Seismic Tomography and Surface Deformation in Krfsuvik, SW Iceland by Jing Liu B.S

Home , Fagradalsfjall, Krafla

Seismic Tomography and Surface Deformation in Krfsuvik, SW Iceland by Jing Liu B.S. Geophysics, China University of Geosciences (Wuhan), 2008

Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree ACHUES of INS1W Master of Science in Geophysics MSSACHUSMOF TECHNOLOGY

Signature of Author ...... Department of Earth, Atmospheric, and Planetary Sciences November. 21, 2013

C ertified by ...... Michael C. Fehler Senior Research Scientist-Earth Resources Laboratory Thesis Supervisor /7 J & Certified by...... Bradford H. Hager Cecil & Ida Green Pro r, Director of Earth Resources Laboratory Thesis Co-Supervisor 7

Accepted by Y Rob Van der Hilst Schlumberger Professor Head, Department of Earth, Atmospheric and Planetary Sciences Seismic Tomography and Surface Deformation in Krfsuvik, SW Iceland by Jing Liu Submitted to the Department of Earth, Atmospheric, and Planetary Sciences On November 21, 2013, in partial fulfillment of the requirements for the Degree of Master of Science in Geophysics

Abstract

The Krfsuvik region of southwestern Iceland is a region of high potential for geothermal energy that is currently experiencing seismic swarm activity and active surface deformation. Understanding the subsurface structure of the area is of great scientific and practical significance. Using permanent and temporary seismic stations deployed in the region, we captured an earthquake swarm from Nov. 2010 to Feb. 2011 clustered around the center of the Krfsuvik volcanic system. We studied the seismicity and Vp, Vs and Vp/Vs ratio in this region by applying double difference tomography. Our tomography result indicates a low velocity zone at a depth of about 6 kin, directly beneath the earthquake swarm. At the same time, our relocation result delineates strike-slip and dip- slip faults above and around this low velocity zone. Brittle-ductile transition is delineated based on the distribution of the earthquakes in this area. In order to understand the relation between the subsurface structure and the surface deformation, we modeled surface deformation using the input parameters constrained from our tomography results. We found that the main deformation is well captured by a pressure source yielding a volume expansion of about 30x 106 m 3 at the depth of about 6 km, centered on the low velocity zone detected in tomography. And the secondary deformation could be explained by the normal and the right-lateral slip faults, whose patterns are delineated by the earthquake relocations. The combination of the local stress caused by the expanding source and regional stress that yields a combination of left-lateral shear and extension might have triggered the earthquakes. Based on the low Vp, Vs and possibly high Vp/Vs ratio at depth of ~6 km and its expanding property, the possibilities of supercritical water, H2 0-rich partial melting with magma intrusion are discussed. The results of this thesis provide new insights to understand the seismicity and surface deformation in volcanic zones as well as provided important reference in exploration of new geothermal areas.

Thesis Supervisors:

Michael C. Fehler . Title: Senior Research Scientisph- EarthResouycpe Laboratory Bradford H. Hager Title: Cecil & Ida Green Professor, birector of Earth Resources Laboratory

2 Acknowledgements

I would like to thank the people for their help towards the completion of this thesis during my two years of studying in the Earth Resource Laboratory at MIT.

First of all, I would like to thank my advisor, Dr. Michael Fehler for his support and guidance throughout my study at MIT. I always feel lucky to meet and work with such a reputable scientist as Dr. Fehler who offered me the chance to enter MIT as well as the field of geophysics. I would also like to thank Prof. Brad Hager for his fruitful and patient suggestions regarding my current as well as future work.

I would like to acknowledge the people in Earth Resource Laboratory (Micheal

Fehler, Bradford Hager, Maria Zuber, Robert van der Hilst, Alison Malcolm, Anna

Shaughnessy, Haijiang Zhang, Yingcai Zheng, Xingding Fang, Qin Cao, Hui Huang,

Xuefeng Shang, Chunquan Yu, Di Yang, Haoyue Wang, Lucas Willemsen, Alan

Richardson) for many helpful comments and exposure to different aspects of the seismology and geodynamics problems in my work. I would also like to thank my husband, Rong, for his support during difficult times.

This work was funded by US DOE, Iceland GEORG Program, Iceland & Swedish

National Science Foundations.

3 Contents

Chapter 1...... 8 Introduction...... 9 1.1 Plate Boundary of Iceland ...... 9 1.2 Reykjanes Peninsula, SW Iceland ...... 10 1.3 Previous Study of Seismicity, Deformation and Structure in Krfsuvik...... 12 1.4 Surface Manifestations of Krysuvik Geotherm al Field ...... 15 1.5 Our Study of the Krf'suvik Geotherm al Field ...... 16 Chapter 2 ...... 19 Seism ic Event Relocation and Tom ography...... 19 2.1 Seism ic Data and Stations ...... 19 2.2 Theory of Event Relocation and Tom ography...... 21 2.3 Results of Event Relocation and Tom ography...... 22 2.3.1 Travel times used in this study...... 22 2.3.2 Event Relocation Results ...... 23 2.3.3 Checkerboard Test ...... 26 2.3.4 Tom ography Results...... 31 Chapter 3 ...... 36 Surface Deform ation Observation and M odeling...... 36 3.1 Geodetic Observation ...... 36 3.2 Surface Deform ation Modeling...... 40 Chapter 4 ...... 45 Discussion...... 45 4.1 Brittle-ductile Transition ...... 45 4.2 Possibilities of the low velocity zone...... 48 Chapter 5 ...... 54 Conclusion ...... 54 Appendix A ...... 56 Geotherm al M anifestation in Krysuvik...... 56 Appendix B ...... 57 GPS m easurem ents in Krysuvik ...... 57 References ...... 59

4 List of Figures

Figure 1-1: Iceland plate boundary. (Hjaltadottir, 2010). Fissure swarms are shown as grey areas and central volcanoes enclosed with grey, thin circles. The black dotted lines are the transform zones. HM: Hreppar micro-plate...... 10

Figure 1-2: Tectonic map of Reykjanes Peninsula and its location in SW Iceland (inset). The blue lines are the plate boundary (solid-axial rifts, long dashed line-propagating rift, dotted line-fracture zones). The hatched areas indicate the high-temperature geothermal fields. Fissure swarms are shown as grey areas in the inset, along the NE-SW direction. RR, Reykjanes Ridge; H, Hengill volcano zone; WVZ, west Volcanic Zone; SISZ, South Iceland Seismic Zone. M odified after Keiding et al., (2009) ...... 12

Figure 1-3: Historical earthquakes in Krfsuvik from 1997 to 2006 (Keiding et al., 2009). The two stars indicated the Mw5.9 earthquake in June 17, 2000 and Mw5.0 earthquake in August 23, 2003 respectively...... 14

Figure 1-4: Cumulative number of earthquakes in the Krfsuvik region from 2007 to Sep. 2012 (continue with fig.3). The two diamonds indicate the two big swarm events that happened in 2009 and 2011...... 15

Figure 1-5: Geology map of the Krfsuvik area in our study. K: Lake Kleifarvatn; KV (orange dot): Krysuvik Volcano...... 18

Figure 2-1: a. Seismic station distribution on the Reyjanes Peninsula from 2010 to 2011 (courtesy of Oli Gudmundsson). b. Seismic stations we used in this study. The dashed black line indicates our study area. The orange dot is the location of the Krfsuvik volcano. The small black dots are the earthquakes from the SIL catalog...... 20

Figure 2-2: a. Cumulative number of events from 1 't July 2010 to 3 1st December 2011 and the blue circle indicates the biggest jump that occurred on 2 7th February 2011; b. Histogram of earthquake magnitudes from 1't July 2010 to 31st D ecem ber 2011...... 20

Figure 2-3: The residual histograms of Absolute P, S, Catalog differential P, S, and C ross-correlation P, S...... 24

Figure 2-4: Earthquake events distribution in Krfsuvik area from Catalogue...... 25

5 Figure 2-5: Earthquake events distribution in Krfsuvik area after relocation. The arrows on the top figure indicate the directions of N-S trending fault and NE- trending fault...... 25

Figure 2-6: Map view and cross sections presented in the following checkerboard test and tom ography results...... 27

Figure 2-7: Checkerboard test of Vp for the map view in figure 2-6. Figures of the top row are the input model; Figures of the middle row are the corresponding recovered model using our acquisition geometry; Figures of the last row are the corresponding resolvability distribution...... 28

Figure 2-8: Checkerboard test of Vp for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8 ...... 28

Figure 2-9: Checkerboard test of Vs for the map view in figure 2-6. Figures have the sam e m eanings as in figure 2-8...... 29

Figure 2-10: Checkerboard test of Vs for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8 ...... 29

Figure 2-11: Checkerboard test of Vp/Vs for the map view in figure 2-6. Figures have the sam e m eanings as in figure 2-8 ...... 30

Figure 2-12: Checkerboard test of Vp/Vs for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8...... 30

Figure 2-13: Horizontal profiles of Vp, Vs and Vp/Vs ratio models at depths of 2,4, and 6 km. The black solid line indicates the resolvability threshold of 0.7. The black dashed lines are the other resolvability values...... 34

