentiometric Principal Aquifer- Purple

I K % %

% \

14»

Vànésiè Contour Interval

Figure 41. Shaded relief map of the Las Vegas Valley depicting the major contours of the depth to the water table in feet for the shallow system (blue contours) and the principal aquifer (purple contour). Major roads are shown as gray lines (from Zikmund, 1996).

102

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

DISCUSSION AND CONCLUSIONS

P-wave Travel Time Delay Discussion

Travel time delays and basin depths estimated using the teleseismic earthquakes

recorded by the LVVBBl are comparable to similar studies of the Santa Clara Valley

(SCV) (Dolenc, 2001; Dolenc et al., 2005) as well as earlier studies of the Las Vegas

basin (Tkalcic, et al., 2003). Detection of the Las Vegas basin using this method indicates

that regional and local geology have an effect on travel times and need to be considered

in global travel time measiuements (Tkalcic et al., 2003).

Comparison to the Santa Clara Valiev. California

The Las Vegas basin is an asymmetric basin with its deepest portion, approximately 5

km, lying just west of Frenchman Mountain (Langenheim et al., 2001a; 2001b) (Figure

1). Observations of travel time residuals in the Las Vegas basin show significant delays

up to -0.45 s (Table 7). Similar travel time delay studies have been conducted in the

LVV (Tkalcic et al., 2003) as well as the SCV (Dolenc, 2001; Dolenc et al, 2005). The

SCV is composed of two elongated basins, the Cupertino basin and the Evergreen basin.

Delays observed in the SCV show travel time delays up to about 0.3 s (Dolenc, 2001;

Dolenc et al, 2005).

Studies in the SCV typically show negative residuals at stations located on hard-rock

and positive residuals at stations located within the basins (Dolenc, 2001; Dolenc et al.,

103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2005); these results are consistent with observations in the LVV. The array coverage in

the SCV was fairly dense and maps of travel time residuals clearly image both the

Cupertino and Evergreen basins showing longer delays in the deeper parts of the basins

respectively (Dolenc et al., 2005). Although the residual gradient observed in the Las

Vegas basin is thought to correspond to the basin’s depth contours on the Valley’s eastern

border, more extensive array coverage to the western and northern part of the Valley is

needed to determine the full extent of the residual pattern across the basin. A dense array

in the LVV would confirm the relationship between increased travel time delay and basin

depth.

In the studies of the SCV, travel times residuals were found to increase with an

increase in basin depth up to 2 km (Dolenc, 2001; Dolenc et al., 2005). Travel time

delays over areas of the basins with depths greater than 2 km did not perceive longer

delays (Dolenc, 2001; Dolenc et al., 2005). Observed delays were associated with

unconsolidated low velocity sediments overlying more consolidated sediment with a

similar velocity to the bedrock (Dolenc, 2001; Dolenc et al., 2005). Observed delays in

the LVV indicate that the calculated basin depths, based on an average P-wave velocity

of 4.37 km/s, were shallower than predicted by the Langenheim et al. (2001a and 2001b)

basin model. Similarly to the results of the SCV experiments, depth estimates in the LVV

from this analysis are thought to correspond to the upper unconsolidated late Neogene

alluvial fill as modeled by Taylor et al. (2004) (Figure 40).

104

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Impact on Global Travel Time Studies

Travel times delays observed in the Las Vegas basin are due to near surface, low

velocity sediments. Near surface crustal features, such as the Las Vegas basin, are smaller

than those resolved on regional maps and are capable of affecting global travel time

measurements (Tkalcic et al., 2003). Small basins similar to those in the LVV and SCV

are easily detected using travel time measurements and their effects need to be considered

and addressed in global travel time measurements (Tkalcic et al., 2003).

Interstation Group Velocity and Vs Inversion Discussion

Interstation group velocities and Vs profiles calculated for the LVV offer preliminary

estimate of the basin fill properties. Measurements of interstation group velocity are the

first recorded in the LVV. In addition. Vs profiles at depths exceeding 300 m have never

been previously published for the Las Vegas basin.

