INSAR AND GPS MEASUREMENTS OF THE 29 MAY 2008 SOUTH ICELANDEARTHQUAKE SEQUENCE

J. Decriem1, T. Arnadottir 1, A. Hooper2, H. Geirsson3, F. Sigmundsson1, M. Keiding1, B. G. Ofeigsson1, S.Hreinsdottir 4, P. Einarsson1, P. LaFemina5, and R. A. Bennett4

1Nordic Volcanological Center,Institute of Earth Sciences,University of Iceland,Reykjavık, Iceland2Dept. of Earth Observation and Space Systems, Delft University of Technology, Delft, Netherlands

3Physics Department, Icelandic Meteorological Office,Reykjavık, Iceland4Dept. of Geosciences, University of Arizona, Tucson, Arizona, USA

5Dept. of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA

ABSTRACT

The South Iceland Seismic Zone (SISZ) is an≃ 80 km1

long E-W transform zone accommodating the relative2

motion of the North American and Eurasian plates across3

southern Iceland. The accumulated stress due to the plate4

motion is released by earthquakes in the zone. The SISZ5

does not rupture along its whole length, but on many6

parallel N-S right-lateral strike slip faults. The largest7

earthquakes are therefore limited to moderate magnitudes8

(M∼6-7), and often occur in sequences of similar size9

events, at times propagating from East to West. After10

a quiet period of 88 years two Mw=6.5 events struck11

the eastern and central part of the SISZ in June 2000.12

The main shocks ruptured two parallel N-S faults, spaced13

about 17 km apart, occurring 3 1/2 days apart [1, 2] . The14

sequence continued on May 29, 2008 when two Mw615

events occurred in the western part of the SISZ, ruptur-16

ing two parallel N-S faults located about 5 km from each17

other. The small time delay (∼3 sec) between the two18

events suggests that the western one was triggered by the19

initial event [3]. Here, we present a geodetic study of20

the May 2008 earthquakes based on continuous and an-21

nual GPS measurements and InSAR images. We propose22

a dislocation model for the fault geometry, location and23

the slip distribution over the fault planes that best fit the24

geodetic data. [4].25

Key words: Satellite geodesy; Earthquake source obser-26

vations; Seismicity and tectonics; Space geodetic sur-27

veys.28

1. INTRODUCTION

Earthquake activity in south Iceland has been recorded by29

the SIL seismic network [5] operated by the IMO, since30

1989. The SISZ is characterized by arrays of N–S right31

lateral strike slip faults, spaced 2–5 km apart [6, 7]. Each32

−23˚ −22.5˚ −22˚ −21.5˚ −21˚ −20.5˚ −20˚ −19.5˚

63.75˚

64˚

64.25˚

64.5˚

29/05/2008

17/06/2000

21/06/2000

He

SISZ

WVZ

RPEVZ

0 10 20

km

NA

EU0.97cm/yr

0.97cm/yr

.

.

Figure 1. Tectonic features SW Iceland: The ReykjanesPeninsula (RP), the Hengill triple junction (He), the West-ern and Eastern Volcanic zone (WVZ/EVZ) and the SouthIceland Seismic Zone (SISZ). The volcanic systems areoutlined in yellow with the fissure swarms shown as darkshaded areas. Mapped surface faults of Holocene earth-quakes are shown with pink lines. The epicenters ofthe June 2000 and May 2008 main shocks are indicatedwith black and white stars, respectively, and focal mecha-nisms (USGS). Aftershocks recorded between June 2 andJuly 10, 2008 are represented with white dots. The lo-cations of GPS benchmarks are shown with diamonds,green for continuous sites and purple for stations mea-sured after the May 29 main shocks. The inset shows theplate boundary across Iceland with the study area out-lined with a white rectangle. The red arrows show theplate spreading between the North American (NA) andEurasian (EU) plates according to the NUVEL-1A platemotion model [9].

