(Mw 6.8), Algeria, earthquake from joint inversion of interferometric synthetic aperture

16 Pages·2009·1.41 MB·English

JOURNAL OF GEOPHYSICAL RESEARCH, VOL.114, B03406,doi:10.1029/2008JB005912, 2009
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Rupture parameters of the 2003 Zemmouri (M 6.8), Algeria,
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earthquake from joint inversion of interferometric synthetic
aperture radar, coastal uplift, and GPS
Samir Belabbe`s,1 Charles Wicks,2 Ziyadin C¸akir,3 and Mustapha Meghraoui1
Received3July2008;revised6December2008;accepted12January2009;published20March2009.
[1] We study the surface deformation associated with the 21 May 2003 (Mw = 6.8)
Zemmouri (Algeria) earthquake, the strongest seismic event felt in the Algiers region
since 1716. The thrust earthquake mechanism and related surface deformation revealed
an average 0.50 m coastal uplift along (cid:1)55-km-long coastline. We obtain coseismic
interferograms using Envisat advanced synthetic aperture radar (ASAR) (IS2) and
RADARSATstandard beam (ST4) data from both the ascending and descending orbits of
Envisat satellite, whereas the RADARSAT data proved useful only in the descending
mode. While the two RADARSATinterferograms cover the earthquake area, Envisat data
cover only the western half of the rupture zone. Although the interferometric synthetic
aperture radar (InSAR) coherence in the epicenter area is poor, deformation fringes are
observed along the coast in different patches. In the Boumerdes area, the maximum
coseismic deformation is indicated by the high gradient of fringes visible in all
interferograms in agreement with field measurements (tape, differential GPS, leveling,
and GPS). To constrain the earthquake rupture parameters, we model the interferograms
and uplift measurements using elastic dislocations on triangular fault patches in an
elastic and homogeneous half-space. We invert the coseismic slip using first, a planar
surface and second, a curved fault, both constructed from triangular elements using
Poly3Dinv program that uses a damped least square minimization. The best fit of
InSAR, coastal uplift, and GPS data corresponds to a 65-km-long fault rupture dipping
40(cid:1) to 50(cid:1) SE, located at 8 to 13 km offshore with a change in strike west of Boumerdes
from N60(cid:1)–65(cid:1) to N95(cid:1)–105(cid:1). The inferred rupture geometry at depth correlates well
with the seismological results and may have critical implications for the seismic
hazard assessment of the Algiers region.
Citation: Belabbe`s,S.,C.Wicks,Z.C¸akir,andM.Meghraoui(2009),Ruptureparametersofthe2003Zemmouri(M 6.8),Algeria,
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earthquakefrom jointinversionof interferometric syntheticaperture radar, coastaluplift, andGPS, J.Geophys. Res.,114,B03406,
doi:10.1029/2008JB005912.
1. Introduction the Africa-Eurasia plate boundary (Figure 1a) [Meghraoui
et al., 1996; Nocquet and Calais, 2004; Serpelloni et al.,
[2] ThethrustandfoldsystemoftheTellAtlas(northern 2007]. Its coastal location hindered, however, the direct
Algeria) has been the site of several large and moderate
observation of surface ruptures and has left open questions
earthquakes in the last decades (Table 1). This shallow
on the probable fault scarp location, structural character-
seismic activity was very often associated with surface
isticsandgeometryofthecausativefault[Meghraouietal.,
faulting, the conspicuous example being the El Asnam
2004; De´verche`re et al., 2005].
