A possible physical mechanism for the unusually long sequence of seismic activity following the 2001 Bhuj Mw7.7 earthquake, Gujarat, India

The 2001 Bhuj earthquake took place in a region away from the active plate boundaries, which qualifies this earthquake to be a typical intraplate event that claimed a death toll of 20,000 people. The aftershock sequence of this earthquake is still continuing for the last eleven years, which makes this aftershock sequence as a unique intraplate earthquake sequence in the world. This sequence is being monitored by a close digital seismic network consisting of 5-12 three-component broadband seismographs and 10-20 accelerographs that led to a homogeneous earthquake catalog consisting of precise and accurate estimates of hypocentral and source parameters. This catalog enabled us to examine the evolution of this earthquake sequence. Our study reveals a few interconnected regularities in the rate of earthquake occurrence, b-values, fractal dimensions, stress drops, and the mean earthquake depth values. We observe that a slow decrease in background seismic activity and a number of bursts in seismic activity characterize the 2001 Bhuj earthquake sequence. We also notice that the background seismic activity is associated with a monotonic decrease in the mean earthquake depth values while the bursts in seismic activity are found to be associated with a decrease in fractal dimension, b-value and mean earthquake depth estimates, and an increase in stress drop values. We propose that these bursts in seismic activity are related to the episodes of break in deep fluid circulation in the direction of the Earth's surface. The effective background permeability of the mid-crust in the 2001 Bhuj earthquake region is evaluated to be about k approximate to 10(-13) m(2) based on the estimates of changes in mean earthquake depth. Thus, we infer that seismogenesis of the 2001 Bhuj earthquake sequence is probably connected with active deep fluid circulation underlying the Kachchh seismic zone. (C) 2012 Elsevier B.V. All rights reserved.