Figure 2-14: Cross section profiles of Vp, Vs and Vp/Vs ratio models. The black solid line indicates the resolvability threshold of 0.7. The black dashed lines are the other resolvability values...... 34

Figure 2-15: The isosurfaces of the Vp, Vs and Vp/Vs ratio. The regions where Vp < 5.75 km/s, Vs < 3.3 km/s and Vp/Vs > 1.89 are shown...... 35

Figure 2-16: Resistivity at the depth of 5 km (Hersir et al., 2013). A low resistivity body was observed at the similar location with the low velocity zone discovered in this study...... 35

Figure 3-1: Displacement of GPS measurements in Krfsuvik area. The grey circles indicate the uncertainties of each measurement (Michalczewska, 2012). Different color arrows indicate measurements in different periods...... 37

Figure 3-2: The mean deformation in the InSAR Line of Sight direction from 2009 to 2011. The solid black lines indicate the volcanic systems (Michalczewska,

6 2012). The dashed black line reveals the secondary deformaton (dislocated deformation of northwestern part of the main deformation)...... 37

Figure 3-3: The algorithm of the cosine and sine rule on a sphere (Essentials of G eophysics, M IT) ...... 38

Figure 3-4: Projection of the GPS measurements on the descending look angle of TerraSA R -X ...... 39

Figure 3-5: Models and results simulated as the InSAR observation. Boxes al-3 are map views of the deformation source geometry, and the red dot is the reference point where deformation is taken to be 0. The numbers indicate the sources, 1 (black dot) for inflation source, 2 for normal fault and 3 for strike-slip fault. The green lines indicate the strike directions of the faults and the red box represents the fault plane for the normal fault. Plots bl-3 are the 3D views of the corresponding map views. The cl-3 are the calculated displacement results projected on the LOS of TerraSAR-X (descending). The silver boxes indicate the corresponding area of the InSAR observation...... 43

Figure 3-6: Seismic moment release starting from 1s July 2010 to 3 1" December 2011. The biggest jump occurred on 27th February 2011...... 43

Figure 3-7: Comparison between the observed InSAR deformation and the simulated deformation. The location of the expanding source mapped on the right picture is (63.910N , 22.050W )...... 44

Figure 4-1: Longitude view of 3D plot with Vp (5.75 km/s) isosurface body and earthquakes in our study area. The blue lines indicate the brittle-ductile transition. Green, blue and red lines indicate BDT at 4, 5 and 6 km, respectively. K : Lake Kleifarvatn...... 47

Figure 4-2: Temperature profiles. Red, green and blue lines indicate temperature of 5500C at 4, 5 and 6 km , respectively...... 47

Figure 4-3: Rock melting studies (Grove et al., 2006). We focus on the dashed curve of basalt melting with 1-2 wt% water. Other curved lines indicated different kinds of rock in various studies. The red horizontal indicates the 0.2 GPa at ~6 km in our interested depth...... 51

Figure 4-4: Relation between Vp, Vp/Vs ratio and volume of fraction of fluids (Nakajima et al., 2001) in basaltic material (amphibolite). Solid and dashed curves show the water inclusion and melt inclusion, respectively. Thin, moderate, and thick curves denote the aspect ratio of 0.001, 0.01 and 0.1, respectively. The black and gray stars indicate the possibly obtained Vp/Vs ratio in this study. The horizontal dotted line indicates the Vp value (5.75 - 5.8 km/s) obtained in our tomography...... 51

7 List of Tables

Table 2-1: List of the RMS of residuals...... 24

Table 2-2: Initial model (one-dimensional velocity model)...... 26

8 Chapter 1 Introduction

1.1 Plate Boundary of Iceland

Iceland (fig. 1-1), between the Reykjanes ridge (RR) in the south and the Kolbeinsey ridge (KR) in the north, is located at the intersection of the Mid-Atlantic Ridge and the North-Atlantic mantle plume. Four rift zones constitute the plate boundary: The

Reykjanes Peninsula (RP), which is the on-land continuation of the RR, the western volcanic zone (WVZ), which is a northward continuation of the RP, the eastern volcanic zone (EVZ) and the northern volcanic zone (NVZ) (Einarsson, 1991). Most seismicity and geothermal activity in Iceland is in some way related to the Mid-

Atlantic plate boundary.

The northwest motion of the Mid-Atlantic Ridge relative to the mantle plume has induced considerable offset of the active plate boundary from the Mid-Atlantic ridge.

The REVEL plate motion model indicates a relative North America (NA) - Eurasia

(EU) motion rate about 19.9 mm/yr in the direction about N102 0E at 64 0N, 20.5 0W

(Sella et al., 2002). In response to this unstable situation, two transform zones have developed: the south Iceland seismic zone (SISZ), connecting the RP to EVZ, the

Tjornes fracture zone (TFZ), which is composed of three lineaments: the Grimsey lineament (GL), the Husavik-Flatey fault (HFF) and the less active Dalvik lineament

(DL), connecting the NVZ to the KR. Differential movements are taken up by these fracture zones. Because the fracture zones are very young and stress field that

9 induced them is changing with time, a complex series of faults have formed, which are not oriented parallel to the spreading direction (Clifton and Einarsson, 2005;

Einarsson, 1976; Einarsson and Eiriksson, 1982).

KR - TFZ G. 66'

651

HM 64* . E VZ km RRt-- Z 0 50s 100-

-24' -22* -20' -18' -16' -14.

1.2 Reykjanes Peninsula, SW Iceland

The Reykjanes Peninsula (RP) (fig.1-2) represents the northward prolongation of the Reykjanes Ridge (RR). The plate boundary along the Peninsula is a zone of high seismicity and recent volcanism, forming a transition between the Reykjanes Ridge

(RR) to the southwest, the Western Volcanic Zone (WVZ) to the northeast, and the

South Iceland Seismic Zone (SISZ) to the east. The zone of seismicity trends around

N80 0E on the central of the Rekjanes Peninsula, but bends toward the south on the western part of the peninsula before connecting to the Rekjanes Ridge. The obliquity

10 of the plate boundary indicates a combination of left-lateral shear and extension, which was observed on the RP by a geodetic study conducted around 1968-1972

(Brander et al., 1976). Recently detailed studies by Keiding et al. (2008) show a left- lateral motion of ~18 mm/yr and opening of ~7 mm/yr below a locking depth of ~7 km.

The superposition and relative motion of the spreading plate boundary over the mantle plume are manifested by the faults and volcanic systems in Iceland, typically characterized by a central volcano and a fissure (dyke) swarm (Thordarson and

Larsen, 2007). Thus, the main tectonic features on the Reykjanes peninsula are a large number of NE-SW trending volcanic fissures, a series of NE-SW strking normal faults and N-S striking right-lateral strike-slip faults crosscutting the normal faults

(Clifton and Kattenhorn, 2006). The fissures and faults are made up of right-stepping en-echelon volcano-tectonic segments from west to east, the Reykjanes, Svartsengi,

Fagradalsfjall, Krfsuvik, Brennisteinsfjoll, Hvalhnukur and Hengill fissure swarms

(fig.1-2). In these fissure swarms, both earthquake swarms and main shock-aftershock sequences are observed.

11 tI 'Ijai

RR T-~

Bre n q~joII

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63,8 I km -22.8" -22.6" -22.4* -22.2' -22* -21.8 -21.6 Figure 1-2: Tectonic map of Reykjanes Peninsula and its location in SW Iceland (inset). The blue lines are the plate boundary (solid-axial rifts, long dashed line- propagating rift, dotted line-fracture zones). The hatched areas indicate the high- temperature geothermal fields. Fissure swarms are shown as grey areas in the inset, along the NE-SW direction. RR, Reykjanes Ridge; H, Hengill volcano zone; WVZ, west Volcanic Zone; SISZ, South Iceland Seismic Zone. Modified after Keiding et al., (2009)

1.3 Previous Study of Seismicity, Deformation and Structure in Krfsuvik

As one of the active volcanic systems on the Reykjanes Peninsula, the Krysuvik volcanic system located in the center of the Peninsula consists of the Kr suvik volcano (63.930N, 22.10 0 W) and NE-SW trending eruptive fissures and fractures with a total length of about 55 km and width of about 13 km (Clifton and Kattenhorn, 2006; Thordarson and Larsen, 2007). The fissure swarms are intersected by a series of N-S trending faults (Clifton and Kattenhorn, 2006), which are known in a few cases to be right-lateral strike slip faults (Einarsson, 1991). The first seismic station was installed on Reykjanes Peninsula in 1991 by the

South Iceland Lowland (SIL) seismic network that operated by the Icelandic

Meteorological Office (IMO) and increased to seven by 1997. Since then, detailed

12 seismicity has been recorded. In 1997-2000, a high background seismicity rate

(Clifton and Kattenhorn, 2006; Einarsson, 1991) was observed in Krfsuvik. During

June-July 1999, an intense swarm occurred during a period of long-lived swarms

(fig.1-3). On June 17, 2000, a big earthquake up to Mw5.9 occurred on a N-S striking strike-slip fault that was believed to be triggered by an Mw6.5 earthquake in the

South Iceland Seismic zone. However, following the June 2000 events, there was a sharp decrease in the seismicity rate in Krfsuvik (fig.1-3). On August 23, 2003, an event with Mw5.0 occurred at a N-S striking strike-slip fault in the Kr suvik region.