thrustfaultingandrelated1980,M 7.3majorevent[Philip
w [3] The 2003 Zemmouri earthquake (also called some-
and Meghraoui, 1983; Yielding et al., 1989]. The thrust
timestheBoumerdesearthquake)affectedtheeasternedgeof
mechanism (global centroid moment tensor (CMT)) of the
the Mitidja Quaternary basin (Figure 1b), and was respon-
M 6.8, 2003 Zemmouri earthquake which occurred in the
w sible for severe damage (cid:1)20 km east of the capital city
Tell Atlas tectonic belt, confirms the pattern of active
Algiers [Ayadi et al., 2003; Harbi et al., 2007]. The NE-
deformation that predicts 4–6 mm/a of convergence along
SW striking fault is consistent with the Tell Atlas fold and
thrust tectonics, the uplifted 55-km-long coastal shoreline
1InstitutdePhysiqueduGlobedeStrasbourg,UMR7516,Strasbourg, [Meghraoui et al., 2004], the (cid:1)40(cid:1) SE dipping aftershocks
France. [Ayadi et al., 2008] and the inversion of body waves
2U.S.GeologicalSurvey,MenloPark,California,USA. [Delouis et al., 2004; Yagi, 2003]. A detailed bathymetric
3FacultyofMines,IstanbulTechnicalUniversity,Istanbul,Turkey.
survey and seismic profiles offshore describe the morpho-
logical and structural features in the Mediterranean Sea
Copyright2009bytheAmericanGeophysicalUnion.
0148-0227/09/2008JB005912$09.00 [Domzig et al., 2006]. A downdip model of uniform slip
B03406 1 of 16
B03406 BELABBE`SETAL.:INSAR OF THEZEMMOURI EARTHQUAKE B03406
Table 1. Large and Moderate Earthquakes With Thrust Mechan- 2008] and for the M 6.0, 1994 and M 6.4, 2004 Al
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ism Alongthe Tell Atlas Hoceima earthquakes whichrevealed theexistence of blind
Location Date Longitude(deg) Latitude(deg) M and conjugate seismic ruptures in the Rif Mountain belt
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[Cakir et al., 2006; Akoglu et al., 2006] (Figure 1a). The
Orle´ansville 9Sep1954 1.47 36.28 6.7
ElAsnam 10Oct1980 1.36 36.18 7.3 coastallocationoftheZemmouriearthquakeisnotanideal
Tipaza 29Oct1989 2.92 36.84 5.9 configuration for producing interferograms since most of
Mascara 18Aug1994 (cid:2)0.03 35.40 5.7 the earthquake deformation occurred offshore. Neverthe-
AinTemouchent 22Dec1999 (cid:2)1.45 35.34 5.7
less, the impressive coastal uplift and epicenter location
BeniOurtilane 10Nov2000 4.69 36.71 5.7
Zemmouri 21May2003 3.65 36.83 6.8 implythatsignificantsurfacedeformationtookplaceinland
as well and can be studied using SAR images.
[5] In this paper, we first present the seismotectonic
characteristics of the Zemmouri earthquake and provide a
on a rectangular dislocation deduced from direct measure- detailed study of interferograms obtained from RADAR-
ments of coastal uplift (using differential GPS (DGPS) and SAT and Envisat images. The InSAR data document the
conventional leveling [Meghraoui et al., 2004]) is consis- measurements of displacement field in the earthquake area
tent with the slip distribution at depth inferred from the in agreement with previous studies of coastal uplift. Mod-
inversion of seismic and geodetic data [Delouis et al., elingoftheInSARdatasettogetherwiththeupliftandGPS
2004]. These models of seismic source include surface slip measurements allow us to deduce the fault parameters and
and comply with the observed tsunami effects and related associated coseismic slip distribution. Finally, open ques-
tidegaugerecordsinthewesternMediterraneanSea[Alasset tions on the earthquake fault geometry and the importance
etal.,2006]. ofInSARanalysisarediscussedemphasizingtheconstraint
[4] The analysis of SAR images and related interfero- ofahiddenseismogenicthrustrupturewithcoastalgeodetic,
grams in the last decade has led to a significant progress in tectonic and seismologic data.
