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Coulomb static stress variations in the Kachchh, Gujarat, India: Implications for the occurrences of two recent earthquakes (Mw = 5.6) in the 2001 Bhuj earthquake region

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Double difference algorithm was used to relocate aftershock sequences of two Mw= 5.6 Kachchh earthquakes (India), which have occurred on 2006 March 7 and April 6, along almost vertical strike-slip Gedi fault (GF) and south dipping reverse North Wagad
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  Geophys. J. Int.  (2007)  169,  281–285 doi: 10.1111/j.1365-246X.2006.03301.x       G      J      I      S    e     i    s    m    o      l    o    g    y Coulomb static stress variations in the Kachchh, Gujarat, India:Implications for the occurrences of two recent earthquakes( M  w = 5.6) in the 2001 Bhuj earthquake region Prantik Mandal, R. K. Chadha, I. P. Raju, N. Kumar, C. Satyamurty, R. Narsaiahand A. Maji  National Geophysical Research Institute, Uppal Road, Hyderabad   500007,  India. E-mail: prantikmandal@yahoo.com Accepted 2006 November 16. Received 2006 November 16; in srcinal form 2006 May 31 SUMMARY Double difference algorithm was used to relocate aftershock sequences of two  M  w  =  5.6Kachchhearthquakes(India),whichhaveoccurredon2006March7andApril6,alongalmostvertical strike-slip Gedi fault (GF) and south dipping reverse North Wagad fault (NWF),respectively. The relocated focal depths delineate a marked variation of 4 and 7 km in the brittle–ductiletransitiondepthsbeneathGFandNWF,respectively.WemodelledtheCoulombfailure stress change (  CFS) produced by the 2001 mainshock and 2006 GEDI events on anoptimally oriented plane. A strong correlation with occurrences of aftershocks and regions of increased    CFS is obtained on both the faults. The predicted    CFS on the GF increased by0.14 MPa at 3 km depth, where the 2006 March 7  M  w  = 5.6 event occurred. The variationin brittle–ductile transition depths may be related to the perturbation in geothermal gradientresultedfromthepresenceofintrusivesorvaryingsaturationstateoffluid-filledfracturedrock matrix beneath both the fault zones. Key words:  aftershocks, double-difference relocation, local earthquake moment tensor in-version, static stress changes, triggered aftershocks. 1 INTRODUCTION The  M  w = 7.7 2001 Bhuj earthquake sequence occurred along thesouth dipping north Wagad fault (NWF), a blind dip-slip fault sys-tem in Kachchh, Gujarat, which slips in a reverse sense of motionin response to the prevailing N–S compression due to the north-ward motion of the Indian plate (Fig. 1; Bodin & Horton 2004;Mandal  et al.  2006). This 2001 January 26  M  w  = 7.7 event (Har-vard hypocentre: 23.40 ◦  N, 70.23 ◦ E, 23.4 km depth) was the largestearthquake known to have occurred in Kachchh, Gujarat, sinceat least the 1989 Allah Bund event of   M  w  =  7.8 (Rajendran &Rajendran 2001). The 2001 Bhuj earthquake was unique for its un-usualfocaldepth,intraplatelocationandsmallsourcearea(Wallace et al.  2006). The main rupture zone was confined to a south-dippingarea of 40 × 40 km, with the 70 per cent primary slip concentrated at depths of 10–35 km (Antolik & Dreger 2003; Bodin & Horton2004; Mandal  et al.  2004).The Kachchh region has experienced two  M  w ≥ 7.7 earthquakesinaspanof182yr(Mandal etal. 2004).Likealllarge(  M   > 7.5)con-tinental midplate earthquakes, both the 1819 Kachchh and the 2001Bhujearthquakesalsotookplacewithinanoldriftthathasbeentec-tonically reactivated in compression (Bodin & Horton 2004). Thesetwo earthquakes were caused by movement along pre-existing re-verse faults in the intruded crust of a failed intracontinental rift, be-neath the 2–4 km thick Palaeozoic and Mesozoic sediments of theRann, and their violent shaking resulted in massive and extensiveliquefaction (Mandal 2006; Mandal & Pujol 2006). However, whilethe1819earthquakedidcausesurfacerupture,thecausativefaultof the 2001 earthquake did not reach the surface (Rajendran & Rajen-dran 2001; Wesnousky  et al.  2001). The far field stresses that could drivetheKachchhriftareinfluencedbytheIndo-Eurasiancollision, by the push from the Indian oceanic ridges, and by the lithosphericstructures inherited from previous orogenic fabrics (Cloetingh &Wortel 1986; Gambos  et al.  1985; Mishra  et al.  2005). However,Bilham  et al.  (2003) also suggested that the Bhuj earthquake and other intraplate Indian earthquakes could be related to the stressregime governed by the flexure of the Indian plate. It has also been proposed that the Bhuj earthquake was triggered by an increase inCoulomb stress due to the 1819 event (To  et al.  2006).After more than five years of the aftershock activity of 2001 Bhujearthquake, an  M  w  =  5.6 event (hypocentre: 23.84 ◦  N, 70.72 ◦ E,3.0 km depth) occurred along the neighbouring GEDI fault (GF,30 km north of NWF). This  M  w  = 5.6 GEDI mainshock occurred on 2006 March 7 and it was followed by another   M  w  = 5.6 eventon 2006 April 6 that located on the NWF (hypocentre: 23.35 ◦  N,70.32 ◦ E, 28.2 km depth), which was the second largest aftershock of the 2001 Bhuj sequence. The 2001 Bhuj mainshock and its sec-ond largest aftershock were more complex than the GEDI event,rupturing about 50 × 40 km area between Kachchh mainland faultand NWF, with a mixture of reverse and right-lateral motion. The C   2007 The Authors  281 Journal compilation  C   2007 RAS  282  P. Mandal   et al. Figure 1.  A plot showing seven seismograph stations (seismograph sta-tions marked by the solid triangles). KVD: Kavada; KOL: Kolvadi; JAH:Jahwarnagar; TAP: Tapar; BEL: Bela; VJP: Vajepar; BHA: Bhachau. KMU,Kachchh mainland uplift. Major faults (lines): ABF, Allah Bund Fault; IBF,Islandbeltfault;KMF,Kachchhmainlandfault;KHF,KatrolHillfault;NPF, Nagar Parkar fault; NWF, North Wagad fault (dotted line), GF, GEDI fault(dotted line). The inset is showing the key map for the area with referenceto Indian plate boundaries (dark lines). The prevailing compression from  in situ  stress measurement and focal mechanism data has been shown by anarrow symbol (after Gowd   et al.  1992). generation of these kind of intraplate earthquakes could be con-trolled by the variation in the brittle–ductile transition in the riftzone,whichwillbeinfluencedbythecrustalthermalconditionsper-haps controlled by the mafic intrusives as revealed by the detailed velocity tomography and modelling of gravity as well as magneticdata (Manglik & Singh 2002; Mishra  et al.  2005; Mishra & Zhao2003; Mandal & Pujol 2006). The maximum estimated slip fromthe InSAR data was about 14.8 m at the source of the 2001 Bhujmainshock (Schmidt & Burggmann 2006), where the presence of a fluid-filled, fractured rock matrix at 23–28 km depth (character-ized by high crack density, high saturation rate and high porosity)was inferred from the earlier tomographic studies (Mishra & Zhao2003;Mandal etal. 2004).Thus,thereisapossibilitythatthisfluid-filled rock matrix will increase the fluid pressure resulting in the perturbation of the stress regime at the hypocentre of the 2001 Bhujmainshock (Mishra & Zhao 2003). The presence of fluids could also cause local rising in temperature along the fault zones dueto upward advection fluids through fractures that can also explainthe local variation in the brittle–ductile transitions along the faultzones (Bickle & McKenzie 1987; Hoisch 1991). Further, the close proximity of the NWF and GF earthquakes along with the presenceof interconnected rupture nucleated trends (Mishra & Zhao 2003)suggestapossiblelinkbetweenthem,whichweinvestigatebymod-ellingCoulombstresstransferfromthe2001Bhujmainshocktothe2006 March 7  M  w = 5.6 GF event. We also examine the combined effect of the NWF and GF events and the role of the brittle–ductiletransition in the occurrences of aftershocks in these fault zones. 