Bulletin of the Seismological Society of America, Vol. 94, No. 2, pp , April 2004

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1 Bulletin of the Seismological Society of America, Vol. 94, No. 2, pp , April 2004 Results from Local Earthquake Velocity Tomography: Implications toward the Source Process Involved in Generating the 2001 Bhuj Earthquake in the Lower Crust beneath Kachchh (India) by Prantik Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Abstract To comprehend the source processes of the 26 January 2001 Bhuj earthquake sequence of M W 7.7 and its influence on the seismic hazard of the Kachchh, we estimate various seismological parameters using reliable and accurate aftershock data. The estimated parameters led to several important findings including the delineation of an east west trending, south-dipping ( 45 ) fault (North Wagad fault [NWF]), which touches the surface about 25 km north of Kachchh Mainland fault (KMF). The aftershock zone is confined to a 60-km-long and 40-km-wide region lying between the KMF to the south and NWF to the north, extending from 10 to 45 km depth. Focal mechanism solutions of the mainshock and 25 significant aftershocks of M W 3.0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the KMF and NWF is mainly characterized by reverse faults with east west trend and southerly dip, matching with the geological faults in the region. The tomographic inversion technique is used to invert 5516 P-wave travel times and 4061 S-P travel-time differences from 600 aftershocks recorded at 8 18 stations. Tomographic results suggest a regional high-velocity body (characterized by high V p [ km/sec], high V s [ km/sec], and low r [ ]) with a head extending 60 km in north south and 40 km in east west at km depths. This high-velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time ( 135 Ma). This crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area. Another important result of our study is the detection of a low-velocity zone (low V p [ km/sec], low V s [ km/sec], large r [ ]) within the mafic body at the hypocentral depth of the mainshock ( km), which is inferred to be a fluid-filled (trapped aqueous fluid resulting from metamorphism) fractured rock mass. The analysis of depth distribution of b-values suggests a high b-value zone between 15 and 25 km depths, which further supports this contention. Hence, the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 Bhuj earthquake within the inferred mafic body in the lower crust. Introduction The Kachchh region (Gujarat, India) is a unique intraplate seismic site on Earth that has experienced two large (M W 7.8 and 7.7) earthquakes within a time span of 182 yr (Gupta et al., 2001). The region lies in zone V (highest seismicity and potential for M 8 earthquakes) on the seismic zoning map of India (BIS, 2002). The latest earthquake of M W 7.7 (intensity X on the modified Mercalli [MM] scale) shook the whole of the Indian region at 8:46 (IST) on26 January 2001 with an epicenter at ( N, E) in the Kachchh region, Gujarat, India (Fig. 1) (Rastogi et al., 2001). Seismicity and first motion indicate that the Kachchh seismic zone is a zone of both strike-slip and reverse faulting (Chung and Gao, 1995). Available Global Positioning System (GPS) campaign data reveal at best a very weak signal and suggest very slow strain accumulation (Sridevi et al., 2001). Thus, the most important question related to the tectonics of Kachchh to be answered is how the large strains accumulate in this intraplate region (the Himalayan plate boundary in the north is more than 1000 km away, and the Herat Chaman plate boundary in the west is 500 km away) 633