the understanding of earthquake deformation [Massonnet
and Feigl, 1998; Bu¨rgmann et al., 2000; Wright et al.,
2. Seismotectonic Setting
2004]. The phase interference of spaceborne radar images
hastheoutstandingadvantagetolocateanddisplaythefield [6] The Tell Atlas of northern Algeria experienced the
displacement of large sections of any earthquake area, largestrecordedearthquakeatElAsnam(10October1980,
providing that an appropriate coherence level exists be- M 7.3)relatedtoaNE-SWtrendingreversefault[Ouyedet
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tween SAR images. The generation of SAR interferograms al., 1981; Philip and Meghraoui, 1983]. Other recent large
isthereforeparticularlyimportantinthrustandfoldsystems and moderate seismic events of this region (Figures 1a and
with complex surface deformation (seismic and aseismic) 1b and Table 1), including the Zemmouri earthquake of
[Fielding et al., 2004] as it may lead to a better seismic 2003revealedacomparablepatternofactivedeformationin
hazard assessment. The application of interferometric syn- agreement with the NNW-SSE to NW-SE convergence and
thetic aperture radar (InSAR) in active zones of North transpressive tectonics along the Africa-Eurasia plate
Africa has been recently performed for the M 5.7, 1999 boundary[Meghraouietal.,1996;Stichetal.,2003].These
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Ain Temouchent blind thrust earthquake [Belabbes et al., seismogenic structures of the Tell Atlas are related to a
Figure1a. ShadedreliefmapofeasternMediterraneanwithfocalmechanismsolutionsofearthquakes
between 1980 and 2005 (data from Global CMT catalog). Gray mechanism corresponds to the 2003
Zemmouriearthquake.ThegraylinerepresentstheplateboundaryaccordingtoMeghraouietal.[1996].
Black arrows are direction of convergence, and the number is plate velocity in mm/a [Nocquet and
Calais, 2004]. The squared area is for Figure 1b.
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B03406 BELABBE`SETAL.:INSAR OF THEZEMMOURI EARTHQUAKE B03406
Figure 1b. Envisat/RADARSAT radar frames (dashed rectangles) with arrows indicating the satellite
flightdirectionforascendinganddescendingorbitsofthe21May2003Zemmouriearthquakearea.Red,
black, and blue arrows indicate flight direction for satellites. The red star indicates the earthquake
epicenterlocation[Bounifetal.,2004],andfocalmechanismsarefromtheGlobalCMTsolutionbetween
1980 and 2003 (size of beach balls is according to magnitude and red mechanism is for the Zemmouri
earthquake).Blacklinesindicateactivethrustfaults,anddashedarrowedlinesindicateactivefoldingand
west of the 2003 epicenter TF is for Thenia Fault [Meghraoui, 1988].
complex‘‘enechelon’’thrustandfoldsystemlocatedalong rupturewith(cid:1)1msurfaceslip,and2.1mmaximumslipat
an E-W trending narrow strip parallel to the coastline. The depth with bilateral rupture propagation along two patches
lateQuaternaryactivedeformation thatindicates arateof2 (Table 2).Yagi [2003] also constructed source models from
to 3 mm/a. for compressive movements estimated from the body wave inversion and obtained similar results pointing
shortening of folded units and paleoseismic investigations out the bilateral rupture propagation on a 60-km-long fault
[Meghraoui and Doumaz, 1996], is comparable to the 4 to with2patchesand2.3m maximum slipatdepth(Table 2).
6 mm/a. convergence rate obtained from NUVEL-1A and Theinversion ofgeodeticdata(coastalupliftandGPS)and
global GPS solutions along the plate boundary [Nocquet accelerogramsalsopointsoutthetwopatcheswithupto1m
and Calais, 2004; Serpelloni et al., 2007]. surface slip SWof the earthquake rupture [Semmane et al.,
[7] The 2003 Zemmouri earthquake affected (cid:1)55-km- 2005]. According to the fault geometry, seismic moment,
long coastline and (cid:1)15-km-thick uppermost crustal struc- main shock location at depth and field observations, the
turethatbelongstotheeasternregionsoftheMitidja Basin earthquake fault did not rupture the surface along the
andrelatedBlidathrustandfoldsystem[Ayadietal.,2008]. coastline but likely offshore at the seafloor. Detailed field
Adetailedstudyofthemainshockrelocation[Bounifetal., investigations made immediately after the main shock indi-
2004] and aftershock distribution (that covers 2 months) cate,however,theexistenceof2-to3-km-longN95(cid:1)–100(cid:1)
using the tomography analysis reveals a NE-SW trending trending surface cracks observed along the Thenia fault
and 40(cid:1)SE dipping fault zone with two seismicity patches (Figure 1b).
