Characterization of the causative fault system for the 2001 Bhuj earthquake of Mw 7.7

Abstract

Precise hypocenters (ERH<0.5 km, ERZ<1.0 km) of 600 aftershocks ( M w 2.0–5.3) delineate a east–west trending blind thrust dipping (∼45°) towards south (named as North Wagad Fault, NWF), about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake of M w 7.7. The aftershock zone involves a crustal volume of 60×40×35 km 3, lying between KMF and NWF. The waveform inversion of 45 aftershocks of M 3.0–5.3 suggest that the E–W trending south-dipping reverse faulting mainly characterizes the aftershock zone; however, some reverse faulting along NW–SE as well as NE–SW planes and some strike-slip faulting along NW–SE trending vertical plane are also noticed. The estimated P-axes point on an average towards N–S, while, T-axes orient in E–W agreeing well with the prevailing regional plate tectonic stress directions. The estimated velocity model from one-dimensional inversion of 8000 P and 5000 S travel times from 600 aftershocks deciphers a detailed crustal structure of the region. Upper 0–6 km, on an average suggests a low velocity zone characterizing the Jurassic and younger sediments. The depth range 6–42 km is characterized on an average by larger values of V p (6.31 to 6.98 km/s with an average of 6.71 km/s), V s (3.64 to 4.05 km/s with an average of 3.85 km/s) and V p/ V s (1.69 to 1.81 with an average of 1.75). The high crustal velocities can be attributed to the existence of a high velocity mafic intrusive/rift pillow structure beneath the region probably emplaced during the rifting time. This, brittle, competent high velocity zone beneath the epicentral area in response to the compression due to the northward movement of the Indian plate could induce sufficient local stress perturbation for generating large intraplate earthquakes of M w≥7.5 in the lower crust. Further within the overall higher velocity depth range a layer characterized by large values of V p (6.98 km/s), V s (3.854 km/s) and V p/ V s (1.81) is found in the depth range of 20.5–30 km, which is inferred to be fractured and saturated (high crack density and probably fluid-filled). Presence of numerous fractures in this layer is confirmed by a high b-value (0.75–1.1) in this layer. This layer might have facilitated the nucleation process of the 2001 Bhuj earthquake. The major rupture and 47% of the aftershocks have occurred within this layer.