_____________________________________________________ Proc. ‘Fringe 2009 Workshop’, Frascati, Italy, 30 November – 4 December 2009 (ESA SP-677, March 2010)

of the N–S faults is composed of an array of left step-33

ping en echelonsurface fractures, separated by push-up34

structures [e.g., 7, 8].35

We study the surface deformation caused by the 29 May36

earthquake doublet. The first mainshock was located37

beneath the Ingolfsfjall mountain (63.972◦N, 21.072◦W38

and∼5 km depth) at 15:45:58.9 UTC according to the39

SIL seismic catalog (IMO). Within seconds another rup-40

ture occurs on the Kross fault, located approximately 541

km west of the Ingolfsfjall event. Co-seismic offsets up to42

200mm were measured by the CGPS network in the area43

[3]. We use Interferometric Synthetic Aperture Radar (In-44

SAR) images as well as continuous and campaign GPS45

measurements to constraint the fault location as well as46

the distribution of the slip over the fault plane.47

2. DATA ANALYSIS

Several radar images acquired by ENVISAT and ALOS48

satellites span the time of the May 2008 earthquakes both49

along ascending and descending tracks. The ASAR in-50

strument onboard the ENVISAT satellite operates in C-51

band with a wavelength of 56.2 mm, whereas the PAL-52

SAR instrument onboard the ALOS satellite operates in53

L-band with a wavelength of 236.0 mm. The SISZ is54

one of the most extensive agricultural area in Iceland, it55

implies a general decrease of the coherence due to the56

vegetation compared to the rest of the country. Winter57

snow is often causing decorrelation in C-band interfer-58

ograms between October and April. In the other hand,59

dry snowpacks are optically thin at L-band. The PAL-60

SAR images gives invaluable interferometric information61

in Iceland during the winter time. Here we use images62

from 2007 to 2008 for three ENVISAT tracks, two de-63

scending(T138, T367) and one ascending (T402) in addi-64

tion to an ascending ALOS track (T20) (Figure 2).65

We use the Stanford Method for Persistent Scatterer66

(StaMPS) to generate interferogram time series. The un-67

wrapped phase ( Figure 3) is calculated with a statistical68

cost flow algorithm as described in Hooper et al. [10].69

The StaMPS method accounts for several error terms in-70

cluding uncorrelated noise, spatially correlated look an-71

gle errors (almost entirely due to DEM errors), and atmo-72

spheric and orbit errors due to the master image [11]. We73

statistically estimate the remaining empirical covariance74

function for each interferogram, following [12].75

3. MODELLING

We use the software PSCMP/PSGRN [13] to calculate76

the surface displacements and the stresses due to slip on77

rectangular dislocations in a layered half-space. We first78

estimate the dislocation geometry and location assuming79

constant slip using non-linear optimization and then use80

a regularized linear inversion algorithm to solve for vari-81

Figure 2. Wrapped phase interferograms spanning the29 May 2008 earthquakes. Each color cycle represents28 mm of motion in the LOS between the ground and theENVISAT satellite and 120 mm for the ALOS. The yellowarrows show the surface projection of the mean LOS unitvectors.

-21.4˚ -21.2˚ -21˚ -20.8˚63.8˚

63.9˚

64˚

64.1˚

ENVISAT T138 / Descending / 2008/05/01 - 2008/06/05

-0.1 0.0 0.1LOS Displacement (m)

0 5

km

-21.4˚ -21.2˚ -21˚ -20.8˚63.8˚

63.9˚

64˚

64.1˚

ENVISAT T367 / Descending / 2007/07/07 - 2008/06/21

-0.1 0.0 0.1LOS Displacement (m)

0 5

km

-21.4˚ -21.2˚ -21˚ -20.8˚63.8˚

63.9˚

64˚

64.1˚

ENVISAT T402 / Ascending / 2008/05/19 - 2008/06/23

-0.1 0.0 0.1LOS Displacement (m)

0 5

km

-21.4˚ -21.2˚ -21˚ -20.8˚63.8˚

63.9˚

64˚

64.1˚

ALOS T20 / Ascending / 2007/07/28 - 2008/09/14

-0.1 0.0 0.1LOS Displacement (m)

0 5

km

Figure 3. Unwrapped InSAR interferograms. The LOSdisplacements during the time interval given on each in-terferogram are shown by color, where blue indicates anincrease in the LOS (motion away from the satellite) andred represents a LOS decrease (motion toward the satel-lite). The colored dots give the GPS station displace-ments projected onto the unit vector of the LOS for eachinterferogram.