Group Velocities for the LVVBBl and LVVBB2

Rayleigh wave dispersion observed in the LVV consists of a mix of higher mode

surface waves and scattered waves making it difficult in some cases to isolate the

fundamental mode Rayleigh wave. Previous studies have shown that Rayleigh waves

excited within basin sediments often interact with higher mode arrivals and scattered

waves causing difficulty in identifying and isolating fundamental mode Rayleigh waves

(e.g., McEvilly and Stauder, 1965; Châvez-Pérez et al., 1992; Savage and Helmberger,

2004). This interaction or inference needs to be considered as a possible error when

measuring fundamental mode Rayleigh wave dispersion curves. As part of this study,

group velocities were calculated for three regional earthquakes (< 300 km) along source-

105

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. receiver paths at seven different stations in the LVV (Figures 10a and 10b; Table 7).

Group velocities range from 1.8 km/s to 2.9 km/s over periods of 1.3 s to 4.0 s (Table 7).

An early study by Bennett (1974) examined Rayleigh waves recorded in the LVV

generated by nuclear tests at the NTS. In this study, group velocities along source-

receiver paths extending from NTS to the LVV were calculated. Group velocities

presented for the Carpetbag event range from 1.7 km/s to 2.5 km/s over periods of 2.2 s

to 4.0 s (Bennett, 1974). Regional group velocities presented by Bennett (1974) and

recorded in the LVV from the earlier Blume array are similar to those observed in this

study over the same period range. This correlation suggests that the fundamental mode

Rayleigh waves were correctly identified and picked for the dispersion analysis.

Interstation Group Velocities of the LVVBBl and LVVBB2

Interstation group velocities, determined as part of this study, are the first group

velocities directly calculated for and contained within the Las Vegas basin. Interstation

group velocities were determined along five interstation paths. Group velocities are

typically lower than those observed along source-receiver paths with values ranging from

0.25 km/s to 2.14 km/s over periods of 1.3 s to 4.0 s (Table 8).

Group velocities determined along interstation paths need to be supported by more

data. Ideally another semi-permanent or permanent array of at least 10 seismometers

should be deployed in the LVV with a more intentional geometry to maximize the

number of interstation paths captured. An array with a geometry in which stations are

located on both the eastern and western sides of the basin separated by distances of 5 km

to 10 km would be ideal for capturing surface waves from earthquakes originating from

western or eastern back-azimuths. Longer interstation distances would allow longer

106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. wavelength Rayleigh waves to be captured increasing the depth of penetration in hopes of

capturing the basin-bedrock contact. In addition, determination of group velocities of

higher mode Rayleigh waves and Love waves as well as phase velocities would provide

further constraints for the Vs inversions.

Vs Inversions

I-D models of shear velocity structure were determined along five interstation paths

within the Las Vegas basin as part of this study (Figure 27). Models of Vs extended to

depths as long as the longest wavelength with an average depth of 2 km (Figures 28, 30,

32, 34, and 36). Modeled data fit the observed dispersion curves well. Shear velocities

observed are low with values ranging from 0.28 km/s to 2.85 km/s. Since the number of

interstation paths was limited, more interstation group velocities need to be inverted to

confirm values reported in this study. However, these low shear velocities are within an

acceptable range for shear velocities measured within basin sediments.

Shallow shear wave velocities studies have been conducted in the Las Vegas basin by

Liu et al. (2005) and Scott et al. (2005). These studies characterize Vs within the upper

300 m of the basin fill. Near surface shear velocities (< 350 m) typically range from 0.25

km/s to 3.00 km/s (Liu et al., 2005; Scott et al., 2005). This study suggests that basin

shear velocities correlate well with these shallow shear velocities and carry down to

depths as great as 4 km. However as previously mentioned, the determination of Vs

through inversion is non-unique and more interstation paths are needed to constrain Vs.

An array with a geometry needed to capture the numerous earthquakes originating in

California is ideal. In addition, longer interstation paths would be needed to image the

basin-bedrock contact.