[Ayadietal.,2008]. The doubledifferenceseismic analysis [8] Coseismic uplift of marine terraces along the 55-km-
shows a large concentration of aftershocks and depth long coastline (Figure 3) combined with GPS and conven-
distribution SW of the fault rupture and indicates a SE tional leveling indicate an average of 0.55 m vertical
dipping thrust geometry in agreement with the centroid movement with two slip patches from which a SE dipping
moment tensor (CMT) solution (Figure 2 and Table 2) planar dislocation model is resolved with 2.8 (cid:3) 1019 N m
[Ayadi et al., 2008]. Using an analysis of body waves and geodeticmoment[Meghraouietal.,2004].Althoughbased
surface waves of teleseismic records, Delouis et al. [2004] on a 60-km-long and 15-km-wide simple rectangular dislo-
found 2.86 (cid:3) 1019 N m seismic moment and a (cid:1)15 s fault cation, the model suggests a seismogenic thrust rupture
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Figure 2. The 21 May 2003 Zemmouri earthquake (black star, M 6.8) and aftershock distribution
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(whitecircles) ofseismiceventsfrom25Mayto31July2003[Ayadietal.,2008].Themainshockand
mainaftershocks(blackcircles)arerelocatedusingthedoubledifferenceseismicanalysisofBounifetal.
[2004]. Focal mechanism solutions are Global CMT.
dipping 50(cid:1) SE consistent with the seismological results 25(cid:1) SE dipping rupture (Table 2) that suggests a fault
(Table 2). The coastal deformation and rupture history surface trace at 15 to 20 km offshore limited to the SW
associatedwiththeaftershockdistribution(Figure2),show- by a NNE trending transform fault and forms a step over
ingaclearconcentrationofseismiceventsontheSWpatch with the Blida thrust system [Braunmiller and Bernardi,
suggest, however, a complex fault rupture likely made of 2005].
several asperities along strike. Early results from geodetic
measurements (coastal uplift and GPS) and seismicity
3. The InSAR Analysis
analysis [Bounif et al., 2004; Meghraoui et al., 2004;
Delouis et al., 2004; Yelles et al., 2004; Semmane et al., [9] We have calculated four coseismic interferograms,
2005; Alasset et al., 2006] imply the existence of a N54(cid:1)– oneascendingandthreedescending,usingsyntheticaperture
N70(cid:1) striking and 40(cid:1) to 50(cid:1) SE dipping fault plane with radar(SAR)dataacquiredbytheCanadianSpaceAgency’s
surfaceslipat5to15kmoffshore.Fromthehigh-resolution RADARSATsatelliteandEuropeanSpaceAgency’sEnvisat
swath bathymetry and seismic profiles De´verche`re et al. satellite (Table 3). The raw data were processed using the
[2005] study the seafloor of the continental slope offshore commercialGAMMAsoftwarewith5azimuth1rangelook
the 2003 earthquake area, identify outcropping thrust fault (i.e.,averagedto20(cid:3)20mofgroundpixelsize)andfiltered
andscarpslocatedat(cid:1)20and32kmfromtheshorelineand usingweightedpowerspectrumtechniqueofGoldsteinand
infer a flat and ramp rupture geometry. The moment tensor Werner [1998]. The effect of topography that depends on
analysis obtained from broadband seismic records yields a the perpendicular separation between orbital trajectories is
Table 2. FaultPlane Parameters of the 21May2003Zemmouri Earthquake
Plane1a Plane2
Longitude Latitude Depth Strike Dip Rake Strike Dip Rake
Source (deg) (deg) (km) (deg) (deg) (deg) (deg) (deg) (deg) M (Nm)
0
HRV 3.58 36.93 15 57 44 71 262 49 107 2.01(cid:3)1019
USGS 3.78 36.89 9 54 47 88 237 43 92 1.30(cid:3)1019
INGV 3.61 36.9 15 65 27 86 250 63 92 1.80(cid:3)1019
Yagi[2003] 3.65b 36.83b - 54 47 86 - - - 2.40(cid:3)1019
Delouisetal.[2004] 3.65b 36.83b - 70 40 95 - - - 2.80(cid:3)1019
Meghraouietal.[2004] 3.65b 36.83b - 54 50 88 - - - 2.75(cid:3)1019
BraunmillerandBernardi[2005] 3.65b 36.83b - 62 25 82 - - - 3.48(cid:3)1019
Thisstudyplanarmodel 3.65b 36.83b 8 65 40 90 - - - 1.78(cid:3)1019
Thisstudycurvedmodel 3.65b 36.83b 10 65 40 90 - - - 2.15(cid:3)1019
aPlane1istheprincipalfaultaccordingtofieldobservations.
bEpicenterrelocationbyBounifetal.[2004].
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Figure3. Coastalupliftat6kmwestofDellys(Figure1b).Thephotographshowstheelevatedmarine
terrace and platform (black arrow is the former shoreline and white arrow is the new shoreline) with an
average 0.55 m of uplift [Meghraoui et al., 2004].
removedfromtheinterferogramsusingtheSRTM3-arc-sec regionexistsinalltheinterferogramsandismostlikelydue
((cid:1)90m)postingdigitalelevationmodel[Farretal.,2007]. to the high agricultural activity along the Oued Isser River.
Envisat interferograms were obtained using precise orbits SignaldecorrelationintheEnvisatinterferogramsalsooccurs
fromDelftUniversity[ScharrooandVisser,1998]whilefor to the south in the mountainous regions due to the low
RADARSAT interferograms orbital information are from altitude of ambiguity of the interferometric pairs (Table 3).
the data header. Because of the significant change in the look direction
[10] Baselines for Envisat data were not reestimated (Table 4), the interferograms naturally differ from each
because there are no orbital residuals visible farther south other. However, the difference is not remarkable since the
oftheearthquakeareawherethereissupposedlynosurface deformation is overwhelmingly vertical, which can also be
deformation. The available RADARSAT orbits are impre- seeninFigures4e,4f,and4gthatshowdigitizedfringesof
cise and processing them leaves orbital errors. Therefore, all interferograms. The difference in the fringe pattern
we have modeled the orbital residuals with a 2-D quadratic between the interferograms is mostly due to the horizontal
surface andsubtracteditfromtheinterferograms[Zebkeret component of the surface displacement. RADARSAT
al., 1994]. interferograms indicate that there are two lobes of defor-
[11] While the descending RADARSAT frame covers mation centered in the Boumerdes and Cap Djenet regions
entirely the uplifted coastal region from Cap Matifou to with the earthquake epicenter being located in the area of
the west and Tighzirt to the east, both the ascending and lower deformation in between them (Figures 4a and 4b).
descending Envisat frames cover only the western part of All interferograms indicate that the maximum surface
the earthquake area (Figure 1b). Despite the atmospheric deformation occurred indeed in the Zemmouri-Boumerdes
noise and low coherence particularly in the epicentral area, region, confirming the field measurement of the coastal
the interferograms display clear coseismic fringes all along uplift [Meghraoui et al., 2004]. In this region, up to 14
the coast (Figure 4). The poor coherence in the epicentral fringes can be counted in the RADARSAT interferograms
Table 3. Characteristics of SARImages Coveringthe 2003Zemmouri Earthquake Area
Altitudeof
Satellite BeamMode MasterOrbit MasterDate SlaveOrbit SlaveDate Trajectory Ambiguity(m) Bperp(m)
RADARSAT1 ST4 35996 27Sep2002 40798 29Aug2003 Descending 1358 12
RADARSAT1 ST4 35996 27Sep2002 41141 22Sep2003 Descending 84 194
Envisat IS2 4900 6Feb2003 6904 26Jun2003 Descending 22 238
Envisat IS2 5308 6Mar2003 7312 24Jul2003 Ascending 40 419
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Figure 4
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Table 4. Unit Vectors of SARScenes curve; this selected factor (i.e., 0.4) represents the best
Satellite FlightDirection East North Up compromise between the roughness of slip and misfit to
the data (Figures 5a and 5b). In our inversions, we use
RADARSAT Descending 0.59339 (cid:2)0.10980 0.79739
Envisat Descending 0.37188 (cid:2)0.07946 0.92487 digitizedfringesbecausesomeofthemcannotbeunwrapped
Envisat Ascending (cid:2)0.38526 (cid:2)0.08173 0.91918 duetonoiseandunwrappingerrors.Inthiscase,theresidual
interferograms are obtained by a multiplication in the
complexdomainofthedataandthesyntheticinterferograms.