2 634 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 1. A plot showing 28 seismograph stations (marked by solid triangles) along with epicenters of 600 aftershocks (black dots) and the mainshock epicenter (star symbol). The U.S. Geological Survey focal mechanism solution for the 2001 Bhuj mainshock is also shown. KMU-Kachchh mainland uplift. Major faults (solid lines): ABF, Allah Bund fault; IBF, Island belt fault; KMF, Kachchh mainland fault; KTF, Katrol fault; NPK, Nagar Parkar fault; NWF, North Wagad fault; ENWF, an unnamed fault mapped by Biswas (1987) in the Wagad uplift, which is inferred to be the probable eastward extension of the NWF. The inset shows the key map for the area with reference to Indian plate boundaries (solid lines). AMD, Ahmedabad; CHN, Chennai (modified after Malik et al. [1999]; faults are from Biswas and Deshpande [1970]). (Gupta et al., 2001). Hence, the mechanism of stress concentration at the aftershock zone of the 2001 Bhuj earthquake is an important outstanding geodynamic problem, which perhaps can be clarified by focused investigation of subsurface velocity structures using reliable and accurate aftershock data that delineated a hidden east west trending fault (the North Wagad fault [NWF]) about 25 km north of the Kachchh Mainland fault (KMF) as the causative fault for the 2001 Bhuj earthquake of M W 7.7 (Rastogi et al., 2001). Prior crustal velocity investigations in the Kachchh region are not very extensive but involve seismic refraction studies, receiver transfer function analysis of teleseismic P waves, regional surface wave dispersion study, and regional earthquake tomography (Kennet and Widiyantoro, 1999; Singh et al., 1999; Gupta et al., 2001; Kumar et al., 2001; Reddy et al., 2001). Integrated geophysical modeling of seismic refraction, gravity, magnetotelluric, and electrical data provided detailed information concerning velocity structure shallower than basement depth (3 6 km) (Gupta et al., 2001). The controlled source seismic refraction and receiver transfer function studies have provided the estimation of Moho depth that varies from 37 to 45 km (Kumar et al., 2001; Reddy et al., 2001). Further, marked crustal underplating has been seen in the form of a high seismic velocity layer and/or reflector at the lower crustal depths (Reddy et al., 2001). A surface wave dispersion study gave the average regional 1D velocity structure of the area (Singh et al., 1999). Regional earthquake tomography revealed that the deeper part of the lithosphere beneath the region is characterized by a marked low seismic velocity zone in the upper part of the mantle ( km) possibly related to the Deccan plume activity (Kennett and Widiyantoro, 1999). In this article, 3D V p and V p /V s models for the aftershock zone of the 2001 Bhuj earthquake have been obtained using Thurber s (SIMULPS12) travel-time tomography method. The inversion involved 5516 P travel times and 4061 S-P travel-time differences from 600 aftershocks recorded by 8 18 digital, three-component stations deployed in the aftershock zone over the time period 13 February 7 March Analysis of ray coverage and inversion of the synthetic data set showed that the models have high resolution down to a depth of 20 km. Further, an effort has been made to obtain

3 Results from Local Earthquake Velocity Tomography 635 a possible model for a lower crustal Bhuj earthquake in terms of velocity anomalies and seismotectonics of the region. Geology Geologically, Quaternary/Tertiary sediments, Deccan volcanic rocks, and Jurassic sandstones resting on Archean basement mainly characterize the region (Gupta et al., 2001). The surface geology of the 2001 earthquake epicentral region is constituted of Mesozoic (135 to 65 million year old) sediments ( 2 6 km thick) overlying an uplifted granitic basement (Gupta et al., 2001). The region lies just north of the basalt-covered areas of southern Kachchh, which are part of the 65- to 60-million-year-old Deccan traps, one of the largest volcanic provinces in the world. Major structural features of the Kachchh region include several east west trending faults/folds, as shown in Figure 1. The rift zone is bounded by a north-dipping Nagar Parkar fault in the north and a south-dipping Kathiawar fault in the south. Other major faults in the region are the east west trending Allah Bund fault, Island belt fault, KMF, and Katrol hill fault. Additionally, several northeast- and northwest-trending small faults/lineaments are also reported. A group of east west trending uplifts, namely Kachchh Mainland uplift, Pachham, Khadir Bela, Wagad, and Chorar uplifts, surrounded by depressions like the Banni plain and Rann of Kachchh, mainly characterizes the structural features of the Kachchh rift basin. The Bouguer gravity anomaly map of the area suggests that these uplifts are mainly associated with gravity highs and basement upwarping/volcanic plugs (Chandrasekhar and Mishra, 2002). Past Seismicity and Tectonics Large but infrequent earthquakes characterize the Kachchh region (Fig. 1). It has been inferred, based on the radiocarbon age data, that there is evidence of the occurrence of an earthquake in this region between A.D. 885 and 1035 (Rajendran and Rajendran, 2001). In 1668, a moderate earthquake occurred in the region with an epicenter at (24 N, 68 E). The really large one for the region was that of 6 June 1819, which killed 2000 people and threw up a 100-km ridge and created what is known as the Allah Bund, now in Gujarat (Rajendran and Rajendran, 2001). The moment magnitude of the 1819 earthquake was estimated to be 7.8 (Johnston, 1994). It is generally believed that the coseismic uplift evidenced by the Allah Bund is the expression of a fault; the block to the south of the Allah Bund is moving down with respect to the northern block (Gaur, 2001). However, based on observations in the shallow trenches across this structure, Rajendran and Rajendran (2001) suggested that this scarp may represent a fold, as evidenced by the morphological features such as the progressive migration of streams and formation of a lake at the down-dip side of the upthrust block. The Rann of Kachchh earthquake of 16 June 1819 (latitude N, longitude E, M W 7.8, and origin time 6:50 p.m.) took a toll of 1500 lives in Kachchh and 500 in Ahmedabad. Between 1821 and 1996, 16 moderate earthquakes of magnitude varying from 4.2 to 6.1 have occurred in this region (Rajendran and Rajendran, 2001). The last damaging earthquake in this region, prior to the 2001 Bhuj earthquake, was the Anjar earthquake of 21 July 1956 of moment magnitude 6.1, which caused 115 deaths. The focal mechanism of Kachchh earthquakes, such as the 1956 Anjar earthquake (Chung and Gao, 1995), the 1819 Rann of Kachchh earthquake (associated with the Allah Bund fault) (Gaur, 2001; Rajendran and Rajendran, 2001), and a few others, indicates reverse faulting, suggesting the presence of a compressional stress field, caused by the northward collision of the Indian plate with Eurasia since 40 million years ago. Nevertheless, the pre-existing normal faults associated with the possible plume related to Early Mesozoic rifting may be getting reactivated as reverse faults, suggesting inverse tectonics (Biswas, 1987; Chung and Gao, 1995; Rastogi et al., 2001). Source Parameters of the Mainshock The mainshock occurred at 23.4 N, E, which is approximately 4 km northwest of Bhachau, as shown in Figure 1 (Rastogi et al., 2001). This region is quite far from the active plate boundaries such as the Herat Chaman and Himalayan plate boundaries, which qualifies this earthquake as an intraplate earthquake. Such earthquakes are rare, accounting for less than 0.5% of global seismicity (Johnston, 1994). On the seismic zoning map of India (Gupta et al., 2001), the Kachchh region lies in zone V. The maximum damage was of MM intensity X in an area 100 km 70 km encompassing Bhuj, Bhachau, Anjar, and Gandhidham, as shown in Figure 2 (Rastogi et al., 2001). The zone of intense deformation of intensity XI (on the MM scale) is in an area of about 15 km 60 km. In this area, ground uplifts of 1 m or so extending hundreds of meters at several places (Budharmora, Chopadwa, and Lunwa), ground slumping (for example, at Amarsar lying closest to the epicenter), and ground cracks of over 1 m width and 5 m deep are noticed in Chopadwa (Fig. 2). Intensity X is assigned in an area of 100 km 70 km, which encompasses Bhuj, Bhachau, Rapar, Anjar, and Gandhidham. Coseismic ground deformations (lateral spreading/faulting) have been reported at two locations, namely Budharmora and Manfara. The earthquake was felt over a large area, extending as far as Chennai ( 2000 km) on the southeastern coast of the Indian peninsular shield (Rastogi et al., 2001). The earthquake occurred on a southdipping reverse fault (strike 65, dip 50, and slip 50 ) at23 km depth, as shown in Figure 1 (U.S. Geological Survey). The maximum dislocation, considering a southward-dipping fault plane, is estimated as about 8.5 m (Yagi and Kikuchi, 2001). Aftershock activity has been quite intense (Fig. 3a). Up until 15 August 2003, 12 aftershocks of M 5, about 170 aftershocks of M 4, and several thousand aftershocks of M 4 have occurred in the region. The latest M 5.0 af-