107

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vs profiles, determined through the inversion of interstation group velocities as part

of this study, are on the order of a localized basin scale. Studies at this scale are important

because they are an easy way to provide Vs profiles at depths greater than geo technical

surveys. In addition. Vs profiles within the basin sediments contribute to more thorough

3-D basin models within the LVV and are used in 3-D analyses of ground motion

simulation.

Summary

The Las Vegas basin is an asymmetric basin within the central Basin and Range

province of western North America. It is composed of Miocene through Holocene clastic

deposits including Late Neogene alluvial deposits underlain by basement rocks

comprised of Precambrian through Miocene metamorphic, carbonate, clastic and volcanic

rocks (Plume, 1989). In early September 2002, LLNL and UNLV deployed the first set of

two earthquake seismometer arrays known as the LVVBB. In late January 2003, the first

array (LVVBBl) was removed; a second array (LVVBB2) was deployed in July 2003

though August 2004. Data collected by the two arrays were used to calculate travel time

delays from teleseismic earthquakes as well as interstation group velocities and Vs

determined from regional earthquakes.

Travel time delays observed beneath the stations of the LVVBBl perceived delays up

to 0.45 s when compared with arrivals at hard-rock sites SGS and F02 (Tables 6a and 6b).

Observed delays were used to determine the basin depth beneath each station of the

LVVBBL The maximum basin depth calculated based on an average P-wave velocity of

4.37 km/s (Zaragoza et al., 2003), was 1.52 km beneath station SQP. Basin depths were

108

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shallower than predicted by early models (Langenheim et al., 2001a and 2001b) and

attributed to the shallower unconsolidated late Neogene alluvial fill overlying the more

consolidated late Miocene and Pliocene sediments within the basin as modeled by Taylor

et al. (2004) (Figure 40).

Group velocities measured from region earthquakes were determined using MFT and

refined with PMF. Group velocities were initially measured along source-receiver paths

and later confined to interstation paths, or paths lying along the same back-azimuth

within 1° degree of derivation. Interstation group velocities were inverted for I-D models

of Vs along five interstation paths within the LVV. Vs values are fairly low and attributed

to the clays and unconsolidated materials within the upper basin as models extend to

depths less than 4 km. These are the first Vs profiles recorded for deeper profiles within

the basin and can contribute to 3-D models of the LVV for simulations.

Finally, the thesis is the first to apply the interstation group velocity method at a local

or basin scale (<10 km). The shallow shear velocities determined through this method

correlate well with geotechnical surveys and offer greater depths of penetration. This

non-invasive method is an excellent means for calculating shear velocity in an urbanized

basin.

109

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

Bell, J.W., 1981, Subsidence in Las Vegas Valley: Nevada Bureau of Mines and Geology Bulletin, no.95, 84 p.

Bennett, T. J., 1974, Amplitude and propagation characteristics of short-period seismic surface waves in Las Vegas Valley: U.S. Atomic Energy Commission Report, NVO-1163-242.

Bingler, E.C., 1977, Las Vegas SE Folio - geologic map: Nevada Bureau of Mines and Geology, Environmental Series, scale 1:24,000.

Birrell, A.D., 1994, Digital mosaic of the western United States, copyrighted image: http://birrell.org/andrew/reliefMaps/301052020v2.jpg.

Burchfiel, B. C., Fleck, R. J., Secor, D. T., Vincelette, R. R., and Davis, G. A., 1974, Geology of the Spring Mountains, Nevada: Geological Society of America Bulletin, V. 85, p. 1013-1022.

Burchfiel, B. C., Cowan, D. S., and Davis, G. A., 1992, Tectonic overview of the Cordilleran orogen in the western United States, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., eds.. The Cordilleran Orogen: Coterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-3, p. 407-478.

Campagna, D. J. and Aydin, A., 1994, Basin genesis associated with strike-slip faulting in the Basin and Range, southeastern Nevada: Tectonics, v. 13, p. 327-341.

Châvez-Pérez, S., Calderôn-Macfas, C., Romero-Jiménez, E., and Gomez-Gonzalez, J.M., 1992, Dispersion analysis using strong motion data: in Proceedings, Tenth World Conference on Earthquake Engineering, Madrid, v. 3, p.1287-1292.

dePolo, C. M., Jones, L. M., dePolo, D. M., and Tingley, S., 2000, Living with earthquakes in Nevada: A Nevadan’s guide to preparing for, surviving, and recovering from an earthquake: Nevada Bureau of Mines and Geology Special Publication 27, p. 1.