This main shock was followed by more than 1000 aftershocks located at depths of ~1 to ~5 km. Part of the aftershocks were located at the same fault as the main shock, while the other part was located at different NE-SW striking fault planes. Most earthquakes in Kr suvik are centered at ~5 km deep, with a variation from surface to

~10 km (Keiding et al., 2009). The shallowest earthquakes are located in the central

Krf'suvik area, where considerable change in geothermal manifestation was observed at the surface. The 2000-2006 seismicity around Krfsuvik was also accompanied by corresponding strain changes. The observed areal strain rate during 2000-2006 showed pronounced signals of areal expansion (Keiding et al., 2008) in Krfsuvik.

However, the simulated areal strain did not capture this areal expansion when the stress change was only considered to be caused by spreading of the plate boundary.

During this period, a tomography study using five-months of local microearthquake data recorded from April 10 to August 30 in 2005 suggested a strong decrease of P- and S- wave velocity and Vp/Vs ratio at the depth of 3-6 km broadly located beneath

Krfsuvik-Kleifarvatn geothermal area, which was interpreted as zones of increased

13 fluid temperature and pressure (Geoffroy and Dorbath, 2008). However, instead of independently inverting the Vp/Vs, they directly divide the Vp by Vs, which leads to large uncertainties of the result of Vp/Vs.

8000 Krsuvfk

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4000

2000 B U 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

From May to October 2009, over 10,000 earthquakes were recorded by the seismic stations deployed by the HYDRORIFT project (Kristjansdottir, 2013) around

Kr suvik (fig.1-4). The majority of the earthquakes were located at depths from 1.4 km to 4 km and generally delineated N-S striking faults. The focal mechanism study indicated that normal, strike-slip, and reverse faulting events took place on the same apparent faults in the same swarms, which was suggested by the cause of non-optimally oriented faults that could be explained by a component of volume increase. Using this seismicity and the seismic stations deployed by the HYDRORIFT project, they detected a low P-wave velocity anomaly located southwest of the Lake Kleifarvatn at the depth of 5-

8 km (Kristjansdottir, 2013). Meanwhile, a low resistivity anomaly was observed in this area by 3D magnetotelluric method at a shallower depth of ~5 km (Hersir et al., 2013).

14 During the period of the increase of the seismicity, uplift deformation was observed in this area (Michalczewska et al., 2012)

4 1.6x10

> 1.4-

1.2- E

-s 0.8 - E

'207 2008 2009 2010 2011 2012 year Figure 1-4: Cumulative number of earthquakes in the Krfsuvik region from 2007 to Sep. 2012 (continue with fig.3). The two diamonds indicate the two big swarm events that happened in 2009 and 2011.

1.4 Surface Manifestations of Krfsuvik Geothermal Field

The Krysuvik high-temperature area is about 40 km 2, located primarily at the intersections of the fissures and the strike-slip faults (see map in Appendix A). The hyaloclastite unit is by far the most voluminous unit and hosts all geothermal manifestations. Welded lavas and spatter debris or scoria are also observed. Phreatic craters are along zones of weakness as indicated by their linear distribution near faults. It is widely believed that the numerous phreatic craters indicate the presence of some heat source at depth, responsible for these explosive eruptions (Mawejje, 2007).

Most of the area are enclosed by the 15-500 C isotherm lines at the depth of 10-12 cm (Mawejje, 2007). The area with isotherm (50 0C) was observed to be completely altered to clays and, in most cases, it was active with fumaroles, mud pots, boiling

15 springs and sulphur (Appendix A). Steam vents and fumaroles were observed to be

linearly distributed following fractures in some places. Recharge seems to be due to an

inflow of cold water from the northeast, probably from Lake Kleifarvatn. In addition to

steam, other gases are also produced. Gases like hydrogen sulfide could easily be identified by the characteristic smell. Mud pots, like thick porridge boiling in a pot, were only mapped at lower altitudes, especially at Seltnin (Southwest of Lake Kleifarvatn).

The clay material is mainly kaolinite, suggesting alteration by acid leaching. In these mud pots, temperature as high as 950 C was recorded. Like mud pots, boiling springs were also observed at lower altitudes, especially at Seltn'n. The hydrothermal manifestations may be linked to an active heat source at depth.

1.5 Our Study of the Krfsuvik Geothermal Field

In our study, we focus on the Krfsuvik area enclosed in the dashed box in figure

1-5. The hyaloclastite units, surrounded by basaltic lava, are by far the most voluminous units in the center of our study area. In the geologic map of figure 1-5, the rock types are lava shield (> 7000 years old), prehistoric lavas (>2400 years old), hyaloclastite (older and early Bruhnes), prehistoric lavas (>2400 years old), hyaloclastite (older and early

Bruhnes), compound lava on hyaloclastite and lavas of 8 th_9 th century from left to right.

Only a few volcanic eruptions are reported to have occurred in historical times (the last

1100 years) in the vicinity of this area. Shallow level magma chambers and dense dyke swarms are considered as the heat source for the geothermal system.

19 shallow exploration wells were drilled in the Krfsuvik area from 1945 to 1950.

Then another three additional wells were drilled in 1960 and an additional well was

16 drilled in 1995. During most of the 20 century, the local municipalities have been keen on utilizing the geothermal fields in the Krfsuvik area for electricity production, large scale domestic heating, and other purposes such as balneology, green houses, etc. So far, however, no large-scale utilization has been realized.

In order to investigate whether there are some potential geothermal sources or risk of magma intrusion around Krf'suvik, in 2010, Massachusetts Institute of Technology

(MIT), Reykjavik University (RU) and Uppsala University (UU) deployed 38 seismic stations on the Reykjanes Peninsula and recorded the seismicity that occurred from 2010 to 2011 in the area around Krfsuvik. Based on these seismic data, we have studied the seismicity in the region using double difference tomography to better relocate seismic events and determine the velocity structure. The tomography reveals a low velocity zone at the depth of about 6 km and the relocation of seismic events delineates the N-S trending and NE-SW trending faults. Brittle-ductile transition and thermal profiles are created based on the distribution of the earthquakes and the location of the low velocity zone in this area. Interferometric Synthetic Aperture Radar (InSAR) observation suggests 20-30 mm uplift as well as expansion of this area. To better understand the relations among seismicity, velocity structure and the observed deformation, we use Coulomb 3 software (U.S. Geological Survey) to model the surface deformation. Our result reveals that the first order deformation may be caused by the inflation of magma sources at around 6 km, where the seismically determined low velocity zone is located and the secondary deformation can be modeled using a NE-SW trending normal fault and a N-S trending strike slip fault that are constrained from the seismic event relocation. This result indicates that the seismicity in Kr suvik around

17 2010-2011 might be triggered by partial melting or supercritical water activities. Such

discovery provides new insights to understand the seismicity and surface deformation in

volcanic zones as well as provide important reference in exploration of new geothermal

areas.

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Figure 1-5: Geology map of the Kry'suvi'k area in our study. K: Lake Kleifarvatn; KV (orange dot): Krfsuvik Volcano.

Chapter 2 presents an overview and analysis of the seismic data, including the theory we used, the checkerboard test and also the tomography results. Chapter 3 is a discussion of the surface deformation observation and modeling. Finally, chapter 4 contains a discussion of the seismic and deformation results.

18 Chapter 2 Seismic Event Relocation and Tomography

2.1 Seismic Data and Stations

The seismic data used in this study were recorded by the SIL network and temporary seismic stations deployed by Massachusetts Institute of Technology (MIT),

Reykjavik University (RU) and Uppsala University (UU). In 2010, MIT, RU and UU deployed 14, 8, and 16 stations, respectively, on the Rekjanes Peninsula (RP) to study the geothermal areas (fig.2- 1 a). Each station has a three-component broadband seismometer and a GPS synchronized clock. The seismic data are digitally recorded at a sample rate of

100 Hz. In our study, 23 seismic stations (5 from SIL, 5 from MIT, 5 from UU and 8 from RU) are used for the seismic event relocation and tomography in the Krfsuvik area

(fig.2-1b). Figure 2-2 shows the cumulative number of earthquakes and distribution of earthquake magnitudes from 1st July 2010 to 31st December 2011 of the SIL catalog.

One pronounced peak in activity at Krysuvik was observed in Feb. 2011 and one smaller peak around Aug. 2011 (fig.2-2a). About 3853 seismic events recorded from Jul. 2010 to

Dec. 2011 by the SIL seismic network around Krf'suvik were selected for analysis. The magnitude of most events is in the range of 0-2, which are called microearthquakes

(fig.2-2b).

19 / YIMO station V UU station V MIT tation V U sltat N b 64 ash

63.9

~ A svh 63.85-grv ~

-22.6 -22.4 -22.2 -22 -21.8 -21.6 Longitude (degree) Figure 2-1: a. Seismic station distribution on the Reyjanes Peninsula from 2010 to 2011 (courtesy of Oli Gudmundsson). b. Seismic stations we used in this study. The dashed black line indicates our study area. The orange dot is the location of the Krfsuvik volcano. The small black dots are the earthquakes from the SIL catalog.