4 636 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 2. A map showing ground deformation, liquefaction sites, and intensity caused by the 2001 Bhuj mainshock. tershock occurred on 5 August Focal mechanism solutions of a few of the major aftershocks obtained from waveform inversion of broadband data indicate a dominant reverse fault movement on the NWF along east west trending south-dipping planes, as shown in Figure 3b (Mandal et al., 2003). The thrust faulting due to most of the aftershocks along the steeply dipping planes ( 45 ) can be explained in terms of movements along the adjustment thrusts. Focal mechanism solutions for some aftershocks show northwest southeast strike-slip movements along near-vertical planes near Manfara northeast of Bhachau ( N, E, as shown in Fig. 3b) that may be related to a probable northwest southeast striking rightlateral tear fault formed during the mainshock and named the Manfara fault (Srinivasan and Reddy, 2001; Wesnousky et al., 2001). Local Seismic Network and Data Since 4 February 2001, the National Geophysical Research Institute (NGRI), Hyderabad, India, has monitored the aftershock activity of the 2001 Bhuj earthquake, with a close digital network consisting of eight 24-bit recorders with an external hard disk (2 GB) and GPS timing system. Out of these, six were equipped with short-period seismometers (1 40 Hz) and two were equipped with broadband seismometers ( Hz). The distance between recording station and epicentral area varies from 14 to 90 km. The recording was done in a continuous mode at 100 samples per second. Some station locations were shifted during the study period, making a total of 11 station sites. Further, for the P-wave velocity tomographic experiment, we used data from the seismic network of the Center for Earthquake Research and Information, Memphis, Tennessee (USA) (from 13 to 27 February 2001) and Hirosaki University, Japan (from 28 February to 7 March 2001). Thus, we have used data from 18 stations for our tomographic study. One-Dimensional Travel-Time Inversion Over 10,000 aftershocks were recorded in the eightstation digital local seismic network of the NGRI, Hyderabad, during 13 February 7 March For the estimation of 1D velocity structure, we inverted the 5516 P travel times and 4061 S-P travel-time differences from 600 aftershocks recorded at 8 18 three-component digital stations (error in epicentral location [ERH] 0.5 km, error in focal depth [ERZ] 1 km; azimuthal gap between recording stations 180 and root mean square error in P-residual [rms] 0.1 sec). Estimated 1D velocity structure suggests an eight-

