Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: a fluid-driven earthquake?

Abstract

Quantitative estimates of the crack density ( ϵ), saturation rate ( ξ) and porosity ( ψ) parameters from seismic velocities ( V p, V s) and Poisson’s ratio ( σ) in the 2001 Bhuj earthquake area in western India indicate that the 2001 Bhuj earthquake hypocenter is associated with high- ϵ, high- ξ and high- ψ in the depth range of about 23–28 km, extending 15–30 km laterally. These anomalies may be due to a fluid-filled, fractured rock matrix, which might have contributed to trigger the 2001 Bhuj earthquake in the intraplate, stable continental region of the Indian Peninsula. This feature is similar to that of the 1995 Kobe earthquake [D. Zhao et al., Science 274 (1996) 1891–1894]. High- ψ areas are generally consistent with high- ϵ areas, but high- ξ areas have a wider distribution, indicating that micro-cracks exist in more localized areas within the crust, and that permeation of fluids in the hypocenter zone might have occurred extensively through the intergranular and fractured pores due to hidden intersecting fault geometry. Here we suggest the possibility that earthquake occurrence is closely related to in situ material heterogeneities, rather than stress conditions alone.