Source parameters of the 2001 Mw 7.7 Bhuj earthquake, Gujarat, India, aftershock sequence

We present the estimated source parameters from SH-wave spectral modeling of selected 463 aftershocks (2002-06) of the 26 January 2001 Bhuj earthquake, the well-recorded largest continental intraplate earthquake. The estimated seismic moment (M-o), corner frequency (f(c)), source radius (r) and stress drop (Delta sigma) for aftershocks of moment magnitude 1.7 to 5.6 range from 3.55x10(11) to 2.84x10(17) N-m, 1.3 to 11.83 Hz, 107 to 1515 m and 0.13 to 26.7 MPa, respectively, while the errors in f(c) and Delta sigma are found to be 1.1 Hz and 1.1 MPa, respectively. We also notice that the near surface attenuation factor (k) values vary from 0.02 to 0.03. Our estimates reveal that the stress drop values show more scatter (Mo-0.5 to 1 is proportional to Delta sigma) toward the larger M-o values (a parts per thousand yen10(14.5) N-m), while they show a more systematic nature (Mo-3 is proportional to Delta sigma) for smaller M-o values (< 10(14.5) N-m), which can be explained as a consequence of a nearly constant rupture radius for smaller aftershocks in the region. The large stress drops (= 10 MPa) associated with events on the north Wagad fault (at 15-30 km depth) and Gedi fault (at 3-15 km depth) can be attributed to the large stress developed at hypocentral depths as a result of high fluid pressure and the presence of mafic intrusive bodies beneath these two fault zones.