The 2001 Bhuj earthquake: Tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation

The January 26, 2001 Bhuj earthquake (Mw 7.6) is one of the most catastrophic Indian earthquakes. We have investigated the 3-D seismic velocity and Poisson's ratio structures of the Bhuj source area to understand the probable cause of triggering the earthquake. We used 1948 P and 1865 S-wave high-quality arrival times from 331 aftershocks recorded at a temporary seismic network. Significant variations up to 5% in velocity and 10% in Poisson's ratio are revealed in the aftershock area. The mainshock is located in a distinctive zone characterized by high-Vp, low-Vs and high Poisson's ratio (σ) in the depth range of 20 to 30 km and extending 15 to 30 km laterally. This feature is very similar to that of the 1995 Kobe earthquake [Zhao et al., 1996]. The anomaly may be due to a fluid-filled, fractured rock matrix, which might have contributed to the initiation of the Bhuj earthquake.