−21.5˚ −21˚ −20.5˚

63.8˚

64˚

64.2˚

GIVA

FRAM

LAMY

KALT

TBRU

BRJA

265S

KIDJ

GORA

SELH

ONDV

BURF

HAAL ISMA

LJOS

SELF KALD

GLJU

AUDS

KATL

HVER

NV16

VG22

OLKE

MOLD

VG20

KAMB

SAND

HV08

VG24

HH04

HLID

DRAU

THON

VG15 VG13

THRE

KAFF

GEIT

SEHE

BLAF

VOGS

100 ± 2mm

−21.5˚ −21˚ −20.5˚

63.8˚

64˚

64.2˚

0 10 20

km

Figure 4. Co-seismic deformation in theOlfus area fol-lowing the 29 May 2008 earthquake sequence. The ob-served horizontal GPS displacements are shown withblue arrows and 95% confidence ellipses, whereas thered arrows are the predicted displacements for the pre-ferred uniform slip model (thick gray lines). The whitestar indicates the epicenter of the first mainshock andblack crosses show locations of aftershocks recorded bythe SIL seismic network from 2 June to 10 July 2008.

able slip given the optimal dislocation location and ge-82

ometry as described in [15].83

Our best-fit uniform slip model for the Ingolfsfjall fault84

has 1.2 m of right-lateral strike slip on an 7.5 km long85

fault extending from 2 to 4 km. This suggests that most86

of the slip is concentrated below 2 km. For the Kross fault87

we estimate a 8 km long fault surface extending from 0.588

to about 5 km depth with 0.75 m of right-lateral strike89

slip. The fault locations and GPS are shown in Figure 4.90

We solve for the distribution of the slip over the pre-91

viously obtained fault geometry by dividing the rupture92

planes into several patches (0.5× 0.5 km from the sur-93

face to 22 km depth). We assume right lateral strike slip94

and impose a Laplacian smoothing operator choosen as95

the best compromise between the solution roughness and96

the misfit using cross-validation sum of squares [14].97

The resulting slip distributions are shown in Fig. 5. Most98

of the slip on the Kross fault is focused in an area that is99

approximately 5 km long and extends from 3 to 6 km100

depth with a maximum motion of 1.4 m. A second,101

smaller slip maximum is located further north, above a102

cluster of aftershocks. For the Ingolfsfjall event the slip103

is concentrated in a 4× 2 km area centered at approxi-104

mately 3 km depth, and decreases smoothly from a max-105

imum of 1.9 m to less than 0.5 m over a distance of 8 km106

toward north.107

The geodetic moment release of our model for the Kross108

fault is M0 = 8.65 × 1017 Nm corresponding to a seis-109

mic moment magnitude of Mw5.9. For the Ingolfsfjall110

fault we estimate a geodetic moment of5.97 × 1017 Nm111

63.88 63.9 63.92 63.94 63.96 63.98 64 64.02 64.04 64.06−10

−9

−8

−7

−6

−5

0.5

−3

−2

−1

0 m

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

63.88 63.9 63.92 63.94 63.96 63.98 64 64.02 64.04 64.06−10

−9

−8

−7

−6

−5

0.5

−3

−2

−1

0 m

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Figure 5. Variable slip model for the Ingolfsfjall fault(top) and the Kross fault (bottom). The amount of right-lateral strike slip is shown with the color scale (samefor both figures). The black circles denote aftershocksrecorded by the SIL network between 2 June and 10 July2008. The locations of the constant slip models are shownwith a gray outline.

(Mw5.8), giving a cumulative geodetic moment for the112

doublet of M0 = 1.46 × 1018 Nm and Mw6.1.113

4. CONCLUSIONS

The first rupture occurred along the Ingolfsfjall fault and114

our models indicate localized and shallow slip. The sec-115

ond event was triggered on the Kross fault.116

The depth extent of the ruptures are shallower than found117

in models for the June 2000 events, which is consistent118

with the brittle part of the crust thickening toward the east119

along the SISZ.120

The June 2000–2008 earthquake sequence has only re-121

leased about half of the moment accumulated by plate122

motion since the last major earthquake sequence in the123

SISZ (1896–1912) ([15]). It is therefore likely that the124

sequence will continue with moderate size earthquakes125

in the SISZ in the coming years.126

ACKNOWLEDGMENTS

The Envisat and Alos data were provided by the European127

Space Agency. We thank UNAVCO, the National Land128

Survey of Iceland, the Icelandic Road Authority, and129

Landsvirkjun for lending us GPS equipment and assist-130

ing with the GPS post-earthquake fieldwork in 2008. In-131

gvar Magnusson at ISOR provided GPS data from 2007.132

Pierre Dublanchet from ENS helped with the GPS mea-133

surements in 2008. We thank Bryndıs Brandsdottir for134

discussions and sharing of preliminary results from her135

seismic studies and Sigurjon Jonsson for the InSAR pro-136

cessing discussions. This work was funded by grants137

from the Icelandic Research Council, United State Na-138

tional Science Foundation (NSF grants number EAR-139

0711446 and EAR-0711456 ),the University of Arizona,140

Pennsylvania State University, and the University of Ice-141

land Research Fund.142

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