2 AFTERSHOCKS AND RELOCATIONS The double-difference algorithm (Waldhauser & Ellsworth 2000)incorporates traveltime differences formed from  P  - and   S  -wave ar-rival times, which reduces the uncertainties significantly due to theaccurate picks. Further, specifying the traveltime as a double dif-ference minimizes errors due to unmodelled velocity structure. Inthis paper, this algorithm is used to precisely relocate the hypocen-tres of 230 aftershocks of the 2001  M  w  =  7.7 Bhuj earthquake.We used 1600  P   and 1400  S   phases recorded on four to eight three-componentseismographsdeployedinKachchhseismiczoneduring2006 January–July by the National Geophysical Research Institute(NGRI), Hyderabad, India (Fig. 1). We also used phase data from18 strong motion accelerograph stations for strong earthquakes. Ingeneral, the arrival times of the  P   phases could be picked (repeated visualinspection)withinabout0.05s(samplingrate100Hz),whilefor the  S   phases the error may be slightly larger.Initialeventlocationswereobtainedusingtheprogram HYPO 71 PC in-built in the  SEISAN  software. We used a 1-D velocity model ob-tainedfromtraveltimeobservationsoftheaftershocksandraytheory(Bodin & Horton 2004). This model is constrained by the converted  phase observations, which occurs at the Precambrian to Mesozoic boundary, between 0.5 and 2.5 km below the region. The average  P  -wave velocity of the Mesozoic sediments has been estimated to be 3.0 km s − 1 from the surface wave dispersion analysis (Bodin& Horton 2004). The velocity model consists of nine layers, thetop of those layers occur at 0.0, 1.5, 10.0, 15.0, 20.0, 25.0, 30.0,35.0 and 39.0 km, with  P  -wave velocities of 3.0, 6.25, 6.45, 6.48,6.72, 7.15, 7.49, 7.88 and 8.19 km s − 1 , respectively. A  Vp / Vs  ra-tio of 1.73 was used for the location of aftershocks. The averagelocation rms was 0.05s. The mean horizontal and vertical single68 per cent confidence estimates are 1.2 and 2.1 km, respectively,for the aftershocks.We used a total of eight iterations for the conjugate gradientmethod (LSQR) within  HYPODD  inversion technique. In this study,the traveltime differences have been estimated for all the event pairswith an inter-event separation less than 5 km and stations located in 100 km radius from the cluster centroid. A maximum of six toeightneighbouringeventslinkedtoeachotherwereonlyconsidered for the relocation. The condition numbers (i.e. ratio of the largestto smallest eigen value) obtained for eight iterations range from 55to 75.  HYPODD  could relocate 230 out of 300 aftershocks, whichwere sufficiently clustered. The  a priori  weights assigned for   P   and  S   waveswere1.0and0.5,respectively.Afterfouriterations,weightswere assigned for small inter-event distances and vice versa to boththe  P  - and   S  -phase data. We then checked each location obtained bythe LSQR against the SVD result to ensure consistency in hypocen-tral location. The average relative uncertainties for the aftershocksupon relocation are 200 m in epicentral location and 400 m in focaldepth estimation.ThedistributionofrelocatedepicentresdefinestwoE–Wtrendingaftershockzones,oneaftershockzoneiscoveringalmost40 × 20kmarea, which is about 8 ◦ from the WSW–ENE striking nodal planeof the mainshock and another aftershock zone confines to the Gedifault (which is mapped earlier by Biswas (1987) as shown in Figs 1and 2) covering almost 20 × 5 km (Figs 1 and 3a). The relocated hypocentres delineate two distinct zones, one along NWF is southdippingE–Wtrendingaftershockzoneextendingupto35kmdepthand the other along Gedi fault is E–W trending almost vertical af-tershock zone. The pink solid circles in Fig. 2(a) show the relocated   M  w = 5.6 events of 2006 March 7 along GF and 2006 April 6 event C   2007 The Authors,  GJI  ,  169,  281–285Journal compilation  C   2007 RAS  Coulomb static stress variations  283 23.4 23.6 23.8 24.0NW (a)(b)NWF Figure 2.  (a) Relocated epicentres of selected 230 aftershocks during 2006January–April. The large pink solid circles show the epicentres for the  M  w  =  5.6 events. The solid green circles represent locations for M4-4.9events. The open green circles mark the epicentres of   M  w  = 3–3.9 eventsand the open blue circles represent the epicentres of   M  w  =  2–2.9 events.Thesolidblacktrianglesrepresenttheseismographs,whereastheopenblack squaresmarktheaccelerographs.(b)3-Ddepthplotofrelocatedaftershocks.ThedottedellipticalzonesshowaftershockzonesalongtheNWFandGEDIfaults. along NWF. The N–S hypocentral depth distribution also clearlydelineates three fault segments: the south-dipping Kachchh Main-land Fault (KMF) at 0–15 km depth, the south dipping NWF ex-tending up to 37 km depth and an almost vertical fault at 0–15 kmdepth (Fig. 3a).Strike-parallel(E–W)hypocentraldepthsectionsuggestsasignif-icantvariationinbrittle–ductiletransitiondepth(upto7km)beneaththeNWFaftershockzonewheretheearthquakefociinthebothwest-ern and eastern ends are confined up to 25 km depth whilst in thecentralaftershockzonetheyarelimitedupto32kmdepth(Fig.3b).Thisalsosuggestsavariationinbrittle–ductiletransitiondepth(upto4 km) beneath the GEDI aftershock zone where the earthquake fociin the both western and eastern ends are confined up to 7 km depthwhilst in the central aftershock zone they are limited up to 11 kmdepth (Fig. 4). 3 SOURCE PROCESS AND STATICSTRESS CHANGES We used a local earthquake moment tensor least-squares inversionscheme(Ebel1989)tocomputethesourcemoment-tensoraswellas    D  e  p   t   h   (   k  m   )      D    e    p     t     h     (     k    m     )    K   M   F    N   W   F (a)(b) Figure 3.  (a) N–S depth profile of relocated aftershocks. The solid black lines mark the faults. (b) Strike-parallel (E–W) depth profile of relocated aftershocks.Thedottedblacklinesshowthebaseoftheobservedseismicity.The elliptical areas represent the aftershock zones. thedoublecouplecomponentsbyinvertingamplitudesandpolaritiesof 12 direct  P  ’s (from vertical) and eight  SH  ’s (from transverse) for the2006February17,February19,March7(onGF),April6(along NWF), April 6 (along GEDI) events. The inversion led to an rmserroroftheorderof0.2–0.3micron.Thefocalmechanismsolutionsfor the March 7 and April 6 events, which occurred along the GEDIfault, suggest a pure strike-slip fault (strike 88 ◦ , dip 78 ◦ , rake 178 ◦ ),whereasthefocalmechanismsfortheFebruary17,February19and April 6 events, which occurred along the NWF, suggest a reversefault with a minor strike-slip component along a preferred southdipping fault (Fig. 2b). The focal mechanism solution for the 2001  M  w  = 7.7 mainshock (Fig. 1) suggests a reverse motion along thesouthdippingNWF(strike82 ◦ ,dip51 ◦ ,rake77 ◦ )(Antolik&Dreger 2003).WecalculateCoulombfailurestress(  CFS)usingthe GNSTRESS v2.17codeandfollowingtheOkada(1992)method.Weassumedanapparent frictional coefficient value of 0.4 as proposed by To  et al. (2006).Weexperimentedwithdifferentvaluesofapparentfrictionalcoefficient, which did not show significant changes in the results.The geometry and slip direction (strike, dip and rake) of the faultneed to be specified for this calculation. A positive change in CFSindicates the increase in likelihood of failure on the receiver fault.It is given by   CFS = τ  β  − µ ′ σ  β , where  τ  β  is the change inshearstressintheslipdirectiononareceiverfault, σ  β  isthechangein normal stress acting on the receiver fault (tension positive), and  µ ′ [ = µ  (1 − β )] is the apparent frictional coefficient, and includesthe effects of pore fluid and the material property of the fault zone. C   2007 The Authors,  GJI  ,  169,  281–285Journal compilation  C   2007 RAS  284  P. Mandal   et al. Figure 4.  