Results from local earthquake velocity tomography; implications toward the source process involved in generating the 2001 earthquake in the lower crust beneath Kachchh (India)

To comprehend the source processes of the 26 January 2001 Bhuj earthquake sequence of M (sub 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 ( nearly equal 45 degrees ) 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 (sub w) > or =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 (sub p) [7.0-8.5 km/sec], high V (sub s) [4.0-4.8 km/sec], and low sigma [0.24-0.26]) with a head extending 60 km in north-south and 40 km in east-west at 10-40 km depths. This high-velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time ( approximately 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 (sub p) [6.5-7.0 km/sec], low V (sub s) [3.6-4.0 km/sec], large sigma [0.26-0.265]) within the mafic body at the hypocentral depth of the mainshock ( approximately 18-25 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.