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

摘要

We present 1-D models of crustal P and S wave velocity structure in the epicentral region of the devastating 1993 Latur earthquake in southern India. Travel time and relative amplitude modelling of a seismogram section of well located aftershocks out to 80 km offset, by reflectivity synthetic seismograms, reveal alternating low-velocity layers (LVLs) for P and S waves in the upper crust at depths of 6.5-9.0 km and 12.3-14.5 km with about 7% velocity reduction. A lower crustal LVL at 24-26 km depth is also inferred by modelling. The seismic activity in the Latur region is essentially confined to 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 high-conductivity zone at a shallow depth of 6-10 km brought out by broad band MT soundings in this region, is found to be coincident with the upper crustal LVLs. These coincident anomalous properties of the upper crust suggest the presence of fluids (probably in a partially- or slightly under-saturated state) below the hypocentral region of the Latur earthquake and its aftershocks.