5 Results from Local Earthquake Velocity Tomography 637 Figure 3. (a) A histogram showing the decay of the frequency of occurrences of M 3 aftershocks with time (in years), where the x-axis represents the number of aftershock. (b) Spatial distribution of focal mechanism solutions determined from waveform inversion of broadband data of Gandhidham (GDM) and Chopadwa (CHP) stations (Figs. 1 and 5a) and local earthquake moment tensor inversion of P- and S-wave amplitudes and polarities. MNF and BDM represent Manfara and Budharmora, respectively. An unnamed fault mapped by Biswas (1987) in the Wagad uplift is inferred to be the probable eastward extension of NWF and marked by ENWF. The solution of the mainshock was obtained from centroid moment tensor inversion of eight broadband stations data of the India Meterological Department. layered crust beneath the region, which is constrained by integrated geophysical survey results and controlled source seismic results, as shown in Figure 4a (Kaila et al., 1981; Gupta et al., 2001). The integrated geophysical survey revealed a five-layered structure overlying the granitic basement (3 6 km), which consists of a high-velocity lead of Deccan basalts sandwiched between the Quaternary sediments and Jurassic sediments (Gupta et al., 2001). The Amreli controlled source seismic profile in the Saurashtra suggests two deeper crustal layers at 20.5 and 30 km depth (Kaila et al., 1981). The results obtained from controlled source seismic sounding close to the aftershock zone suggest a Moho depth of km and a variation from 37 to 45 km in most of the Kachchh region (Reddy et al., 2001). Further, the receiver transfer function analysis of teleseismic P waves recorded at the Bhuj station revealed a Moho thickness of 42 km and an upper mantle velocity of the order of 8.20 km/ sec (Kumar et al., 2001). The velocity model discussed earlier has been used as the initial model for our 1D travel-time inversion. The details of the 1D travel-time inversion can be found in Mandal et al. (2003). The new V p and V s models for the region obtained by them are shown in Table 1. This model suggests a significant increase in V p at 6 30 km depths ( 1.1 km/sec) and a decrease in V p at km depth (0.3 km/sec). This 1D model is used as an initial model for 3D tomographic inversion. Well-located aftershocks using 3D velocity structure (ERH 0.5 km, ERZ 1 km, and rms sec) define an east west trending plane extending from 10 to 40 km depth and covering an area of km 2 that dips toward the south at an angle of 45 (Fig. 4b,c). This could be a hidden fault beneath the Banni plains (of soft sediments) that may be extending in the east as an unnamed fault (ENWF) in the middle of the Wagad uplift as reported by Biswas (1987) (Fig. 1). Talwani and Gangopadhayay (2001) have expressed the possibility of such faults in the south of the Island belt fault. We interpret this south-dipping hidden fault as the NWF representing the fault plane of the mainshock (Figs. 1 and 3b). The majority of aftershocks are confined between the KMF and NWF (Fig. 4b,c). The orientations and depth extents of the South Wagad fault and KMF as given by Biswas and Khattri (2002) are not well established by any geophysical or seismological evidences, hence they are not shown in depth section (Fig. 4c). Nevertheless, there are some aftershocks that are off this trend, which can be explained in terms of sudden movements along adjustment thrusts due to the stress perturbation caused by the occurrence of the mainshock in the area. V p and V s Local Earthquake Tomography The source processes for large strain energy accumulation, which are enabling the stress regime of the Kachchh region to generate large intraplate earthquakes in the lower