Static stress changes due to the  M  w = 7.7 2001 Bhuj earthquakeandthe  M  w = 5.62006GEDIearthquakeat(a)3km(consideringdislocationmodel only along NWF, without considering the dislocation along GF for the 2006 March 7  M  w  =  5.6 event). Considering two dislocation modelsalong NWF and GF (for the 2006 March 7 event of   M  w  = 5.6), (b) 3 km,(c) 10 km and (d) 24 km. The rectangle suggests the orientation of the NWF. µ ′ values of 0.2–0.8 are widely used in other studies (Harris 1998).We present calculated    CFS for   µ ′ = 0.4,  β  = 0.5, and pore fluid  pressure) = 0.38. Further, for CFS calculation we considered a  σ  1 (azimuth 181 ◦ , plunge 14 ◦ ), a  σ  3  (azimuth 300 ◦ , plunge 61 ◦ ) and   R = 0.4asobtainedfromthestressinversionof444well-constrained focalmechanismsolutionsofBhujaftershocks(Mandal etal. 2006).ThereceiverfaultgeometryofAntolik&Dreger(2003)fortheBhujearthquakeisadopted(strike = 82 ◦ ,dip = 51 ◦ ,rake = 77 ◦ ),whereasthereceiverfaultgeometryforGEDIfaultasobtainedfromthemo-ment tensor analysis of the local earthquake is considered (strike = 90 ◦ , dip = 78 ◦ , rake = 178 ◦ ). For the estimation of    CFS, we used the distributed slip model as obtained from the inversion of InSAR data where the majority of slip confines within a 20  ×  20 km re-gioncentredatadepthof20km(Schmidt&Burggmann2006).Theconsideredblindthrustfaultplanewhosedown-dipwidthandalongstrike length are 20 km and 20 km, respectively, is discretized intosquaresubfaultswithadimensionof1kmresultinginanaverageslipof14.8m(Schmidt&Burggmann2006).Itwillbeimportanttonotethat the GF began to experience  M  w ≥ 4 events since 2003 August5,anduntil2006March6,anotherfourearthquakesof   M  w = 4–4.7have occurred along GF. Thus, we assume that the occurrences of five  M  w = 4–4.9 events would produce an approximate 0.05-m slipon the GF (20 km long and 2 km wide extending from the surfaceto 15 km depth). Hence, with a view to experiment the influence of 2001  M  w = 7.7 event on GF, initially we used a smaller slip of 0.05onGF(beforetheoccurrenceof2006March7  M  w = 5.6event),and later after the occurrence of   M  w  =  5.6 event we applied a slip of 0.7 m along GF. We also experimented with the slip distributionobtained from teleseismic waveform inversion of 2001 mainshock (Antolik & Dreger 2003), which resulted in weak correlation be-tween   CFS and aftershock occurrences. 4 RESULTS AND DISCUSSION In accordance with the above-mentioned seismological observa-tions, precisely relocated 1172 aftershocks (by  HYPODD ) delineatetwo distinct zones, one along NWF is south dipping E–W trendingaftershock zone extending up to 35 km depth and the other alongGEDIfaultisE–Wtrendingalmostverticalaftershockzoneextend-ing up to 15 km depth.The maximum depth of seismogenic faulting can be interpreted as the transition from brittle faulting to plastic deformation (Sibson1982). The maximum depth at which crustal earthquakes occur de- pends on factors like geometry and mode of faulting, geothermalgradient,lithology,porefluidpressureandstrainrate(Sibson1982).There are few published heat flow measurements in Kachchh; Roy(2003)notedasomewhathigherthannormalheatflowfromtheBhujearthquake source region (55–93 mWm − 2 ), which suggests higher rather than lower deep crust temperatures unless heat production isunusually high in shallow rocks. Sibson (1984) has shown that achange in geothermal gradient along strike from 20 ◦ to 30 ◦ C km − 1 would cause the base of seismogenic zone to shallow by 4–6 kmdepending on rock type. Our results suggest a 7 km deepening of the seismogenic base beneath the central aftershock zone, wherethe 2001 Bhuj mainshock and 2006 April 6  M  w  = 5.6 aftershock occurred.Thetomographicstudyrevealedhigh-velocitymaficintrusivesontheeasternandwesternsidesofthecentralaftershockzone(Mandal& Pujol 2006). The