Crustal seismic velocity structure in the epicentral region of the Latur earthquake (September 29, 1993), southern India: inferences from modelling of the aftershock seismograms

We present 1-D models of crustal P and S wave velocity structure in the epi central region of the devastating 1993 Latur earthquake in southern India. Travel time and relative amplitude modelling of a seismogram section of wel l located aftershocks out to 80 km offset, by reflectivity synthetic seismo grams, reveal alternating low-velocity layers (LVLs) for P and S waves in t he upper crust at depths of 6.5-9.0 km and 12.3-14.5 km with about 7% veloc ity reduction. A lower crustal LVL at 24-26 km depth is also inferred by mo delling. The seismic activity in the Latur region is essentially confined t o shallow upper crustal depths of less than 6 km, above the LVLs. The upper crustal velocity models further reveal a relatively low V-p/ V-s ratio of 1.65, while synthetic seismogram models consistent with the upper crustal P wave reflection phases reveal a Q(p)/Q(s) ratio certainly less than 1. A h igh-conductivity zone at a shallow depth of 6-10 km brought out by broad ba nd MT soundings in this region, is found to be coincident with the upper cr ustal LVLs. These coincident anomalous properties of the upper crust sugges t the presence of fluids (probably in a partially- or slightly under-satura ted state) below the hypocentral region of the Latur earthquake and its aft ershocks. (C) 1999 Elsevier Science B.V. All rights reserved.