Heat flow measurements have been made in four boreholes in the epicentral area of the Latur earthquake (Mw 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 granite-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 (Mw 6.3, 1967) in the western part of the DVP, and at another locality, Lonar, 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 Dharwar 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 μW 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 Mw>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.
Sukanta Roy, R.U.M. Rao