6 638 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 4. (a) Epicenters of selected 600 aftershocks of M during 4 February 7 March A solid star shows the epicenter for the 2001 Bhuj mainshock. The inferred causative fault is shown by the dotted line and marked as NWF. Hypocentral depth plots of selected earthquakes in the (b) east-west direction and, (c) north-south direction. Geologically valid inferred fault trace is shown by dotted line and marked as NWF. Surface traces of KMF and SWF are marked by arrows (after Biswas, 1987). (d) Histrogram of focal depth distribution for 2001 Bhuj aftershocks. crust, remain unexplained. In order to obtain a better understanding of the processes involved in generating high strain energy, a 3D velocity (V p, V p /V s ) structure is determined using travel-time inversion, as described in the following section. For tomograpic inversion, 10,000 P and S arrival times from 600 well-located Bhuj aftershocks (with rms 0.01 sec) are used. We have applied the tomographic inversion method developed by Thurber (1983) and Eberhart-Phillips and Michael (1993) for the iterative damped least-squares inversion of P- and S-wave arrival time data for simultaneous estimation of a 3D crustal seismic velocity (V p and V p /V s ) structure and hypocentral parameters. Tomographic inversion mainly depends on the grid spacing, the damping values, the number of iterations, and the initial 1D velocity model. Our starting velocity model was obtained using a travel-time inversion, as discussed in the previous section. We investigated several nodal configurations for the estimation of the optimal velocity model parameterization. The grid configuration (with an origin at N and E) showed the most pronounced diagonal element resolution. It contains six layers of constant velocity; the tops of those layers occur at

7 Results from Local Earthquake Velocity Tomography 639 Table 1 1D Velocity Model for the Kachchh, Gujarat, Obtained from 1D Travel-Time Inversion Depth to the bottom of the layer (km) V p (km/sec) V s Poisson s (km/sec) V p /V s ratio , 3.0, 6.0, 20.0, 40.0, and km, with P-wave (Swave) velocities of 2.0 (1.12), 3.72 (2.09), 6.35 (3.57), 6.97 (3.92), 6.84 (3.84), and 8.2 (4.19) km/sec, respectively. This configuration is aligned parallel to the major tectonic features in the area (like the KMF, Katrol, NWF, Wagad uplift, Banni, and mainland where much of the seismicity is concentrated) covering a square area of 200 km 200 km in the east west and north south directions, respectively. The grid separation is 2 5 km within the seismic zone and km for the rest of the model volume (Fig. 5). In this study, blocks that are hit with a significant number of rays (85% 95%) are distributed in the aftershock zone, which seems to be controlled by the density of station distributions indicating a larger number of stations in the aftershock zone. For both V p and V s tomography, the number of hits per block in the upper 0 6 km is 90% 92% for the central aftershock zone, except near three corners (northeast, northwest, and southwest), whereas in the 6 40 km depths it is about 90% for the central aftershock zone, except near four corners (Figs. 6 and 7). However, the number of hits per block for the last layer (20 40 km) is quite small, except beneath the seismic zone, where it is about 90%. These maps provide an estimate of the reliability of the solution because resolution depends upon the number of rays that pass through each block. In this study, blocks that are hit with a significant number of rays are distributed in a circular region covering the main aftershock zone. The model resolution for the V p as well as V p /V s inversion is highest in the aftershock zone. In the 0 20 km depth range, the percentage of power in the main diagonal varies between 24% and 50% for the best-resolved part of the model (aftershock zone) and 5% to 20% for less-resolved portions. Nevertheless, the resolution of the main diagonal elements is quite poor (maximum 4% in the aftershock zone) in the km depth range, which is perhaps due to relatively poor ray coverage. Several tests are performed to check the reliability of our tomography, which are discussed later. In order to test the effect of variation of the depth distribution of grid points Figure 5. A plot showing the grid parameterization used for V p and V p /V s tomography.

8 640 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 6. P-wave observation matrices (hit maps) showing the number of rays passing through each block of the 3D model at 0 3 km, 3 6 km, 6 20 km, and km depths. on the final velocity model, a test inversion was made with a starting model in which the depths of the layers were assigned at 4, 8.0, 22.0, and 44.0 km. This model produced similar patterns of velocity anomalies, except at 44 km depth, where resolution is very poor. At different depth levels, that is, 4, 8, and 22 km depths, the change in velocity varies from 5% to 10%. In another test, a small change in the initial applied seismic velocity model (e.g., 10%) also had no significant effect on the final velocity values, that is, the final velocity model suggests a similar pattern with an 8% 10% increase in velocity in comparison to the tomograms obtained for the initial velocity model. Nevertheless, it gave rise to a change in depths of the event foci within 1.5 km as well as a change in epicentral location of the event within 0.5 km. In another test, with a view to check the effect of event distribution, a test inversion was performed using two different subsets of the data. One subset contains 290 events, and the other one consists of 310 aftershocks. Both subsets produced similar patterns of the seismic velocity perturbations with a variation of 2% 5% over the whole study area. Another test involved increasing or decreasing the horizontal area by 5%, which produced no effect on the event distribution and had a negligible effect ( 1%) on the final seismic velocity model. Finally, the checkerboard test is performed (Humpreys and Clayton, 1988). Our synthetic model is designed by assigning alternate high- ( 6%) and low-velocity ( 6%) perturbations to the 3D grids. Then travel time for this model is computed using 3D approximate raytracing method for the same configuration of stations, events, and ray paths as used in the field experiment. Finally, random errors with a standard deviation of 0.1 sec were added to the synthetic data. Grid spacing showed that the resolution is generally good over the entire region but was consistently superior in its central part. For V p tomography, the power of resolution for this checkerboard model suggests that 80% 90% and 90% 96% of the assumed velocity anomaly can be retrieved for the aftershock zone in the topmost (0 3 km) and upper crustal layers (3 20 km), respectively (Fig. 8a,b). However, for the surrounding regions of the aftershock zone, only 60% 70% of the assumed velocity is retrieved. The resolu-