modelling of gravity and magnetic data indi-cates a crustal thickening of 8–10 km resulted from the existenceof volcanic plugs of alkaline magmatic composition beneath theepicentral zone of the 2001 Bhuj earthquake (Mishra  et al.  2005).Therefore, the compositional changes due to the intruded mafic in-trusives can lead to a marked increase in the geothermal gradient inthe central aftershock zone, which can explain the 7 km deepeningof the seismogenic base. Hence, the deepest portion of the base of the seismogenic zone beneath the central fault segment will be themaximum probable zone for the future frictional failure (where the2001  M  w  = 7.7 Bhuj mainshock as well as 2006  M  w  = 5.6 eventtook place).Similarly,thehighgravityassociatedwiththeWagaduplift(southof GF) has been interpreted as an intrusive body at 1–7 km (Mishra et al.  2005), which might have resulted in 4 km deepening beneaththe main GEDI aftershock zone where the  M  w = 5.6 event of 2006March 7 took place. Further, it would be important to mentionhere that the rift zone, such as the Narmada–Son lineament area,was shown to have two brittle–ductile transitions at shallow ( ∼ 12– 15 km) and at deeper ( ∼ 20–25 km) depths in presence of crustalintrusive bodies (Manglik & Singh 2002). Similarly, two brittle– ductile zones could characterize GF zone. Presently, it seems thatthe shallow brittle–ductile zone got perturbed resulting in the recentoccurrence of earthquakes along GF.Alternatively, the presence of fluid-filled fractured rock matrixat 23–25 km depth, which is characterized by high crack density,highsaturationrateandhighporosityasrevealedbythetomographicstudy,couldenhancethefluidpressureresultinginlubricationofthefault zone (Mishra & Zhao 2003). This presence of fluids has beenobserved to lead to positive static stress changes at the hypocentraldepth of the 2001 Bhuj mainshock and the 2006 April 6  M  w = 5.6aftershock. Further, the upward advection of fluids along fractures C   2007 The Authors,  GJI  ,  169,  281–285Journal compilation  C   2007 RAS  Coulomb static stress variations  285 could locally raise the temperature along the fault zone (Bickle &McKenzie 1987; Hoisch 1991), which could also provide an alter-native explanation for the variation in seismogenic base along thenorth Wagad and GEDI fault zones.We present   CFS computed (considering both NWF and GF) inthe horizontal planes lying at 3, 5, 10 and 24 km depths (Figs 4a–d).Thecorrelationbetweentheaftershocksandincreased   CFSregionis observed to be good for the assumed slip model, which suggeststhe rupture model buried at 10 km depth as the preferred rupturemodel for the 2001 Bhuj earthquake. The predicted    CFS valuessuggest an increase of 0.14 MPa along GF before the occurrence of 2006March7  M  w = 5.6event;thus,itcanbeinferredthatthiseventwas triggered by the 2001 Bhuj mainshock. It is also important tonote that an increased    CFS region along GEDI fault continues upto a depth of 10 km showing a strong correlation with occurrenceof events along GF. At 24 km depth,   CFS values show a zone of negative values along GF suggesting a very weak correlation withthe occurrence of events that could be an indicative of the decreasein the seismicity rate along the GF due to the local stress shadow.However, these stress changes show a zone of positive values along NWF favouring a strong correlation with the occurrence of after-shocks, where the 2001 Bhuj mainshock and most of its aftershockstook place. ACKNOWLEDGMENTS The authors are thankful to the Director, NGRI for his kind permis-sion to publish this work. The authors would like to express their gratitude to the Editor, Geophysical Journal International and thereviewers for their critical and thorough review of this manuscript,which has improved the quality of this manuscript significantly. 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