110

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dolenc, D., 2001, Basin structure influences on the teleseismic wave propagation in the Santa Clara Valley, California [M.S. Thesis]: Berkeley, University of California, 151 p.

Dolenc, D., Dreger, D., and Larsen, S., 2005, Basin structure influences on the propagation of teleseismic waves in the Santa Clara Valley, California: Bulletin of the Seismological Society of America, v. 95, p. 1120-1136.

Duebendorfer, E. M., and Wallin, E. T., 1991, Basin development and syntectonic sedimentation associated with kinematically coupled strike-slip and detachment faulting, southern Nevada: Geology, v. 19, p. 87-90.

Duebendorfer, E. M., and Black, R. A., 1992, Kinematic role of transverse structures in continental extension; an example from the Las Vegas Valley shear zone, Nevada: Geology, V. 20, p. 1107-1110.

Duebendorfer, E. M., and Simpson, D. A., 1994, Kinematics and timing of Tertiary extension in the western Lake Mead region, Nevada: Geological Society of America Bulletin, V. 106, p. 1057-1073.

Duebendorfer, E. M., Beard, L. S., and Smith, E. L, 1998, Restoration of Tertiary deformation in the Lake Mead region, southern Nevada: The role of strike-slip transfer faults, in Faulds, J. E., and Stewart, J. H., eds.. Accommodation zones and transfer zones: The regional segmentation of the Basin and Range Province: Geological Society of America Special Paper 323, p. 127-148.

Dziewonski, A., Block, A., and Landisman, M., 1969, A technique for the analysis of transient seismic signals: Bulletin of the Seismological Society of America, v. 59, p. 427-444.

Federal Emergency Management Agency, Mitigation Directorate, 2000, HAZUS 99 Estimated Annualized Earthquake Losses for the United States: FEMA 366

Goldstein, P., and Minner, L., 1996, SAC2000; seismic signal processing and analysis tools for the 2L ‘ century: Seismological Research Letters, v. 67, p. 39.

Guth, P. L., 1981, Tertiary extension north of the Las Vegas Valley shear zone. Sheep and Desert Ranges, Clark County, Nevada: Geological Society of America Bulletin, v. 92, p. 763-771.

Herrin, E., and Goforth, T., 1977, Phase-matched filters: Application to the study of Rayleigh waves: Bulletin of the Seismological Society of America, v. 67, p. 1259- 1275.

Herrmann, R. B., and Ammon, C. J., 2002, Computer programs in seismology. Version 3.20: http://www.eas.slu.edu/People/RBHerrmann/ComputerPrograms.html.

I l l

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Jeffreys, H., and Bullen, K. E., 1940, Seismological Tables: British Association Seismological Committee, London.

Kennett, B. L. N., and Engdahl, E. R., 1991, Traveltimes for global earthquake location and phase identification. Geophysical Journal International, v.l05, p.429-465.

Langenheim, V. E., Grow, J. A., Jachens, R. C., Dixon, G. L., and Miller, J. J., 2001a, Geophysical constraints on the location and geometry of the Las Vegas Valley Shear Zone, Nevada: Tectonics, v. 20, p. 189-209.

Langenheim, V. E., Grow, J. A., Jachens, R. C., Dixon, G. L., Miller, J. J., Lundstrom, S. C., and Page, W. R., 2001b, Basin configuration beneath Las Vegas Valley, Nevada: Implications for seismic hazard evaluation, in Luke, B., Jacobson, E., and Werle, J., eds.. Proceedings, 36* annual Symposium on Engineering Geology and Geotechnical Engineering: Las Vegas, Nevada, p. 755-764.

Lay, T., and Wallace, T. C., 1995, Modem global seismology: San Diego, Academic Press, 521 p.