9 Results from Local Earthquake Velocity Tomography 641 Figure 7. V p /V s observation matrices (hit maps) showing the number of rays passing through each block of the 3D model at 0 3 km, 3 6 km, 6 20 km, and km depths. tion in the last (20 40 km) crustal layers indicates a relatively poor resolution (70% 80%) for the main aftershock zone and a relatively poor resolution at the surrounding regions (Fig. 8c,d), while for V p /V s tomography, the checkerboard model suggests that 88% and 90% of the assumed velocity anomaly is retrieved for the topmost (0 3 km) and upper crustal layers (3 20 km), respectively (Fig. 9a,b). And, the resolution in the last (20 40 km) crustal layer indicates 60% 80% resolution for the main Bhuj aftershock zone (Fig. 9c,d). A similar test with 10-km grid spacing showed similar patterns of checkerboard with poor resolution. Tomography of Bhuj Aftershock Data Three-dimensional P and S velocity structures at 0 3, 3 6, 6 20, and km depths are obtained by using Thurber s (1983) tomographic inversion (SIMUL12) of 5516 travel times of P wave and 4061 S-P travel-time differences from 600 aftershocks recorded at 8 18 stations during 6 February 31 March 2002 (Figs. 10 and 11). V p and V p /V s values obtained from tomography were used to compute tomograms of V s and Poisson s ratio. In the upper 0 6 km depths beneath the aftershock zone, our tomographic results suggest an increase of 4% 6% in both V p and V s that is in good agreement with the areas of exposed Deccan basalts and rocky mainland of Kachchh (Figs. 2, 10a,b and 11a,b). Surrounding low-velocity zones (V p and V s ) in the 0 6 km depth range can be attributed to the Jurrasic and Tertiary sediments or water-saturated cracks as obtained by integrated geophysical survey results (Gupta et al., 2001). In the deeper 6 40 km depth range, results suggest on average an increase in V p (0.5% 2%) as well as V s (1.3% 3%) beneath the aftershock zone of 60 km 40 km encompassing Bhachau, Rapar, and Lodai (Figs. 10c,d and 11c,d). Most interestingly at 3 40 km depths, V p tomograms clearly brought out a highvelocity anomaly in the region northwest of the aftershock zone that is very well correlated with the location of the Patcham uplift/gravity high. This is not reflected in the V s

10 642 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 8. P-wave checkerboard results showing V p perturbation in percentage at (a) 0 3 km, (b) 3 6 km, (c) 6 20 km, and (d) km depths. tomograms. However at 6 40 km depth, a clear high V s anomaly is seen in the region northeast of the aftershock zone that correlates with the location of the Kadir uplift/ gravity high where V p tomograms also suggest a highvelocity anomaly in the 0 20 km depth range. These highvelocity bodies might be representing crustal intrusive as revealed by modeling of gravity and seismic data of the region (Reddy et al., 2001; Chandrasekar and Mishra, 2002). It will be important to note that majority of the aftershocks including the mainshock have occurred in a zone of low velocity (low V p, low V s ) in the km depth range, suggesting the key role played by the presence of fluids in generating these events. The east west sections of (V p, V s, and r) and north south depth sections of V p clearly delineate a bell-shaped high-velocity body beneath the aftershock zone characterized by upper mantle velocities at km depths (V p km/sec, V s km/sec, and r ) with a prominent head extending 60 km in the north south and 40 km in east west directions (Figs. 12 and 13). This body is observed to have a regional base extending over the entire study area of 100 km 100 km. Thus, this body may possibly represent a mafic pluton/frozen rift pillow from previous episodes of rifting ( 135 Ma). At the hypocentral depth of the mainshock (18 25 km), a zone of low velocity characterized by low V p ( km/sec), low V s ( km/sec), and large r ( ) is noticed (Fig. 12). This zone, extending 40 km in east west and 15 km in north south, is inferred to be a fluid-filled (trapped aqueous fluid released from metamorphic reactions) fractured rock matrix.

