Geothermal investigations in the 1993 Latur earthquake area, Deccan Volcanic Province, India

Heat flow measurements have been made in four boreholes in the epicentral area of the Latur earthquake (M-w 6.2, 1993) in the southeastern part of the Cretaceous-Eocene Deccan Volcanic Province (DVP), central India. Three of these, located 4 to 8 km from the surface rupture zone near Killari, were about 180 m deep and yielded heat flow values in the range 33 to 40 mW m(-2). A hole was specially drilled in the surface rupture zone and penetrating the entire 338 m Deccan basalt cover and a further 271 m into the Archaean granite-gneiss basement. Temperature gradients over both the basaltic as well as the ganite-gneiss sections, which have a large thermal conductivity contrast, yielded a consistent heat flow value of 43 mW m(-2). Earlier measurements at Koyna, the site of another stable continental region (SCR) earthquake (M-w 6.3, 1967) in the western part of the DVP, and at another locality, Loner, in the central part of the DVP had yielded heat flow values of 41 mW m(-2) and 47 mW m(-2), respectively. Thus the southern part of the DVP is characterised by a low heat flow regime, similar to that over the Archaean Dhanwar province immediately to the south. The 617 m deep hole at Killari provided an opportunity for carrying out measurements of radioactive heat production on core samples of the 271 m long basement section. Thirty-nine core samples of the migmatitic gneisses and the granites encountered in the 271 m long basement section of KLR-1 were analysed for U, Th and K, and resulted in estimates of heat production of 0.5 and 2.6 mu W m(-3), respectively, for the two types. A crustal heat production model is envisaged, with the thicknesses of the gneisses and the granites constrained from available pressure estimates in the Dharwar craton, and also considering a 17 km 'granitic' upper crust and a 20 km 'granulitic' lower crust based on deep seismic sounding data. Using this crustal heat production model and the heat flow value of 43 mW m(-2), the estimated crustal temperatures imply a deep (>30 km) brittle-ductile transition. Even though heat flow-crustal temperature considerations allow for deeper, crustal earthquake foci, most of the M-w > 6 events of the last three decades in the Australian and Indian SCRs show shallow foci (<10 km). This calls for seeking possible mechanisms for frictional failure to take place in the top part of a crust which is largely brittle. (C) 1999 Elsevier Science B.V. All rights reserved.