Liu, Y., Luke, B., Pullammanappallil, S., Louie, J., and Bay, J., 2005, Combining active- and passive-source measurements to profile shear wave velocities for seismic microzonation, in Earthquake Engineering and Soil Dynamics, eds. Boulanger, R., Dewoolkar, M., Gucunski, N., Juang, C., Kalinski, M., Kramer, S., Manzari, M., and Pauschke, J., ASCE Geotechnical Special Publication 133, 14 p.

Matti, J.C., and Bachuber, F.W., 1985, Las Vegas SW Quadrangle geologic map: Nevada Bureau of Mines and Geology, Las Vegas area map 3Bg, scale 1:24,000.

Matti, J.C., Bachuber, F.W., Morton, D.M., and Bell, J.W., 1987, Las Vegas NW Quadrangle geologic map: Nevada Bureau of Mines and Geology, Las Vegas area map 3Dg, scale 1:24,000.

Matti, J.C., Castor, S.B., Bell, J.W., and Rowland, S.M., 1999, Las Vegas NE Quadrangle geologic map: Nevada Bureau of Mines and Geology, Las Vegas area map 3Cg, scale 1:24,000.

McEvilly, T.V., and Stauder, W., 1965, Effect of sedimentary thickness on short-period Rayleigh-wave dispersion: Geophysics, v. 30, p. 198-203.

Murphy, J. R., and Hewlett, R. A., 1975, Analysis of seismic response in the city of Las Vegas, Nevada: A preliminary microzonation: Bulletin of the Seismological Society of America, v. 65, p. 1575-1597.

Mussett, A.E., and Khan, M.A., 2000, Looking into the Earth: An introduction to geological geophysics: New York, Cambridge University Press, 470 p.

112

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Perry, J. J., and O’Donnell, J., 2001, Loss estimation from large earthquakes occurring on faults in the Las Vegas Basin, Nevada using HAZUS, in Luke, B., Jacobson, E., and Werle, J., eds.. Proceedings, 36* annual Symposium on Engineering Geology and Geotechnical Engineering: Las Vegas, Nevada, p. 661-669.

Plume, R. W., 1989, Ground-water conditions in Las Vegas Valley, Clark County, Nevada: U.S. Geological Survey Water-supply Paper, v. 2320-A, p. A1-A15.

Rodgers, A., McCallen, D., Tkalcic, H., Wagoner, J., Louie, J., Anderson, J., Luke, B., Snelson, C., and Taylor, W., 2004, Seismic wave amplification in Las Vegas: Site response and empirical estimates of ground motion: Eos (Transactions, American Geophysical Union) v. 85, n. 47, Abtract S41C-05.

Savage, B., and Helmberger, D.V., 2004, Complex Rayleigh waves resulting from deep sedimentary basins: Earth and Planetary Science Letters, v. 218, p. 229-239.

Scott, J. B., Rasmussen, T., Luke, B., Taylor, W. J., Wagoner, J. L., Smith, S. B., and Louie, J. N., 2005, Shallow shear velocity and seismic microzonation of the urban Las Vegas, Nevada basin: in review Bulletin of the Seismological Society of America.

Shearer, P. M., 1999, Introduction to Seismology: Cambridge, Cambridge University Press, 260 p.

Slemmons, D. B., Bell, J. W., dePolo, C. M., Ramelli, A. R., Rasmussen, G. S., Langenheim, V. E., Jachens, R. C., Smith, K., and O’Donnell, J., 2001, Earthquake hazard in Las Vegas, Nevada, in Luke, B., Jacobson, E., and Werle, J., eds.. Proceedings, 36* annual Symposium on Engineering Geology and Geotechnical Engineering: Las Vegas, Nevada, p. 447-460.

Smith, K., O’Donnell, J., and Slemmons, D. B., 2001, Seismicity and ground motion hazards in the Las Vegas area, Nevada, in Luke, B., Jacobson, E., and Werle, J., eds.. Proceedings, 36* annual Symposium on Engineering Geology and Geotechnical Engineering: Las Vegas, Nevada, p. 587-598.

Snelson, C. M., McEwan, D. J., Hirsch, A. C., and Zaragoza, S. A., 2004, Imaging the Las Vegas Basin: Results From Recent Seismic Refractions Experiments: Eos (Transactions, American Geophysical Union) v. 85, n. 47, Abstract S34A-07.