4000 1000 a 3000 800f 6001 E 2000: Cz *400

N 1000 200f

100 200 300 400 500 0 1 2 3 4 days Magnitude Figure 2-2: a. Cumulative number of events from I't July 2010 to 3lst December 2011 and the blue circle indicates the biggest jump that occurred on 2 7 February 2011; b.

Histogram of earthquake magnitudes from 1 't July 2010 to 31 ' December 2011.

20 2.2 Theory of Event Relocation and Tomography

We applied the Double Difference Tomography (Zhang et al., 2009; Zhang and

Thurber, 2003) to simultaneously relocate the seismic events and construct travel time

tomography (Vp, Vs and Vp/Vs). Based on seismic ray theory, the arrival time T of a

body wave from an earthquake i to a seismic station k is written as a path integral,

k T, =t +f sdI, (1)

S - T = - 1)s dl, (2) where t' is the original time of the event i , s is the slowness vector and dl is the element length of the integral path. A first order truncated Taylor series is used to linearize the highly non-linear relationship in equation (1,2). For equation (2), S and P waves from the same events are with the same origin times, thus the unknown origin times are removed from this equation. The misfit between the observed (To ) and predicted (T cal) arrival time rk can be expressed by the perturbations of the hypocenter parameters ( , dt') and slowness parameters (ds):

" - ca = d + dt ds d. (3) n=1 14

The double difference, the difference of residuals between the observed and theoretical

travel times in linked event pairs (ij) at one station (k), drk , are used in the DD

earthquake location algorithm (Waldhauser and Ellsworth, 2000), which could be written

as:

dr = -=rk-k=(T-T- =T- obs~(Tk -Tlj)cal.- (4

21 The observed travel time difference (T - Tk) can be calculated by both the waveform cross-correlation for similar waveforms and absolute catalog arrival times. A pseudo-bending ray tracing algorithm (Um and Thurber, 1987), which finds the ray paths and calculates the travel times, is applied to update the velocity models. In order to calculate the relative, absolute event locations and the subsurface velocity structures, the absolute arrival times, the waveform cross-correlation data, and the catalogue differential arrival times are used in the inversion. For the Vp/Vs ratio in equation (2), in order to avoid the problems that P and S have different raypaths and Vs structure can not be inferred reliably from Vp and Vp/Vs values, we check P and S raypaths and remove the corresponding S-P times from the Vp/Vs inversion if the paths differ by more than a specified threshold. The complete inversion system is solved by the LSQR algorithm for sparse linear equations and least-squares problems (Paige and Saunders, 1982).

2.3 Results of Event Relocation and Tomography

2.3.1 Travel times used in this study

Due to the small magnitude of most events, P and S arrival time picking become the most difficult but important part in our seismic study. Two rounds of arrival time picking were performed. In the first round, we automatically picked the P arrivals using the AR-

AIC method (Sleeman and van Eck, 1999) and then applied it to identify the S arrivals in a window after the P arrivals. In the second round, we manually corrected the P and S automatic picks by visually inspecting the three-component recordings. Thus we ensured the high quality of the final event catalog. For the P arrivals, the arrival times on vertical components are selected for their clear onset. For the S arrivals, the arrival times on east or north components, whichever are more visible, are selected. Therefore, shear wave splitting is not taken into account. We used two kinds of differential travel times in our

22 calculation. One is catalog differential time, which is obtained by directly subtracting the corresponding catalog travel time of the linked neighbor (Waldhauser, 2001). The other one is cross-correlation differential time, which is obtained by cross-correlation between two waves in our specified threshold. In summary, we have totally 37,459 and 35,493 absolute catalog P and S arrives, 628,764 and 511,162 pairs of catalog differential P and

S arrives, 7,485,607 and 9,559,917 pairs of cross-correlation differential P and S arrives,

32,925 absolute catalog S-P times, 451,370 pairs of catalog differential S-P times, and

2,280,581 pairs of cross-correlation differential S-P times.

2.3.2 Event Relocation Results

We analyze the residuals between the calculated and measured travel times to evaluate the convergence of inversion. The root-mean-square (RMS) residuals after the first, eleventh and twelfth round iterations for absolute P, S, catalog differential P, S and cross-correlation P, S are listed in Table 2-1. From the RMS of the residuals, we could find that the inversion converged till the 12th round iteration. Figure 2-3 shows the histograms of the corresponding residuals after 12 iterations. The residuals of S spread more widely than P, which might be contributed by the anisotropy in this area.

Figure 2-4 and 2-5 show the catalogue event distribution and the relocation result, respectively. From the map view of the two results, we find that our relocation result better delineates the N-S trending and NE-SW trending faults on the surface. In the SIL catalog, the event depths are widely scattered in a range of 2-10 km. However, in our relocation result, most earthquakes are concentrated at the depth of 1-6 km and are more clustered. From the plot of the cross section Al-B1 in the relocation result, we find that the NE-SW trending fault is deeper than the N-S trending faults. Meanwhile, the average

23 event depth after relocation is about 2 km shallower than the catalog events. Systematic shifts in event depths might be caused by the difference in the velocity models. In our calculation, the velocity model is updated.

Table 2-1: List of the RMS of residuals.

RMS (s) Absolute Absolute Catalog Catalog Cross- Cross- P S differential differential correlation correlation P S P S

1"t round 0.107 0.172 0.086 0.119 0.087 0.135

1 1 th round 0.038 0.061 0.029 0.045 0.008 0.011

121h round 0.038 0.061 0.029 0.045 0.008 0.010

Absolute P Absolute S 8000

6000[ 4000 3000 ~4000f*f E I

1000 .82y 2 -0.1 0 0 1 0 2 01 02 residuals (s) residuals (s)

Catalog differential P X 104 I Catalog differential S 6- 4.x & 51 3. 4.

82 -01 0 01 02 2 -01 0 01 02 residuals (a) .Aresiduals (s)

x Cross-coalaion P S Cross-correlation 5 61 4

3; 4

1 2

0 5 0 0065 01 481 -005 0 00 5 0.1 residuals (s) residuals (s)

Figure 2-3: The residual histograms of Absolute P, S, Catalog differential P, S, and Cross-correlation P, S.

24 63.9 6 C%4 Al B1

63.9 I: A.B1_

-22.19 -22.03 -21.87 *W * * * * o t 0 2 Al -11

4 - E o - . 65

10 5 km . 1 -22.19 -22.03 -21.87 Longftute (degree) Figure 2-4: Earthquake events distribution in Krysuvik area from Catalogue.

63.96 BA .

A1 -- - As6- 1 -E 63.91 --

06 km A2 WI 63.86 -0 -22.19 -22.03 -21.87 (w) WdeQ

Al B1 2 - 41N-S trendihg fault ted6 - If-trending. 5km - 1 5 kmfault 8t -22.19 -22.03 -21.87 Longitute (degree) Figure 2-5: Earthquake events distribution in Krysuvik area after relocation. The arrows on the top figure indicate the directions of N-S trending fault and NE-trending fault.

25 2.3.3 Checkerboard Test

The starting model for the inversion is a one-dimensional model (Table 2-2)

(Tryggvason et al., 2002). The X nodes, from west to east, are -50 km, -16 km, -8 kin, -4

km, -2km, 0 km, 2 km, 4 km, 8 km, 16 km, 50 km; The Y nodes, from south to north, are

-40 km, -6 km, -2 km, 0 km, 2 kn, 6 kin, 40 km; The Z nodes, in depth, are -100 km, 0

km, 2 km, 4 km, 6 km, 9 km, 12 km, 16 km, 21 km, 32 km, 200 km. The reference

location (0, 0) is (63.91 0N, 22.030 W).

Table 2-2: Initial model (one-dimensional velocity model)

Depth (km) -100 0 2 4 6 9 12 16 21 32 200

P velocity 3.4 3.6 5.6 6.4 6.6 6.7 6.8 7.0 7.1 7.4 8.0

(km/s)

Vp/Vs ratio 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78 1.78

In order to quantitatively understand our 3D tomography results, we evaluate the resolution of our tomography model using the acquisition geometry through a checkerboard test. The synthetic velocity model includes a 3D checkerboard pattern of positive and negative 5% perturbations in P velocity and negative and positive 5% perturbations in S velocity relative to the 1D velocity model. Therefore the constructed synthetic model of Vp/Vs is on the order of positive and negative 10% perturbations.

Synthetic travel times are computed in the perturbed model for the relocated hypocenters and station distribution. In order to assess the recovered checkerboard models quantitatively, we calculated the semblance between the exact and recovered

26 checkerboard anomalies using a circular operator in the (x, y)-plane with a radius of 5 km

centered on each node (Zelt, 1998).

Z'=(ti + nri)2 R = i= (5) 2 EiM(t.2 + nrt2)'

where tj and ri are the true and recovered velocity anomalies at the ith node inside the

circle consisting of M nodes. R-values, the resolvability, are in the range of 0.1-1.0. The

results are considered resolvable when values are larger than 0.7. R-values are used to

evaluate the resolution for the following studies of the slices of 3D tomography results.