(Figures 4a and 4b) and 16 fringes in the Envisat interfero- On the basis of the fringe gradient, the southernmost fringe
grams (Figures 4c and 4d). Assuming purely vertical in the isolated patch located near Cap Djenet (Figure 4e) is
deformation,theseline-of-sight(LOS)displacementscorre- assumed to be the third fringe (i.e., 8.49 cm) and the large
spond approximately to 0.50 m of uplift since the Envisat curved fringe southeast of Boumerdes (Figure 4f) is as-
interferograms are more sensitive to vertical deformation sumedtobethefirstfringe(i.e.,2.83cm).Aconstantoffset
thanthe RADARSATinterferograms. Captured onlybythe in the InSAR data is not solved as we think it is negligible
RADARSAT interferograms, the LOS displacement in the since one may observe to the south areas of nominal
Cap Djenet region reaches to (cid:1)0.25 m (i.e., 9 fringes). deformation.
[12] An examination of interferograms reveals some [14] Inordertoconstrainthelocationandgeometryofthe
anomalies in the fringe pattern of the Cap Matifou region fault rupture, we run several inversions using digitized
(black arrows in Figure 4). Fringes are clearly disturbed, fringes(Figures4e,4f,and4g),coastalupliftmeasurements
offsetandinvertedalongalineamentthatcoincideswiththe [Meghraouietal.,2004]andcoseismicGPSvectors[Yelles
Thenia fault. This implies that the Thenia fault must have et al., 2004]. The InSAR, coastal uplift and GPS data were
experienced some triggered slip during or after the main treated as equally weighted in all inversions. The Poisson
shock, a phenomenon commonly observed due to large ratio in the elastic half-space is given as 0.25. A tilt for
earthquakes [Fialko et al., 2002]. orbital residuals is solved by the Poly3D inversion but
found to be insignificant. Several attempts with no con-
straints on the fault rake are performed but the inversion
4. Modeling the InSAR Data
predicts abnormally high strike-slip (both right and left
[13] We used the Poly3Dinv slip inversion method to lateral) components. Therefore, we keep the N60(cid:1)–65(cid:1)
model geodetic data using realistic fault surfaces (see also strike and rake as pure thrust (90(cid:1)) fixed and test several
maps in Figures 5a and 5b) [Thomas,1993; Maerten et al., fault plane dips (20(cid:1)–60(cid:1)) in agreement with most of focal
2005]. The method is based on the analytical solution for a mechanismparametersofthe2003mainshock(Table2).In
triangular dislocation in a linear, elastic, homogeneous and the absence of any observed seafloor rupture and related
isotropic half-space, which uses triangular surfaces as dis- SAR data, no surface slip on the uppermost patches is
continuities [Maerten et al., 2005]. Hence, the use of allowed in the inversions. However, it is possible that
triangular elements allowed us to construct fault models some slip reached the seafloor, but neither the SAR data
that better approximate two-dimensional planar surfaces, nor the uplift measurements can resolve it. Taking into
avoiding gaps and overlaps that are inevitably encountered account the surface deformation and seismotectonic frame-
when modeling highly segmented faults of varying strike work of the epicentral area, we suggest the following two
withsimplerectangulardislocations.Thismethodimproves slip models:
thefittothegeodeticdataparticularlyinthenearfieldwhen [15] 1.Avariableslipmodel isobtainedusinginversions
modeling complicated fault ruptures [Maerten et al., 2005; with various planar faults striking N65(cid:1) and dipping 30(cid:1),