Stammler, K., 1992, Seismic Handler, User’s Manual, [PhD thesis]: Seismological Central Observatory, University of Erlangen.

Stein, S., and Wysession, M., 2003, An introduction to seismology, earthquakes, and earth structure: Oxford, Blackwell Publishing, 498 p.

113

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sterner, R., 1995, Digital elevation tiles of Nevada, California, Utah, and Arizona, copyrighted images; http://fermi.jhuapl.edu/states/states.html.

Su, F., Anderson, J. G., Ni, S., and Zeng, Y., 1998, Effect of site amplification and basin response on strong motion in Las Vegas, Nevada: Earthquake Spectra, v. 14, p. 357-376.

Tabor, L. L., 1982, Geology of the Las Vegas area: Department of Energy Report, JAB- 10145-1.

Taylor, W. J., 1996, Mesozoic and Cenezoic tectonics and structures of southern Nevada, in dePolo, C. M., ed.. Proceedings of a Conference on Seismic Flazards in the Las Vegas Region: Las Vegas, Nevada, Nevada Bureau of Mines and Geology Open- File Report 98-6, p. 12-23.

Taylor, W.J., Luke, B., Snelson, C.M., Liu, Y., Wagoner, J., Rodgers, A., McCallen, D., Rasmussen, T., and Louie, J., 2004, Spatial relations among young faults, basin fill and Vs in Las Vegas Basin: Basin and Range Province Seismic Hazards Summit II

Tkalcic, H., Rodgers, A., Snelson, C. M., McEwan, D. J., 2003, Comprehensive analysis of broadband seismic data in the Las Vegas Valley: Eos (Transactions, American Geophysical Union) v. 85, n. 46, Abstract SI lD-0322.

Wen, K. L., Yeh, Y. T., and Huang, W. G., 1992, Effects of an alluvial basin on strong ground motions: Bulletin of the Seismological Society of America, v. 82, p. 1124- 1133.

Wernicke, B. P., 1992, Cenozoic extensional tectonics of the U.S. Cordillera, in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., eds.. The Cordilleran Orogen; conterminous U.S.: Cambridge, Massachusetts, Geological Society of America, p. 553-581.

Wessel, P., and Smith, W. H. F., 1991, Free software helps map and display data: Eos (Transactions, American Geophysical Union), v. 72, p. 445-446.

Zaragoza, S. A. and Snelson, C. M., 2004, Crustal Velocity Model of Watusi Data Integrated With Legacy Data, Clark County, Nevada: Eos (Transactions, American Geophysical Union) v. 85, n. 47, Abstract S34A-06.

Zhou, R. M., and Stump, B. W., 2004, Rayleigh waves generated by mining explosions and upper crustal structure around the Powder River Basin, Wyoming: Bulletin of the Seismological Society of America, v. 94, p. 1410-1429.

Zikmund, K., 1996, Extent and potential use of the shallow aquifer and Wash flow in Las Vegas Valley, Nevada: Southern Nevada Water Authority.

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Graduate College University of Nevada, Las Vegas

Darlene J. McEwan

Local Address: 5500 Mountain Vista Street Apt. 1222 Las Vegas, Nevada 89120

Home Address: 175 Tyler Street Buffalo, New York 14214

Degree: Bachelor of Science, 2002 State University of New York at Buffalo

Special Honors and Awards: Lilly Fong UNLV Geoscience Department Scholarship (2003) Bemada E. French UNLV Geoscience Department Scholarship (2004) UNLV Graduate Student Association Grant (2004)

Publications: McEwan, D. J., Snelson, C. M., Tkalcic, H., and Rodgers, A., 2004, Shear velocity structure beneath the Las Vegas Valley, Nevada from regional and teleseismic events [abs.]: Eos (Transactions, American Geophysical Union), v. 85, Abstract S43A-0978.