63.96 ./

-63 .91 kmt=63.91 e) -

-j / ,4km/ on3AA -22.19 -22.03 -21.87 Longitute (degree) Figure 2-6: Map view and cross sections presented in the following checkerboard test and tomography results.

27 Deph-2 km 63.96 Deplhv4 km Depth-6 km 63.96 6396-- VP(%) Ph5 63911 63.91 83.91

-2219 83.86 -5

83.96 VP(%) 83.98

8391w G3.91 63.91

83.86 83.88 -5

63.96 M96 83.96

03.91 e3.91 06

63.86 22.3 -21. 83 Longlute Idegre.) Longiout. Idegre ) Longnute (dsgr..) Figure 2-7: Checkerboard test of Vp for the map view in figure 2-6. Figures of the top row are the input model; Figures of the middle row are the corresponding recovered model using our acquisition geometry; Figures of the last row are the corresponding resolvability distribution.

Lattudo=63.91 (dogr..) Latktud0=63.93(dgr..) Vp(%)

10 10

0 Vp(%)

10 1C

resolvabily

06 P .5

10 10 -22.19 -22.03 -21.87 -22.19 -22.03 -21.87 0. Longitute (dogre) Longitute (dsgrW) Figure 2-8: Checkerboard test of Vp for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8.

28 Depth=2 km Depth=4 km Deoth-6km Vs(%) 83.9f En 5

83.91

21 0 03. I51 -2219 -2203 -21.87 VS(%) 63.96 83.96

63.91 83.91

63.96 -22.19 63.86 M3 -2203 -21.87 -21.87 resolvabs~y 63.96 63.98 83.98

.7 63.91 83.91 e3.91;

0.5 -22.19 -2203 -21.87 -cr- I -Cr-%A -r. .O '~~-2219 -22.03 -21.87 04 Longtu* (degree) LoVgNute (degee) Long"ute (degree) Figure 2-9: Checkerboard test of Vs for the map view in figure 2-6. Figures have the same meanings as in figure 2-8.

Latkude=63.91 (deg") Lattude=63.93(degree) Vs(%)

-22.19 -22.03 -21.87 Vs(%)

resovabily 0

0.7

S0.80.6 0.5 10 10 04 -22.19 -22.03 -21.87 -22.19 -22.03 -21.87 Longitute (degre.) Longitute (degree) Figure 2-10: Checkerboard test of Vs for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8.

29 Depth.2 km Depth=4 km Depth-6 km 83.9f 10

- 3.91 0

-10 -22. -2.3-21.07 11

39G 10 5 0 -5

8e -22.03 -21.87 -10

63. e3.96 m94

83.91 e3.91

-2219 -22.03 -21. -22.19 -22.03 -21.87 -2219 Z03.. -21.87 Longotme (dgre) Longstut (dsgre.) Longtikui fdsgrs.) Figure 2-11: Checkerboard test of Vp/Vs for the map view in figure 2-6. Figures have the same meanings as in figure 2-8.

LatItude=63.91 (degree) Latltude=63.93(degree) VpIVS(%) 10

10 101 . -;L.1 -22.19 -22.03 -21.87 VpIVs(%)

5 10

Ic 10

resolvablity 0.8 0.7 0.6 0.5

10 04 -22.19 -22.03 -21.87 -22.19 -22.03 -21.87 Longfitu* (degree) Longitute (degre.) Figure 2-12: Checkerboard test of Vp/Vs for the cross sections in figure 2-6. Figures have the same meanings as in figure 2-8.

30 Figure 2-6 is the map view and cross sections of our study area in this thesis and it is used in the following checkerboard test and tomography results. Figure 2-

7,8,9,10,11,12 show the map view and cross profiles of the checkerboard test results of

Vp, Vs and Vp/Vs. The recovered checkerboard and resolvability values show that the

Vp has resolution down to a depth of ~7 km on both the horizontal and vertical profiles.

The Vs models are resolved well at the depths of 2 and 4 km and lose resolution at the depth of ~ 6km for the horizontal checkerboard test, but have resolution down to a depth of ~6 km. While the Vp/Vs ratio almost loses resolution at ~6 km on both horizontal and vertical profiles. From the analysis of the resolution test, we find that our results have relatively better resolution of vertical profiles than horizontal profiles. This phenomenon can be explained from the ray coverage. From figure 2-5, we find that the earthquakes are concentrated within a depth range of 1-6 km. Thus the limitation of the depth of earthquakes caused that at the depth of ~6 km, the cross rays in vertical directions are more than that in horizontal directions.

2.3.4 Tomography Results

Figure 2-13 presents horizontal slices of the tomography results at various depths.

In general, the resolvability tests validated our results. Clearly higher Vp and Vs are observed beneath Krf'suvik volcano (gray dot) and beneath Lake Kleifarvatn, but there is a region of reduced Vp and Vs between them at the depth of 2 km. These are consistent with the geology study in area. The rocks between the Krfsuvik volcano and Lake

Kleifarvatn are hyaloclastite, while around Krfisuvik volcano they are basaltic lavas. The relatively higher Vp and Vs of the Lake Kleifarvatn might be related with its formation.

Lake Kleifarvatn is an in-land lake, no connections with other rivers or lakes. Thus it

31 might be a crater lake, which was formed when water filled the basin left by the collapse of big magma chamber without sufficient support for its roof after eruption. Therefor a remnant magma chamber might be the possible reason for the higher Vp and Vs. Lower

Vp/Vs ratio has been obtained in the lake and around the southwest part of the lake. After the big earthquake in 17 th June 2000, a lot of cracks and fissures under Lake Kleifarvatn have been created and 12% of by volume of the lake drained down (Clifton et al., 2003).

While because of the high permeability in this region, circulation of water with steam reduces the Vp/Vs ratio slightly. On the surface, hot springs are observed at the corresponding area (Mawejje, 2007). At the depth of 6 km, a low Vp anomaly is detected within the resolvability range, while low Vs and high Vp/Vs anomalies are lost resolvability on the horizontal map view.

On the cross sections of our study area (fig.2-14), a low Vp velocity zone at a depth of about 6 km is detected within the resolvability range. Most part of the low Vs velocity zone is in the resolvability range. While the high Vp/Vs ratio zone is slightly out of, but very close to the resolvability range. Figure 2-15 shows a 3D view of the low Vp

(-5.75 km/s), Vs (~3.3 km/s) and possible high Vp/Vs ratio (-1.89) anomaly isosurfaces along with the locations of earthquakes occurred in the Krfsuvik area. The low Vp anomaly is directly beneath the earthquake swarm. Most of the microearthquake events occurred above or around this low P velocity zone. The low S velocity and high Vp/Vs ratio isosurface bodies are slightly toward the west. The decreased resolvability of Vs and

Vp/Vs ratio at the depth of -6 km and the relatively worse RMS (fig.2-3) caused by anisotropy and noise of the S onset might have caused the slight shift of the Vs and

Vp/Vs ratio isosurface bodies from the center of earthquake swarm.

32 This low Vp and Vs velocity zone was also reported by Geoffroy (2008), which was interpreted as supercritical water because of the low Vp/Vs ratio he obtained by dividing Vp by Vs. However, when directly dividing Vp by Vs, the uncertainties of

Vp/Vs ratio will increase because of both the inversion uncertainties of Vp and Vs. For example, if the uncertainties of both Vp and Vs are +5%, the uncertainties in Vp/Vs ratio will reach ~10%, which is crucial to interpret the change of Vp/Vs ratio. In our tomography, we invert the Vp/Vs ratio directly. At the depth of ~6 kin, our Vp/Vs inversion reveals a possible high Vp/Vs ratio body, but its exact extension is not well resolved because the lack of deeper earthquakes. Meanwhile, the 3D Magneto Telluric

(MT) survey also obtained a low resistivity zone at the same location of our low Vp zone

(Hersir et al., 2013) (fig.2-16). This high conductivity and low velocity zone could be caused by the filling of water in cracks (supercritical water) or partial melting. If it is interpreted as partial melting, the rock matrix will change and Vs will decrease much more than Vp. Thus we could expect a high Vp/Vs ratio, which is consistent with the possible Vp/Vs anomaly detected in our tomography. The detailed discussion of the low

Vp and possible high Vp/Vs ratio around 6 km is addressed in Chapter 4.

33 Deeth.2 km Vs(kmis) Depth.2 km e3-96 3.9W S2 8 5 6 63.91 4 63.91 Vplyi 5

Depth.4 km vp(kwMs) Dopth.4 km Deih-4 km 2

83.91 63.91 Vs(kwih it 8

iskm, Depth-6 km DepthuO km D*Dthu6 km VD(kNVs) 83.9 83 2 (

63.91 4 63.91 Ii 1.8 683 63.86

Longoul*0(d69-1 LongouI idegrw) LongdM* (dso")

L&IM 63.93(deg") Vp(kmws) LaMUdO=63.93(dsar..1 VsfkmIs) LatIude.63.93(degs,*) 2

~ S VP/VS

10 .6IV -22.03 .a:.udA4391(sgm) VP(kmf/s) LaUd*S3.91(dSgeej) VS(km/3) Lautlude=6391(dogt.)

