Surface loading and triggered earthquakes in the Koyna–Warna region, western India

Abstract

Earthquakes of M≥5.0 continue to occur in the vicinity of Koyna reservoir in western India, the largest known case of Reservoir Triggered Earthquakes (RTS). The region remains seismically active even after four decades of the impounding of the Koyna reservoir. The earthquakes occur repeatedly along two major fault systems in NE–SW and NW–SE directions. We performed cross-correlation analysis between time series of the Koyna reservoir levels and the strain factor (energy 1/2) calculated for earthquakes of M≥3.0 occurring in the region during 1963–1999. Four time windows of 5 years or more are selected for cross-correlation to predict the periodicities, if any, in the seismic energy release caused by the annual reservoir water level fluctuations. A similar analysis was performed for the newly-impounded Warna reservoir, 25 km south of the Koyna dam. Our results suggest that the initial seismicity in the Koyna region during 1963 was triggered after the region attained steady state pore pressure by diffusion processes, particularly occurring along vertical strike-slip faults. Subsequently, major episodes of earthquake energy release until 1999 show a periodic behavior related to the annual filling of both the Koyna and Warna reservoirs. Two stages of earthquake energy release are evident till 1996 and coincide with annual filling and draining cycles of the reservoirs. Since 1996, the seismic energy release episodes correlate mostly with the draining cycle of the reservoir levels indicating a shift in the present day earthquake activity in the region, which may be due to a combined effect of the Koyna and Warna reservoirs. To explain the causal relationship between the reservoir water level fluctuations at the surface and earthquakes at hypocentral depths in terms of diffusion processes, we modeled the pore pressure front diffusion with time, in an inhomogeneous medium. It is seen that a water level change of the order of 1 m in 5 days in the surface loading can propagate 5–15% of pore pressure front, corresponding to 0.75–2.25 bar, to the hypocentral depth of 6–8 km in the presence of a vertical conducting fault. These small stress perturbations are sufficient to trigger seismicity on pre-existing, critically stressed faults in the Koyna–Warna region.