Abstract

The generalized inversion of S-wave spectral amplitude data from 18 strong-motion sites in the Kachchh seismic zone, Gujarat, India, has been carried out to estimate source parameters of 38 aftershocks (Mw 2.93–5.32) from the 2001 Bhuj earthquake (Mw 7.7). The result shows that the seismic moment M0 and source radius r of aftershocks are between 3.1×1013 and forumla and 226–889 m, respectively, while stress drops (Δσ) vary from 0.11 to 7.44 MPa. The regression analysis between M0 and r shows a break in linear scaling at forumla. The estimated stress-drop values scatter significantly with M0 (forumla) for larger aftershocks (forumla) but vary in proportion with M0 (forumla) in case of smaller magnitude events. This can be due to the complex rupture process associated with relatively larger aftershocks in comparison to circular rupture for smaller events. The high stress-drop values are observed for events located on the north Wagad fault (at 15–30 km depths) and Gedi fault (at 0–5 km depths). This could be attributed to the large stress develop at the hypocentral depth as a result of high pore-fluid pressure beneath these two fault zones. The regression between M0 and corner frequency fc shows M0 is proportional to forumla, which is consistent for earthquakes with short rupture lengths and high stress-drop values. High stress-drop values commonly associated with the events from Kachchh and adjacent Narmada-Son lineament rift zones indicate strong potential for generating high peak ground accelerations and hence high seismic hazard.

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