Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

The source parameters for 213 Bhuj aftershocks of moment magnitude varying from 2.16 to 5.74 have been estimated using the spectral analysis of the SH- waveform on the transverse component of the three-componnet digital seismograms as well as accelerograms. The estimated stress drop values for Bhuj aftershocks show more scatter (Mo^{0.5 to 1} ∞ Δσ) toward the larger seismic moment values (log Mo ≥ 10^14.5 N-m, larger aftershocks), whereas, they show a more systematic nature (Mo³ ∞ Δσ) for smaller seismic moment (log Mo < 10^14.5 N-m, smaller aftershocks) values. This size dependency of stress drop has also been seen from the relation between our estimated seismic moment and source radius, however, this size-dependent stress drop is not observed for the source parameter estimates for the other stable continental region earthquakes in India and around the world. The estimated seismic moment (Mo), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 2.16 to 5.74 range from 1.95 × 10¹² to 4.5 × 10¹⁷ N-m, 239 to 2835 m and 0.63 to 20.7 MPa, respectively. The near-surface attenuation factor (k) is found to be large of the order of 0.03 for the Kachchh region, suggesting thick low velocity sediments beneath the region. The estimated stress drop values show an increasing trend with the depth indicating the base of seismogenic layer (as characterized by larger stress drop values (>15 MPa)) lying in 22–26km depth range beneath the region. We suggest that the concentration of large stress drop values at 10–36km depth may be related to the large stress/strain associvated with a brittle, competent intrusive body of mafic nature.