Seasonal Variations in the b-Value of the Reservoir-Triggered Seismicity in the Koyna-Warna Region, Western India

The analysis of the local earthquake catalog revealed fine features in the behavior of seasonal components of induced seismicity in the Koyna-Warna region, western India. The seasonal variations in seismicity associated with annual fluctuations in water levels in the Koyna and Warna reservoirs have local maxima during the year, corresponding to the known mechanisms of immediate and delayed responses of reservoir-triggered seismicity (RTS). The superposed epoch analysis combined with the stochastic modeling of random earthquake catalogs revealed regular changes in the b-value of the frequency-magnitude relationship of the earthquakes within the annual cycle of seasonal seismicity fluctuations. The minimum b-values fall in the intervals of the maxima of both the immediate and delayed seismic responses. The maximum b-values are observed during the rise and fall phases of water levels in the reservoirs. The pattern of changes in the seismicity together with the b-value behavior at the activation phase of the immediate RTS response is consistent with the scenario of gradual redistribution of the failure process from the lower to higher scales, characteristic of the activation of transient seismicity. A pilot laboratory experiment on cyclic initiation of fracture process by pore pressure in a granite sample extracted from a well in the Koyna-Warna region of reservoir-triggered seismicity demonstrated the patterns of changes in the acoustic regime similar to the seasonal features of the Koyna-Warna region seismicity.