AN INVESTIGATION INTO THE LATUR EARTHQUAKE OF SEPTEMBER 29, 1993 IN SOUTHERN INDIA

The devastating (M-w 6.2) Latur earthquake of September 29, 1993 in So uth India has claimed an estimated 11,000 human lives. With an I-max o f VIII, the earthquake was felt to an average distance of 750 km. More than 125 shocks were reported to have been felt during August 1992-Ma rch 1993. Out of these,during October-November 1992, several shocks of M greater than or equal to 2.0 were recorded at the NGRI seismic stat ion at Hyderabad which is the closest (220 km) to the epicentre. No su ch shocks occurred for at least 8 months before the Latur earthquake. The aftershocks were monitored by a network of up to 21 stations betwe en October 8, 1993 and January 31, 1994. A majority of the aftershocks occurred within a 10-km radius from the main shock. On the basis of t he location of aftershocks in the first few days, a plane dipping at a n angle of 45 degrees towards the southwest and striking at 135 degree s is inferred to be the fault plane which extends to a depth of 4.5 km and on projection meets the surface in the vicinity of the observed s urface rupture. Assuming the aftershock zone to be the rupture zone, t he stress drop is estimated to be 7 MPa with a maximum displacement of 1.7 m for the main earthquake. A unique discovery is the high concent ration of helium in the soil in the immediate vicinity of the surface rupture indicating that the rupture extends to the surface from a dept h of a few km. A detailed broadband magnetotelluric (MT) investigation revealed the presence of an anomalously high conductive zone at a dep th of 6-10 km. Observation of a Pc phase, lagging behind the Pg phase by about 0.6 to 0.8 s in the seismograms of aftershocks, indicating a low-velocity zone at 7 to 10 km depth, is consistent with the MT resul ts. This highly conductive low-velocity layer is inferred to be fluid- filled. The main stress regime in Peninsular India is NE compressive s tress due to plate tectonic movement. Erosion of the basalt cover in t he Deccan Plateau may be adding additional compressive stress in the r egion. The existence of a low-velocity highly conductive fluid-filled layer will enhance stress concentrations in the uppermost brittle part of the crust causing the earthquake. (C) 1998 Elsevier Science B.V. A ll rights reserved.