Resor et al., 2005]. Fault surfaces meshed with triangles 40(cid:1), and 50(cid:1) SE located from 6 km to 24 km offshore
were constructed using MATLAB1. We meshed the planar (Figure 5a). The best fitting model (RMS = 2.69 cm) is
offshorefaultwith6(cid:3)5quadrangles(i.e.,60triangles)and obtained with 60-km-long and 30-km-wide offshore thrust
2(cid:3)1quadranglesfortheTheniafault.Theslipdistribution fault(i.e.,fault4inFigure5a)strikingN65(cid:1),dipping30(cid:1)to
of the triangle elements was then inverted for, with a 40(cid:1) SE and located at (cid:1)13 km offshore from the epicenter
negativity constraint on the dip slip component (i.e., thrust (Figures 5a and 6a). However, as illustrated in the RMS
only, 90(cid:1) rake). To avoid unphysical oscillatory slip, the versusdistanceplotofFigure5a,thefaulttiplocationisnot
scale-dependentumbrellasmoothingoperatorofPoly3Dinv tightly constrained and could be anywhere between 9 to
is applied to the inverted slip distribution. We choose the 18 km from the shoreline. The best fitting model predicts a
smoothingfactorbasedontheRMSmisfitversusroughness coseismic slip distribution at (cid:1)8- to 10-km-depth on the
Figure4. Coseismicinterferogramsofthewesterndeformationzoneofthe2003Zemmouriearthquakeareain(aandb)
the descending RADARSAT geometry and (c and d) ascending and descending Envisat radar geometry; the image
reference(dateandorbitnumbers)isinthetoprightcorner.Blackarrowsindicatethesatelliteflightdirection.Starshows
the2003Zemmouriearthquakeepicenter[Bounifetal.,2004].(e,f,andg)Digitizedfringeslabeledwithdisplacement(in
cm) in the line-of-sight (LOS) direction for each interferogram (Figure 4e with 4b, Figure 4f with 4c, and Figure 4g with
4d).(h)baselineswithaltitudesofambiguityinm(inblackbox)foreachpairofSARimageswithorbitnumbers.Fringes
areobservedinallinterferogramswith2lobesofhighdeformationcenteredinBoumerdesandCapDjenetregion.N105(cid:1)
trending black arrows near Cap Matifou show the anomalies observed in the fringe pattern along the Thenia Fault
(Figure1b).ThewesternearthquakeareabetweenCapMatifouandZemmourishowsseveralfringesthatcorrespondtothe
zone of maximum deformation.
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fault and shows two linked asperities reaching 4.7 m and (TFinFigure1b)inCapMatifou aremodeledinaforward
(cid:1)3 m maximum slip in the NE and SW patches, respec- manner with a 0.38 m slip on a (cid:1)20-km-long subvertical
tively, and 1.78 (cid:3) 1019 N m (M 6.8) geodetic moment. right-lateral fault as the inversions fail to explain the
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Additionally,offsetfringesobservedalongtheTheniaFault disturbed fringes (Figure 4).
Figure 5
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Figure 6. (a) Slip distribution model of preferred planar fault (4 in Figure 5a) of the Zemmouri
earthquake rupture obtained from the inversion of the digitized fringes and coastal uplift data. Dip
component of the coseismic slip on each triangular element are inverted using Poly3Dinv. Dip-slip
distribution (4.7 m maximum from colored contours on fault surfaces) shows two linked maximum of
slipontheN65(cid:1)trendingand60-km-longmajorfaultrupture.(b)Slipdistributionofthepreferredcurved
fault model striking N60(cid:1) ((cid:1)65km) to N102(cid:1) ((cid:1)13km) onthe western part. The curved fault model is
constructedwithPoly3Dinordertomatchthetwofaultplaneswithdifferentstrikes.Dip-slipdistribution
(2.1mmaximumfromcoloredcontours)withtwodistinctmaximumslipsreachingtheseafloornorthof
Boumerdes and Cap Djenet.