ZI i 0

10 10 10 -22.19 -22.03 -21.57 9.0 -2 -21.07 --. IWV -".%" *Z1.O LongfItw (do"gr@I Longkuw (dog ) Longitui. (degre) Figure 2-14: Cross section profiles of Vp, Vs and Vp/Vs ratio models. The black solid line indicates the resolvability threshold of 0.7. The black dashed lines are the other resolvability values.

34 Isovelocity: Vs=3.3 km/s Isoratio: Vp/Vs=1.89 0 Isovelocity: Vp=5.75- km/s

2 2i - 2 4 -4~ 4

53.__

L0a,9l I YL-k 6391 07 .9 0388-2203~ 1.8 63.8 22.03 -21.7 Lat 86 03 -21,87 (degrool 22,19 Langule (dP ) NdW-) 22 19 Ider I -22 19 2202 2187 Figure 2-15: The isosurfaces of the Vp, Vs and Vp/Vs ratio. The regions where Vp < 5.75 km/s, Vs < 3.3 km/s and Vp/Vs > 1.89 are shown.

1000

7095

- 100 l}70 E 7090

7085 50

Initial model 7060 440 445 450 455 Figure 2-16: Resistivity at the depth of 5 km (Hersir et al., 2013). A low resistivity body was observed at the similar location with the low velocity zone discovered in this study.

35 Chapter 3 Surface Deformation Observation and Modeling

3.1 Geodetic Observation

Further support of the low velocity zone in the tomography result and the relationship between the seismicity and the tectonics of the Krfsuvik region can be obtained by evaluating the surface deformation observed in the region.

We found publicly-available GPS and InSAR analyses showing the ongoing deformation in the region. The geodetic measurements between 2009 and 2011 show significant uplift and time-variable expansion in the Krfsuvik area (fig.3-1,2). The

Global Positioning System (GPS) data recorded by the permanent GPS stations (MOHA,

KRIV) and semi continuous GPS stations (LAMB, SELC), in the Krfsuvik area recorded

20-30mm uplift around Feb. 2011 (Appendix B). This area also exhibits expansion to some extent around the southwest corner of Lake Kleifarvatn (Michalczewska, 2012).

Meanwhile, the InSAR measurements in this region also detected uplift and expansion

(Michalczewska, 2012). Figure 3-2 shows the average unwrapped TerraSAR-X interferograms spanning from 2009 to 2011. The main deformation is observed around the southwest corner of Lake Kleifarvatn and a secondary deformation, a dislocated deformation of northwestern part of the main deformation, is enclosed within the dashed lines in fgiure 3-2. The deformation shows displacements up to 30 mm in the Line-Of-

Sight (LOS) direction, although we do not know whether the orbit is ascending or

36 descending. The inclination of TerraSAR-X is 97.440. The ascending (Jul.18, 2010 -

Jun.14, 2011) and descending (Jul.24, 2010 - Jun.08, 2011) vectors of the look angles are

(-0.5664, -0.1259, 0.8145) and (0.6422, -0.1276, 0.7559) (east, north, vertical) respectively. I- ~ J

6~44000- '-

0 5

Figure 3-1: Displacement of GPS measurements in Krysuvik area. The grey circles indicate the uncertainties of each measurement (Michalczewska, 2012). Different color arrows indicate measurements in different periods.

64' E2! *21

17 11 -~11 S

-22', Figure 3-2: The mean deformation in the InSAR Line of Sight direction from 2009 to 2011. The solid black lines indicate the volcanic systems (Michalczewska, 2012). The dashed black line reveals the secondary deformaton (dislocated deformation of northwestern part of the main deformation).

37 In order to determine the orbit of the InSAR measurements, we project the three- component measurements of the GPS measurements (measured from figures in Appendix

B) of four GPS stations using equation (6) to both the ascending and descending look angles, respectively, of the InSAR in the Krfsuvik area.

dl,, = dh sin(G) + d, cos(G) (6) where G is the look angle and it could be obtained from the vectors of the look angle as

G = atan ((e 2 + n 2)/z) (e, n and z are the unit vectors in east, north and vertical directions), dl, 5 is the displacement projected on the look angle of the satellite, dh and d, are the horizontal and vertical displacement, respectively. d, can be obtained directly from the GPS measurements, and df could be calculated using the east and north measurements, de and dn.

dh = de cos(D) + dn sin(D) (7) where D is the cross-angle between the north and the LOS surface projection vector. We use the cosine and sine rules on a sphere (fig.3-3) to find this cross-angle.

Let a, b. and d be the angular distances between 3 points and A. B. and D the angles as indicated in Figure The cosine rule for a and A is:

cosa = cos bcosd + sinbsindcosA

and the sine rule:

sin a sin b sin d sin A sinB sin!) Figure 3-3: The algorithm of the cosine and sine rule on a sphere (Essentials of Geophysics, MIT).

38 For our problem, compared with the satellite altitude, the Krfsuvik area is considerably small, so we use the central latitude of this area, 63.920, as the average angle we used in cosine and sine rules. d = 97.440 - 900, b = 900 - 63.92 0(if the latitude of the point we want to calculate is 63.920), A = 900, then we get D = 160

(fig.3-3). Therefore we use 160 as the cross angle between the north and the LOS surface projection vector in the area we modeled in next section.

After projecting the GPS measurements onto both the ascending and descending orbits of TerraSAR-X using equation (6), we find that the result presented in Figure 3-4 that is obtained using the descending orbit of TerraSAR-X is fairly consistent with the

InSAR observation in Figure 3-2. Therefore in the following simulation, we project our simulated surface displacements to the descending look angle of TerraSAR-X.

I . rnr n 63.96 /7 .

63.94 2()

~63.92 110

63.9 MSELC 4-J / 0 63.88 *1' WRIV / -10 r-3n86 /__ -22.15 -22.1 -22.05 -22 -21.95 -21.9 longitude (degree,

Figure 3-4: Projection of the GPS measurements on the descending look angle of TerraSAR-X.

39 3.2 Surface Deformation Modeling

We used constraints from our seismic velocity model, which includes the NE-SW

and N-S faults and a low velocity zone, to model the observed deformation. We used the

Coulomb 3 software (U.S. Geological Survey) to set up models and calculate the surface

displacement in a half space using various sources of deformation.

We start our deformation modeling by assuming that there is an isotropic volume

source buried at a depth of d where the low velocity zone is located. While there is generally a trade off between deformation source depth and the volume of the source for generating a certain deformation, we remove this ambiguity by constraining the main

source of deformation within the low velocity zone identified by the seismic inversion at the depth of 6 km. For our deformation modeling, we place an inflation source at the

location of x=l km, y=0 and z=6 km (fig.3-5al, bl) and we choose a reference point (6,

8) km on the surface (Michalczewska, 2012), where the deformation is set to be zero.

When we use a volume change of 30 x 106 iM3 , the surface displacement projected on the

LOS vector (fig.3-5cl) can be compared with the observed result plotted in figure 3-2.

The largest uplift and subsidence in our simulation are around 35 mm and -15 mm respectively. While the largest uplift and subsidence obtained from InSAR are 27 mm and -13 mm respectively, which are in approximate agreement with our model.

Comparison of Figure 3-5cl with Figure 3-2 shows that the deformation predicted using a volume source centered at a depth of 6 km fits the observed pattern of InSAR deformation rather well. However, detailed comparison shows that the northwestern part of the observed deformation extends further to the northwest (dashed black line in fig.3-

2) and this extension is not captured by only using the inflation source. To further

40 investigate this northwest extension of the deformation, we add a dip-slip fault at the

0 depth from 3 km to 6 km, dip angle of ~ 4 'and the length of - 10 km, as delineated from our seismic event relocations (fig.2-5). Input parameters of the fault in our model are set to be 400 of the dip angle, (-4, -4) km and (4, 4) km for the starting and ending points projected onto the surface and 3 and 6 km of the starting and ending points of depth (fig.3-5a2, b2). After testing the normal and thrust faults, we find that a normal fault with slip value of 0.09 m can explain the dislocation of the northwest part from the main area (fig.3-5c2). Using equation MO = MDA (y is rigidity, D is displacement and A is the area of the fault), we obtainM0 = 1.5x 1024 dyne - cm, and M" = 5.4 (M, =

Li cm( )- 10.7 (Hanks and Kanamori, 1979)) from the above input 3 kidyne-cmi nu parameters.

However, compared with the deformation obtained from InSAR, the simulated surface deformation of the northwestern part in the northeast direction is smaller than the observed. To address this discrepancy, our next test is including the strike-slip faults we observed in our relocation results. To simplify our model, we included one strike-slip fault at depth from 0.5 to 3 km and length of -8 km delineated from our seismic relocation results (fig.2-5). Therefore some input parameters of the fault in our model are set to be (0, -4) km and (0.1, 4) km of the starting and ending points projected on the surface and 0.5 and 3 km of the starting and ending points of depth (fig.3-5a3, b3). After testing different parameters of this strike-slip fault, we find that a right lateral strike-slip fault with slip value of 0.09 m can well explain the northeast dislocation of the northwestern part from the main area. Using the input parameters, we obtain MO =

5.8x 1023 dyne - cm, and M, = 5.1. After adding motion along both the normal and

41 strike-slip faults onto the inflation model, the modeled result matches well with both the major as well as the secondary deformation in the InSAR observation (fig.3-5c3). The largest values of the uplift and subsidence on both the simulated deformation are about 27 mm and -13 mm, respectively, which is in reasonable agreement with the observation.

