Low coda Qc in the epicentral region of the 2001 Bhuj earthquake of Mw 7.7

On 26 January, 2001 (03:46:55,UT) a devastating intraplate earthquake of M-w 7.7 occurred in a region about 5 km NW of Bhachau, Gujarat (23.42degreesN, 70.23degreesE). The epicentral distribution of aftershocks defines a marked concentration along an E-W trending and southerly dipping (approximate to45degrees) zone covering an area of (60 x 40) km(2). The presence of high seismicity including two earthquakes of magnitudes exceeding 7.7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the Kutch area; a site lying in the seismic zone V (zone of the highest seismicity for potentially M8 earthquakes) on the seismic zoning map of India. Attenuation property of the medium around the epicentral area of the Bhuj earthquake covering a circular area of 61,500 km(2) with a radius of 140 km is studied by estimating the coda-Q(c) from 200 local earthquakes of magnitudes varying from 3.0-4.6. The estimated Q(0) values at locations in the aftershock zone (high seismicity) are found to be low in comparison to areas at a distance from it. This can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone. Distribution of Q(0) values suggests that the local variation in Q(0) values is probably controlled by local geology. The estimated Q(0) values at different stations suggest a low value of Q=(102 +/- 0.80)*f((0.98 +/- 0.02)) indicating an attenuative crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q(c) at lower frequencies (1-3 Hz) that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and/or intrinsic attenuation due to fluid movement in the fault zone and fluid-filled cracks. The large Q(c) at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected. Based on the attenuation curve estimated for Q(0)=102, the ground acceleration at 240 km distance is 13% of 1 g i.e., 0.13 g agreeing well with the ground acceleration recorded by an accelerograph at Ahmedabad (0.11 g). Hence, it is inferred that the Q(0) value obtained from this study seems to be apt for prediction of ground motion for the region.