11 Results from Local Earthquake Velocity Tomography 643 Figure 9. V p /V s checkerboard results showing V p /V s perturbation in percentage at (a) 0 3 km, (b) 3 6 km, (c) 6 20 km, and (d) km depths. Depth Distribution of b-values The limitations of seismicity catalogs make it difficult to assess the joint magnitude, the spatial distribution of earthquakes across a broad magnitude range. The poor detection capability of the seismological networks often could not record all lower magnitude earthquakes, while the long recurrence intervals and short time span covered by catalogs force a poor sampling of large-magnitude earthquakes. Thus, parameterization of magnitude distribution of earthquakes is generally done using the Gutenberg Richter relationship (Gutenberg and Richter, 1944): log (N) a bm, (1) where N is the number of events, M is the magnitude, and a and b are constants. This parameterization makes it possible to assess the magnitude distribution beyond the empirically characterized segment of the distribution. The maximumlikelihood method is used to determine the b-value. The method gives a better estimate of the b-value than the leastsquares method, and the estimate is stable for the number of events exceeding 50. By this method, the b-value is defined as b log e/(m M ), 10 av c where M av is the average magnitude for each subset and M c is the lower magnitude limit used in the analysis. For b-value

12 644 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Figure 10. P-wave velocity tomograms estimated at (a) 0 3 km, (b) 3 6 km, (c) 6 20 km, and (d) km depths. GDM, RPR, ANJ, LIQ, BAU, and NWF represent the Gandhidham, Rapar, Anjar, Lodai, Bhachau, and North Wagad faults, respectively. study, an M 3 catalog of Bhuj aftershocks is designed using combined hypocenteral data from NGRI and the India Meteorological Department (IMD, 2002). Then, for the M 3 catalog of Bhuj aftershocks, the M c and b-value are estimated to be 3.0 with 90% confidence and , respectively (Fig. 14a,b). Further, a comparison of estimated b-values using the least-squares technique and maximumlikelihood method is shown in Figure 14b,c. The depth distribution of b-values is estimated using both maximumlikelihood and least-squares methods (Fig. 12c). The results indicate a marked increase in b-values in the depth range of km where the maximum occurrence of aftershocks is also noticed. This may be due to the increase in heterogeneity within that layer indicating the presence of more fractures (Aki, 1965; Wyss et al., 1997). Discussion Our tomographic results clearly delineate a regional high-velocity body beneath the aftershock zone at km depths existing in most of the 100 km 100 km area of Kachchh with a prominent head extending 60 km in the north south and 40 km in east west directions. This body is inferred to be a mafic pluton/rift pillow that might have intruded during the previous episodes of rifting ( 135 Ma). The controlled source seismics results in the area also confirm the underplating beneath the aftershock zone in the form of a high seismic velocity layer and/or reflector at the lower crustal depths (Reddy et al., 2001). Further, the gravity modeling along two profiles in the near vicinity of the aftershock zone revealed two regional intrusive bodies/mafic plutons associated with two marked gravity highs, namely the Patcham and Wagad uplifts (Chandrasekar and Misra, 2002). The gravity highs associated with the Patcham, Kadir, and Bela uplifts are circular, similar to those observed over volcanic plugs of Deccan traps in the Saurashtra (Chandrasekhar and Mishra, 2002). Presence of east west trending dykes about km long and patches of Deccan basalts have been reported in the Wagad uplit, evidencing the magmatic/volcanic activity in the region (Biswas and Desh-

