Heat flow and thermal structure in the Koyna seismic zone, western India: Implications for recurrent reservoir triggered seismicity

Abstract

Scientific core drilling to a depth of 1522 m in the Koyna seismic zone, western India, provides a rare opportunity to characterize the geothermal regime and study its implications on seismogenesis. The borehole KBH-1 penetrated the 932.5 m thick Deccan Trap and continued for 589.5 m in the granite-gneiss basement. The equilibrium temperature profile was obtained from measurements made 10 months after drilling. Thermal conductivity was measured on core samples of Deccan basalt and basement granitoids. Radiogenic heat production of the basement granitoids was estimated from U, Th and K analyses. Salient results are as follows. (i) The temperature gradients in Deccan basalt and granite-gneiss basement, 26.5 mKm−1 and 15.7 mKm−1, respectively, combined with thermal conductivity measurements for the two rock types, yield a mean heat flow of 43 mW m−2. (ii) The low heat flow in Koyna is similar to values characteristic of Archaean cratons. (iii) Mean Th concentration of 6.8 ppm, U 0.9 ppm, and K 1.8% yielded a mean radiogenic heat production of 0.9 μWm−3 for the basement granitoids. (iv) New geological and geothermal constraints lead to improved estimates of temperature in the crust. Although geothermal considerations support a ∼ 25 km deep seismic-aseismic transition, the seismic activity is limited to the uppermost 10 km. (v) The infrequent occurrence of earthquakes below ∼10 km could be attributed to the low permeability of the basement granitoids, which reduces the influence of pore fluids in triggering failures along faults at deeper levels.