Occurrences of large-magnitude earthquakes in the Kachchh region, Gujarat, western India: Tectonic implications

- The 2001 Bhuj earthquake originated at > 25 km depth intraplate setting.
- Analysis of aftershock data for 12 years show its fault-bound nature.
- A strain-hardening mechanism is proposed for this deep crustal event.

Moderate-to-large damaging earthquakes in the peninsular part of the Indian plate do not support the long-standing belief of the seismic stability of this region. The historical record shows that about 15 damaging earthquakes with magnitudes from 5.5 to ~ 8.0 occurred in the Indian peninsula. Most of these events were associated with the old rift systems. Our analysis of the 2001 Bhuj earthquake and its 12-year aftershock sequence indicates a seismic zone bound by two linear trends (NNW and NNE) that intersect an E-W-trending graben. The Bouguer gravity values near the epicentre of the Bhuj earthquake are relatively low (~ 2 mgal). The gravity anomaly maps, the distribution of earthquake epicentres, and the crustal strain-rate patterns indicate that the 2001 Bhuj earthquake occurred along a fault within strain-hardened mid-crustal rocks. The collision resistance between the Indian plate and the Eurasian plate along the Himalayas and anticlockwise rotation of the Indian plate provide the far-field stresses that concentrate within a fault-bounded block close to the western margin of the Indian plate and is periodically released during earthquakes, such as the 2001 MW 7.7 Bhuj earthquake. We propose that the moderate-to-large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain-hardened environment.