13 Results from Local Earthquake Velocity Tomography 645 Figure 11. S-wave velocity tomograms estimated at (a) 0 3 km, (b) 3 6 km, (c) 6 20 km, and (d) km depths. GDM, RPR, ANJ, LIQ, BAU, and NWF represent the Gandhidham, Rapar, Anjar, Lodai, Bhachau, and north Wagad faults, respectively. pande, 1970). It is well known that the presence of this type of mafic pluton/frozen rift pillow could also explain the occurrence of large earthquakes in the lower crust beneath the intraplate regions (Kane, 1977; Cambell, 1978). A similar type of causative mechanism has already been proposed for lower crustal intraplate earthquakes like the New Madrid (USA) earthquakes (Pollitz et al., 2002), the 1997 Jabalpur (India) earthquake (Rajendran and Rajendran, 1998), and Amazonian earthquakes (Zoback and Richardson, 1996). The mafic pluton/rift pillow may be acting as an asperity in response to north-northeast compression due to northward movement of the Indian plate, which can explain the large strain accumulation at the 2001 Bhuj aftershock zone. Results also suggest a distinct low-velocity zone of 40 km 15 km within the mafic high-velocity body at the hypocentral depth of the mainshock (18 25 km). This zone can be interpreted as a highly fractured fluid-filled rock matrix. Further, the depth distribution of b-values suggests a marked increase in b-values in the depth range of km, indicating presence of more fractures. The heat flow values measured in Cambay and Rajasthan suggest a high heat flow of milliwatts/m 2 for the Kachchh area, indicating the crustal thermal state for the Mesozoic basin (Panda, 1985). Thus, the source of fluid at km depths could be of deeper origin representing trapped aqueous fluid released during metamorphism/partial melt intruded in the crust during plume activity. It is important to note here that the teleseismic and regional earthquake tomography showed low-velocity zones in the upper part of the mantle ( km), possibly indicating the presence of a plume head centered on the Cambay Kachchh region and the overlying continental lithosphere (Kennett and Widiyantoro, 1999). Hence, it is inferred that the presence of fluids (trapped aqueous fluids released from metamorphic reactions) at the hypocenteral depth of the mainshock may have further facilitated the occurrence of the 2001 Bhuj earthquake (Zhao et al., 1996).

14 646 P. Mandal, B. K. Rastogi, H. V. S. Satyanarayana, and M. Kousalya Conclusion Figure 12. Depth sections of V p, V s, and Poisson s ratio along three east west profiles across the aftershock zone (CC, DD, and EE as shown in Fig. 5): (a) V p along CC, (b) V p along DD, (c) V p along EE, (d) V s along CC, (e) V s along DD, (f) V s along EE, (g) r along CC, (h) r along DD, and (i) r along EE. Results suggest that the presence of a regional highvelocity body (characterized by high V p [ km/sec], high V s [ km/sec], and low r [ ]) beneath the aftershock zone of the 2001 Bhuj earthquake at km depths with a prominent head extending 60 km in the north south and 40 km in east west directions. This body with a base covering the whole study area (100 km 100 km) may be a mafic pluton/frozen rift pillow from previous episodes of rifting. This type of mafic pluton (probably acting as an asperity in response to the north-northeast compression of the Indian plate) seems to be contributing significantly in accumulating large crustal stresses that might be resulting in the generation of large lower crustal intraplate earthquakes in the Kachchh area. At the hypocentral depth of the mainshock ( km), a low-velocity zone within the mafic pluton is inferred to be a fluid-filled (aqueous fluid) fractured rock matrix. Hence, the large stress/strain accumulation associated with the mafic pluton/ridge pillow in response to the north-northeast compression of the Indian plate might have resulted in the occurrence of the 2001 Bhuj earthquake. Further, the weak decoupled fluid-filled lowvelocity zone (aqueous fluid/melt) within the mafic body/rift pillow (at km depth) might have enabled the prevailing north south regional compression to accelerate the reverse movement on the pre-existing NWF in the lower crust. Acknowledgments We are thankful to Dr. V. P. Dimri, Director, NGRI, for his encouragement and kind permission to publish this work. This study was supported by the Department of Science and Technology, New Delhi. We are grateful to CERI, Memphis, USA, and Hirosaki University of Japan for providing their aftershock data. References Aki, K. (1965). Maximum likelihood estimate of b in the formula log N a bm and its confidence limits, Bull. Earthquake Res. Inst. Tokyo Univ. 43, Biswas, S. K. (1987). Regional framework, structure, and evolution of the western marginal basins of India, Tectonophysics 135, Biswas, S. K., and S. V. Deshpande (1970). Geological and tectonic maps of Kachchh, Bull. Oil Natural Gas Comm. 7, Biswas, S. K., and K. N. Khattri (2002). A geological study of earthquakes in Kutch, Gujarat, India, J. Geol. Soc. India 60, Bureau of Indian Standards (2002). Criteria for earthquake resistant design of structures, fifth rev., 39 pp. Campbell, D. L. (1978). Investigation of the stress concentration mechanism for intraplate earthquakes, Geophys. Res. Lett. 5, Chandrasekhar, D. V., and D. C. Mishra (2002). Some geodynamic aspects of Kachchh basin and seismicity: an insight from gravity studies, Curr. Sci. 83, Chung, W.-Y., and H. Gao (1995). Source parameters of the Anjar earthquake of July 21, 1956, India, and its seismotectonic implications for the Kachchh rift basin, Tectonophysics 242, Eberhart-Phillips, D., and A. J. Michael (1993). Three-dimensional velocity structure in the Parkfield region, central California, J. Geophys. Res. 98, 15,737 15,758.

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