FLUIDS BELOW THE HYPOCENTRAL REGION OF LATUR EARTHQUAKE, INDIA - GEOPHYSICAL INDICATORS

A set of geophysical measurements and observations was undertaken to i nvestigate the nature of the crust beneath the epicentral region of th e deadly Mw 6.1 Latur earthquake of September 30, 1993. With an estima ted focal depth of 2.6 km and the associated well defined but subtle s urface ruptures, it is a rare stable continental region (SCR) earthqua ke with surface rupture. The focal depth of 69 out of 73 well located aftershocks is less than 5.5 km. Broad band (10(3) - 10(-3) Hz) magnet otelluric (MT) soundings reveal the presence of an anomalously high co nductivity zone at a shallow depth range of 6-10 km. Consistent with t his result is the observation of a Pc phase, lagging behind the Pg pha se by about 0.6 to 0.8 sec in the aftershock seismograms indicating a low velocity layer (LVL) at 7 to 10 km depth. A Bouguer gravity low of 5 m.gal, nearly coincident with this feature, is also observed. Above evidences indicate that the focal zone of the Latur earthquake sequen ce is limited to depths of about 5 to 6 km in the upper crust by an un derlying low-velocity and high conductivity layer. We interpret this h igh conductive, low velocity layer as a fluid filled fractured rock ma trix. The inferred stress regime, including due to uplift of the Decca n Plateau, triggered by erosion of basalt cover is likely to be confin ed mostly in the upper part of the crust. Existence of a low velocity, high conductivity fluid filled layer will enhance stress concentratio n in the uppermost brittle part of the crust causing mechanical failur e.