The total moment release in our geodetic model is about 2.08x 10 2 4 dyne -cm and the total recorded seismic moment release estimated using equation

MO= 1 0 ( +10.7) (Hanks and Kanamori, 1979) for the recorded moment magnitudes by

SIL is about 2.3x1022 dyne - cm (fig.3-6). This discrepancy might be caused by the aseismicity in this region.

We successfully modeled the observed InSAR deformation by considering the low velocity zone as an expanding source and incorporating the normal and right lateral strike-slip faults (fig.3-7). This expanding source will cause local pressure change in this region. Thus the formation of the earthquakes above and around this expanding source might be related to this local pressure change caused by the expanding source.

An increased pressure of an expanding source could lead to a local stress field above the source whose maximum stress is vertical and compressive and horizontal stresses are extensional. We consider this stress field caused by the expanding source as local stress. Such a stress field would favor the formation of normal faults.

The combination of the left-lateral and expansion tectonic stress field of SW

Iceland is considered as regional stress. In such stress field, right-lateral strike-slip faults as well as normal faults can easily occur. According to the previous seismic and geological observation, right-lateral strike-slip faults already existed in this area (Clifton and Kattenhorn, 2006; Einarsson, 1991).

42 IIS c1 mm 10 al bi 30

01 .5 1 0 .5 10

0 -10 .20 Y (km) 10 -15 -10 0 *10 10 0 X (Wm) X (Smn) 10 X(km)

15 nm b2 c2 15 10 a2

/ 10 / - /o

E-io.

*10 1 0 10 Y (km) -is- -10 0 to -10 X (km) X (kml) 10 X(km)

b3 c3 1! 1a3 10 2t 10 5.10 10 D1 N -15 -10 -20 0 -1( Y (km) -10 .10 0 10 x100 -10 I 0 10 (m) X (SM) 0X(km) Figure 3-5: Models and results simulated as the InSAR observation. Boxes al-3 are map views of the deformation source geometry, and the red dot is the reference point where deformation is taken to be 0. The numbers indicate the sources, 1 (black dot) for inflation source, 2 for normal fault and 3 for strike-slip fault. The green lines indicate the strike directions of the faults and the red box represents the fault plane for the normal fault. Plots bl-3 are the 3D views of the corresponding map views. The cl-3 are the calculated displacement results projected on the LOS of TerraSAR-X (descending). The silver boxes indicate the corresponding area of the InSAR observation.

2.5x 1022

E. 1.5[ C: 1

0.5[

100 200 300 400 500 days Figure 3-6: Seismic moment release starting from 1st July 2010 to 31st December 2011. The biggest jump occurred on 27th February 2011.

43 The NE-SW trending dip faults delineated by seismic relocation, interpreted as normal faults in the surface deformation modeling, and the N-S trending strike-slip faults, interpreted as right lateral strike-slip fault, are consistent with the combination of the regional and local stress. Therefore, the formation of the NE-SW trending normal faults and the N-S trending right lateral strike-slip faults could be explained as two possibilities:

1). The faults are pre-existing faults, and slip is triggered by a combination of the regional and local stress field. 2). The combination of the regional and local stress field generated these faults. A more detailed analysis of the change of strain and stress

(including both the plate spreading and local expanding) in Krfsuvik geothermal field is needed to further confirm the cause of the seismicity.

mn 0

6400 64.00 20

0 10

635 0 to 10 -22.26 -22.05 -21.85 Longitute (degree) Figure 3-7: Comparison between the observed InSAR deformation and the simulated deformation. The location of the expanding source mapped on the right picture is (63.91 0N, 22.050W).

44 Chapter 4 Discussion

4.1 Brittle-ductile Transition

From the 3D view of the low Vp zone and distribution of the earthquake events, we observe that all of the earthquakes have occurred right above or around this low velocity zone (fig.4-1). The occurrence of the earthquakes indicated the local materials to be brittle. The lack of earthquakes in this low velocity zone probably reveals relatively more ductile materials at the depth of about 6 km. According to the distribution of earthquakes we plot a brittle-ductile transition (BDT) boundary in our study area, above which 99% of the earthquakes occurred (fig. 4-1). The depth of BDT boundaries varies in a range of

4-6 km from west to east. The transition from brittle to ductile deformation is most strongly dependent on lithology and temperature. The depth of the BDT can be considered as a first order isotherm (Foulger, 1995). On the surface of our study area,

50 0C temperature at the depth of ~12 cm has been measured and high temperature recorded in well KR-2 in Krfsuvik geothermal field is 250 0C at the depth of -1.2 km

(Mawejje, 2007). The non-glassy basalts may deform in the brittle field up to 5500 C +

100 0 C and in ductile behavior between 8000C and 950 0 C and the state in between is semi-brittle (Violay et al., 2012). Based on the above temperatures and the depths of

BDT and ductile behavior, we can estimate the geothermal profile in this area (fig.4-2).

The geothermal gradient is about 2000C/km between 0 and 1 km. While at the depths deeper than 1 km, based on the different BDT boundaries, three geothermal profiles are

45 determined. To the east area of the earthquake swarm, next to Lake Kleifarvatn, the depth of the BDT is 6 km where the temperature is around 5500 C. Moving to the west, further away from Lake Kleifarvatn, the depth of the BDT, where the temperature is

5500 C, becomes shallower, 5 to 4 km. Below BDT, the materials are semi-brittle or ductile. Due to lack of water convection, the thermal gradient would be higher than that from 1 km to BDT boundary (dashed line in fig.4-2). The closer to Lake Kleifarvatn, the smaller of the thermal gradient is (fig.4-2). This observation implies stronger convection near Lake Kleifarvatn, which can be explained by the high fracture density and high permeability near Lake Kleifarvatn as discussed in previous literatures (Clifton et al.,

2003; Geoffroy and Dorbath, 2008). The rocks with higher permeability and stronger convection usually have lower Vp/Vs raio, which is consistent with our tomography result near Lake Kleifarvatn at the depth of 2-4 km (fig.2-13).

46 63.96

-K

63.91 . ..

4-0'.

km 63.86 0

..4 6 8 -22.19 -22.03 -21.87 Longitute (degree)

2 E

8 0 200 400 600 800 Temperature (degree Celsius)

Figure 4-2: Temperature profiles. Red, green and blue lines indicate temperature of 550 0C at 4, 5 and 6 kin, respectively.

47 4.2 Possibilities of the low velocity zone

Seismic velocity varies depending on several physical conditions, such as composition, saturation condition, temperature, and pressure. Composition might play the fundamental role among the others. Meanwhile, changes of status of the rocks, such as solid versus partial melting, would also influence the velocities. The seismic velocity

anomalies caused by pressure and temperature variations at the same depth are

considerably smaller compared with the effect of saturation of fluid or melting conditions

of inclusions in the rock matrix at the same depth (Nakajima et al., 2001). Through the thermal expansion study (AT = A, a: thermal volume-expansion coefficient, AV, AT:

volume and temperature change, V: Volume), we find that the temperature change in our

low velocity zone is 45-1000 C given the volume change of 30 x 106 m 3 from our

surface deformation, the volume of the low velocity zone of 20 x10 m 3 from our

seismic tomography result and a of 15 - 33x 10-6 /C in most rocks. Then we obtain

the change of the Vp of 0.025-0.057 km/s using the relation of AVp/AT -0.57x

10-3 km/(s'C) (Christensen, 1979). This result is significantly smaller than ~1 km/s

obtained in seismic tomography. Therefore the change of the velocity might be caused by

the effect of saturation of fluid or partial melting in rocks. Here we propose 2 possibilities

for the low Vp in our study: 1) partial melting; 2) supercritical water.

1). Partial melting: Based on the low Vp, Vs and possible high Vp/Vs in our

tomography result, partial melting could be a reasonable explanation, considering the

H2 0-rich condition in our study area. For basaltic rock, partial melting usually happened

at temperature -1200 0 C. However, when mixed with water, the partial melting

temperature would decrease (fig.4-3) (Grove et al., 2006; Liu et al., 1996). If the

48 temperature reaches -800'C at depth ~ 6 km (fig.4-2) and mixes with 1-2 wt% (volume,

3-6%) water, basalt melting could occur. Given the volume of low velocity anomaly

-20 km 3 obtained from the tomography, the volume of water required would be

0.6-1.2 km 3 . After the big earthquake in 17th June 2000, 12% of the volume

(-0.1 km 3 ) of Lake Kleifarvatn disappeared (Clifton et al., 2003). The required 0.6 km 3 of water injection is in reasonable range of the water loss if considering the contribution of Lake Kleifarvatn, ground water and precipitation. According to our possible Vp/Vs ratio of -1.89 (black star in figure 4-4), the volume of the melt would be bigger than

10%. When basaltic rocks are subjected to secondary processes, such as magma differentiation and remelting, rhyolite will be generated. Thus we would predict some volume of silicic rocks exposed in Krf'suvik if rhyolite flowed out on the surface. This kind of rhyolites generated by remelting of basalt has been observed in Iceland by several geologists and geochemists (Gunnarsson et al., 1998; Jonasson, 1994). At the same time, from figure 2-14, a tail of low Vp is observed in deeper (> 7 km) and located in the east of the ~ 6km low velocity zone. If this low velocity tail represents the trace of magma intrusion, then partial melting at the depth of ~ 6 km would be further surpported. If the crustal magma were derived from some deeper magma chamber, such as in shallower mantle (Thordarson and Larsen, 2007), there would be low Vp, Vs and high Vp/Vs ratio zones gradually distributed in deep structure. However, due to the limited depth of earthquakes in this study, our tomography has no resolution when the depth is deeper than 8 km.

