An investigation into the Latur earthquake of September 29, 1993 in southern India

Abstract

The devastating ( M w 6.2) Latur earthquake of September 29, 1993 in South India has claimed an estimated 11.000 human lives. With an I max of 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–March 1993. Out of these, during October–November 1992, several shocks of M ≥ 2.0 were recorded at the NGRI seismic station at Hyderabad which is the closest (220 km) to the epicentre. No such shocks occurred for at least 8 months before the Latur earthquake. The aftershocks were monitored by a network of up to 21 stations between 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 the location of aftershocks in the first few days, a plane dipping at an angle of 45° towards the southwest and striking at 135° 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 surface rupture. Assuming the aftershock zone to be the rupture zone, the 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 concentration of helium in the soil in the immediate vicinity of the surface rupture indicating that the rupture extends to the surface from a depth of a few km. A detailed broadband magnetotelluric (MT) investigation revealed the presence of an anomalously high conductive zone at a depth 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 results. This highly conductive low-velocity layer is inferred to be fluid-filled. The main stress regime in Peninsular India is NE compressive stress due to plate tectonic movement. Erosion of the basalt cover in the Deccan Plateau may be adding additional compressive stress in the region. 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.