[16] 2.Thealternativemodeltakesintoaccountthecom- between8and9kmoffshore(fromtheepicenter)forafault
plicated fringe distribution west of Boumerdes (Figures 4a, dipping 40(cid:1) and 50(cid:1) SE. The RMS misfit plot also shows
4b,4c,and4d)andalsoincludestheinversionofcoseismic minimum values (2.77 cm, 2.81 cm) for 30(cid:1) SE dipping
slip on regularly spaced faults ((cid:1)2 km) from 4 to 20 km faultat11kmoffshoreandfor20(cid:1),30(cid:1),40(cid:1),and50(cid:1)at13km
offshore (Figure 5b). To the NE, faults are planar with an offshore(Figure5b).Takingintoaccountthe40(cid:1)to50(cid:1)SE
azimuth of N60–65(cid:1), consistent with the aftershocks dis- fault dip of most focal mechanism solutions (Table 2) and
tribution[Ayadietal.,2008]andfocalmechanismsolutions the40(cid:1)SEdippingfaultgeometryfromaftershockstomog-
of the main shock (Table 2). The SW fault section is raphy analysis [Ayadi et al., 2008], we select fault 3 in
modeled according to the N95(cid:1)–105(cid:1) trending aftershocks Figure 5b with 40(cid:1) to 50(cid:1) SE dip as our preferred solution.
andrelatedtomography[Ayadietal.,2008]wherethemain The fault is here a 65-km-long and 30-km-wide offshore
NE-SW trending earthquake rupture cannot crosscut the thrustfault striking N65(cid:1) anddipping40(cid:1)SE, tothe nearly
continuous fringes visible particularly in the ascending vertical and (cid:1)N102(cid:1) trending rupture SWof the epicenter
Envisat interferogram west of Boumerdes (Figure 4c). This (Figure5b).Twodistinctslippatchesyield2.15(cid:3)1019Nm
changeinfaultstrikemaybemodeledasacurvedfault.The (M 6.8) geodetic moment with (cid:1)0.5 m slip reaching the
w
sliponeachfaultisinvertedfordips20(cid:1),30(cid:1),40(cid:1),50(cid:1)and seafloor(Figure6b).Additionally,offsetfringesobservedin
60(cid:1)SEusingthesameInSAR,GPS,andcoastalupliftdata. CapMatifou(Figures4a,4b,4c,and4d)maybeexplained
AsshowninFigure5b,theinversionresultssuggestthatthe by 0.15 m triggered slip on the 20-km-long subvertical
minimum RMS misfits improve the fault location between right-lateral secondary Thenia fault (Figure 1b).
8 and 13 km offshore. The lowest 2.67 cm RMS error [17] Figure 7 shows the synthetic interferograms pre-
indicates that the fault rupture is most probably located dicted by the best fitting models with planar (Figures 7a–
Figure5. (a)Mapofthedifferentplanarfaulttippositions(1to8)usedfortheinversions.Thewhitestarrepresentsthe
mainshockepicenter[Bounifetal.,2004]andthereferencepointfordistancecalculation.Notethatfault4(whitedashed
line) represents in this case the preferred fault model taking into account the lowest RMS misfit (right inset). The right
inset shows the RMS error versus distance from the epicenter for different faults (1 to 8) with various dips (30(cid:1), 40(cid:1) and
50(cid:1)).Thearrowshowsthepreferredsolutionat(cid:1)13km.TheleftinsetshowsthemodelroughnessversustheRMSmisfit,
the corresponding smoothing factor for each model roughness is indicated on the right axis. (b) Map of different curved
fault tip positions (dashed lines) used for the inversion with various dips (20(cid:1), 30(cid:1), 40(cid:1), 50(cid:1) and 60(cid:1)). The lowest RMS
error is calculated for the 40(cid:1) and 50(cid:1) curved fault at (cid:1)9 km (white dashed line 3) from the epicenter and represents our
preferred model. The right inset shows the RMS misfit versus the model roughness with 0.4 corresponding smoothing
factor.
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B03406 BELABBE`SETAL.:INSAR OF THEZEMMOURI EARTHQUAKE B03406
Figure 7
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