Magma intrusion is a popular explanation for the observed uplift and expansion deformation by the geodetic measurements. Seismicity and deformation triggered by

49 magma intrusion has been observed in several volcanic systems in Iceland. In Krafla

volcano in north Iceland, the 1974 - 1975 seismic activity accompanied the Krafla rifting

when multiple eruptions and dike injections occurred. The 1994-1995 seismicity and

deformation at the Hengill triple junction, Iceland, is considered to have been triggered

by minor magma injection in a zone of horizontal shear stress (Sigmundsson et al., 1997).

While in Krfsuvik region, the continue recording of GPS and InSAR has been observing

periodic inflation and deflation on the surface (Keiding et al., 2008; Michalczewska et al.,

2012). The intrusion of the magma would cause the inflation of the surface and the

increased pressure would cause the change of the local stress that could trigger

earthquakes when combine with the regional left lateral and expansion stress field. This

continued earthquakes could be observed at the historical seismic recording (fig.1-3,4).

These earthquakes could increase the local permeability and the ground water will reach

as deep as about 6 km. When the water accumulates to be some certain amount, partial

melting at shallow depth (-6 km) occurs, which would further increase the local stress, trigger big seismic swarms and cause significant surface uplift and expansion. The

phenomenon of big seismic swarms inserting in long-live earthquakes has been observed

and recorded (fig. 1,3). During the big earthquake swarm, the water of Lake Kleifarvatn would be injected into underground (Michalczewska, 2012). Due to the circulation of this

cold water (fig. 4-2), partial melting might be cooling down and then surface deflation is

observed. Change of the stress state and precipitation of minerals might make fractures become to seal and then inflation caused by magma intrusion happens, which indicates the beginning of another circulation of inflation and deflation. We speculate that the intrusion of the magma would respond for the long period seismicity and surface uplift

50 and expansion, while the partial melting at shallow depth may be the cause of the big earthquake swarms and the circulation of the cool water is possible the reason for the short period deflation.

4 / I I P&Is B 02 pwwiotmt Sedhnent wet 3 thS study N et al. '94 0- 2 0 L&W 72 4 Loet al. 1 ,98 F\ Basat !! 0.2 - (~6kn ) 600 800 1000 1200 1400 T OC

1.8O.

0,6.0------

- 4F. Irk. 6 70 1 Vp (km/i) volume fraction of fluids (%)

51 2). Supercritical water: The substance that is above its critical temperature and

pressure is defined as a supercritical fluid. The supercritical fluids have a combination of

gas-like mass transfer and liquid-like solvating characteristics, which are considered as

highly compressible. The critical point of pure water occurs at 22.1 MPa and 374 0C from

the laboratory experiments that present the physical properties of supercritical fluids

(Palmer et al., 2004). From the thermal profile in figure 4-2, we could find that the fluids

or ground waters at the depth of about 6 km are probably in a supercritical state with high pressures. However, Vs is more sensitive to rock matrix but less sensitive to the free gas to water ratio. Therefore, we should expect low Vp/Vs ratios in a supercritical case, which is inconsistent with our inversion results. Unfortunately, due to the lack of deep earthquakes, the resolvability of the Vp/Vs ratio at 6 km depth is far from ideal. At the

same time, we have relatively well resolved Vp and Vs and consider the uncertainties are in reasonable range, if we calculate the Vp/Vs ratio by directly dividing Vs by Vp from the tomography result, then the Vp/Vs value at where the low Vp appears would be 1.74, which is actually a relatively lower than the average value (1.8). Therefore, the possibility of supercritical water cannot be ruled out based on currently available data. If we benchmark this Vp/Vs of 1.74 to figure 4-4, as shown by the gray star, we find that the volume fraction of water would be about 1-2%. Given the volume of low velocity anomaly -20 km 3 obtained from the tomography, the volume of water included would

3 be 0.2-0.4 km . As mentioned above, after the big earthquake in 17 th June 2000, 12% of the volume (~0.1 kM 3 ) of Lake Kleifarvatn disappeared (Clifton et al., 2003). The required 0.2 km 3 of water injection is on the same order of the water loss if considering

52 the contribution of Lake Kleifarvatn, ground water and precipitation.

Such supercritical fluid could possibly be responsible for the dilatation of this area and increase the potential of generating or triggering faults as we discussed above. The formation of the supercritical reservoir from water would increase the local pressure and cause slightly surface uplift and expansion. When some deeper earthquakes occur, the dramatic increase of permeability of deeper structure could lead to recharge of the supercritical reservoir. The mixing of supercritical fluid with downward colder water would lead to phase separation and formation of a high dynamic superheated vapor phase, which would convey heat to the upper crust and cause dilatation and seismic events. The releasing of vapor will make pore pressures of the reservoir suddenly go down and then cause surface deflation. After sealing of fractures above the reservoir, another supercritical reservoir begins to format. In this supercritical case, we speculate that the formation of the supercritical reservoir are responsible for the long period seismicity and surface uplift and expansion, while the phase change of the supercritical fluid may be the cause of the big earthquake swarms and the afterward short period deflation.

The understanding of this low velocity zone is of great important for developing potential geothermal heat source, mitigating risk of magma intrusion or understanding the activity of the Mid-Atlantic Ridge. Detailed analysis of the energy distribution and attenuation of S waves at deeper part in our study area would provide more properties of this low velocity zone. Incorporation of deeper earthquakes would not only provide better constraint of Vp/Vs ratio to distinguish whether this low velocity zone is partial melting or supercritical water, but also provide velocity information in deeper structure, which would help us to trace whether there is magma intrusion or not.

53 Chapter 5 Conclusion

The Krfsuvik geothermal field is considered as one of the most active geothermal

fields in SW Iceland. Large earthquake swarms have been recorded from Nov. 2010 -

Feb. 2011 in the Krfsuvik area. Using the seismicity recorded by 23 seismic stations

deployed by SIL, UU, RU and MIT, we delineated shallow N-S trending strike-slip faults

and a relatively deeper NE-trending dip fault through earthquake relocations. We also

detected a low velocity zone around 6 km under the earthquake swarm, which is in the

similar location with a low resistivity zone detected by previous 3D MagnetoTelluric

(MT) survey(Hersir et al., 2011). Resolvability tests were performed to validate the tomography results. Based on the distribution of the earthquakes, we delineated the brittle-ductile boundary in Krysuvik geothermal field.

From the InSAR data, we observed the expansion and uplift from 2009 to 2012 in this area. Such deformation might indicate more complex geodynamic behavior than

simple plate-motion strain accumulation. Using our seismic model as a constraint, we

considered both point source volume change and faults as possible sources of the deformation. A point source at the depth of 6 kin, supported by the seismic tomography, well modeled the main deformation, which further supported the low velocity zone. The

NE-SW trending dip fault, interpreted as a normal fault, and the N-S trending fault,

interpreted as right-lateral strike slip fault, well simulated the secondary order deformation.

54 The potential geological explanations of supercritical water, H2 0-rich partial melting with magma intrusion are discussed for the detected low velocity zone.

Further studies with deeper earthquakes are needed to obtain high fidelity Vp/Vs ratios for better understanding the cause of the low velocity zone. The results of this thesis provided new insights to understand the seismicity and surface deformation in volcanic zones as well as provided important information in exploration of new geothermal areas.

55 ......

Appendix A Geothermal Manifestation in Krysuvik

Geology mapping area

Iy Hot Springs, Mud Pots f f

[ Mud Pot

(Mawejje, Sulphur needles 2007)

56 Appendix B GPS measurements in Krysuvik

LI4dA)a*,nt 21 MR1 , kb t 1

44)

24) 1' r z 1) - 2)) - I --

-4" 2~' .2RIlop

7(917 NON 2409 ZO 10 1 :M2 2013 i

40 410 :(Krziffi _I N 211 A-111 awl

-21,

40 201i7 2(11111 201$) 24410 24)11 2*112 2)MI :IwM$

'41) I iL I 41) 2w'

240 i 60 I, i 2 1 11

2017 20WK 204 2010 2011 2112 201

(http://strokkur.raunvis.hi.is/-sigrun/KRIV.html).

57 ~4kt( 1)'~ N 14~4n~ ~~nddr'

410

-. ~ . 2 . .~ .4 ....

20-17 2"11 2012h 'II o ILt

-40

40) ALI

E E

~~ ~ ~2010 2011 2huI2 2x7 ~ 2~' 2o zmi yo: (http://strokkur.raunvis.hi.is/~sigrun/KRIV.html).

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