McEwan, D.J., Snelson, C.M., and Rodgers, A., 2004, Analysis of Rg wave dispersion for shear velocity structure in northeast Las Vegas Valley, Nevada: Using regional data collected by the Las Vegas Valley Broadband array: Seismological Reseach Letters, v. 74, p. 257.

McEwan, D.J., Snelson, C.M., Tkalcic, H., and Rodgers, A., 2003, Initial results from the Las Vegas Valley Broadband array based on differential travel time residuals and interstation phase velocities [abs.]: Eos (Transactions, American Geophysical Union), V. 84, Abstract SI lD-0323.

McEwan, D. J., Snelson, C. M., Tkalcic, H., Rodgers, A., and Louie, J. N., 2003, Investigating the Las Vegas Valley: analysis from the Las Vegas Valley Broadband array using differential travel time residuals and interstation phase velocities: Geological Society of America Abstracts with Programs, v. 35, p. 477.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hirsch, A. C., McEwan, D. J., Howley, R. A., Mehling, J. B., Snelson, C. M., and Drohan, P., 2004, A geophysical study of fissures in Pahrump, Nevada [abs.]: Eos (Transactions, American Geophysical Union), v. 85, Abstract H21E-1080.

Snelson, C. M., McEwan, D. J., Hirsch, A. C., and Zaragoza, S. A., 2004, Imaging the Las Vegas Basin: Results from recent seismic refraction experiments [abs.]: Eos (Transactions, American Geophysical Union), v. 85, Abstract S34A-07.

Tkalcic, H., Rodgers, A., Snelson, C. M., and McEwan, D. J., 2003, Comprehensive analysis of broadband seismic data in Las Vegas Valley [abs.]: Eos (Transactions, American Geophysical Union), v. 84, Abstract SI lD-0322.

Zaragoza, S. A., Snelson, C. M., McEwan, D. J., Sandru, J., and Hirsch, A. C., 2003, Crustal velocity model of Watusi and legacy seismic refraction data, Clark County, Nevada [abs.]: Eos (Transactions, American Geophysical Union), v. 84, Abstract SllD -0321.

Snelson, C. M., Sandru, J., McEwan, D. J., Hirsch, A. C., Zaragoza, S. A., Draa, A., Hanson, A. D., Kaip, G., Harder, S. H., Azevedo, S., McKibben, W., Rodgers, A., Lewis, J. P., Smith, D., Rock, D., and McCallen, D., 2003, Preliminary results from SILVVER ’03 - Seismic Investigations of the Las Vegas Valley: Evaluating Risk [abs.]: Eos (Transactions, American Geophysical Union), v. 84, Abstract SllD-0325.

Bidgoli, T. S., Fossett, E., Knudsen, T. R., Kubart Dano, R. K., McEwan, D. J., and Taylor, W. J., 2003, Surface rupture, paleoseismology, and seismic hazard assessment of the Holocene California Wash fault, southern Nevada: Implications for risk to the greater Las Vegas area: Geological Society of America Abstracts with Programs, v. 35, p. 476.

Zaragoza, S. A., Snelson, C. M., McEwan, D. J., Hirsch, A. C., Sandru, L, and Draa, A., Las Vegas Valley Seismic Response Project (LVVSRP): Initial results from seismic refraction experiments: Geological Society of America Abstracts with Programs, v. 35, p. 477.

Snelson, C. M., Rodgers, A., Smith, K., Slemmons, D. B., O’Dormell, J., Zaragoza, S. A., Hopkins, J., McEwan, D. J., and Myers, J. R., Seismic hazards in the Las Vegas Valley, NV, USA: preliminary analysis from earthquake and seismic-refraction data: Seismological Research Letters, v. 74, p. 197-198.

116

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thesis Title: A Seismological Study of the Las Vegas Basin, NV: Investigating Shear Velocity Structure and Basin Depth

Thesis Examination Committee: Chairperson, Dr. Catherine M. Snelson, Ph.D. Committee Member, Dr. Andrew D. Hanson, Ph.D. Committee Member, Dr. Wanda J. Taylor, Ph.D. Committee Member, Dr. Arthur Rodgers, Ph.D. Graduate Faculty Representative, Dr. Barbara Luke, Ph.D.

117

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