Source parameters and tectonic implications of aftershocks of the Mw 7.6 Bhuj earthquake of 26 January 2001

We present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period. The data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock. We have calculated high-quality single-event locations, based on a ID velocity model determined for the region for earthquakes with magnitudes between similar to2 and 5. Aftershock locations reveal activity concentrated on a nearly east-striking, south-dipping plane, trapezoidal in outline. The active zone tapers from about 45 km along strike at the shallow end, which is about 5 kin deep, to no more than 25 km long at a depth of 35 km. The total rupture area was about 1300 km(2). We estimate the static stress drop of the mainshock at 16 +/- 2 MPa. Aftershocks extend nearly through the entire crust, with concentrations in the lower crust at about 26 kin and in the upper crust at about 10 km. The fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake. The aftershock data are consistent with the Bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift.