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

Abstract

Abstract—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.

Influence of Rock Watering on Post-Seismic Activity: A Study on the Khibiny Massif

“…Presently, the issue of the influence of watering of the rock on seismicity can be considered well-studied. There are experimental [Board et al, 1992;Kartseva et al, 2022] and field data [Hainzl et al, 2006[Hainzl et al, , 2013Heinicke et al, 2017;Maystrenko et al, 2020;Smirnov et al, 2022;Talwani, 1997;Vorobieva et al, 2020;Zoback and Harjes, 1997] on the influence of moisture content of the geologic material on the parameters of the seismic regime for natural (tectonic) and induced seismicity. The key result of these studies is the increase in seismicity during elevation of pore fluid pressure and the "lubrication" effect due to the increase in the amount of liquid penetrating into the medium caused either by the changes in the reservoir level [Smirnov et al, 2022;Talwani, 1997], or by heavy rainfalls [Hainzl et al, 2006[Hainzl et al, , 2013Heinicke et al, 2017;Maystrenko et al, 2020], or by snowmelt in spring [Zhukova et al, 2023], or by direct liquid injection for extraction of hydrocarbons [Vorobieva et al, 2020;Zoback and Harjes, 1997].…”

“…Porosity and moisture content of rock play a significant role in this process. At the same time, the increase in seismicity during the growth of moisture content is manifested in both the growth in the number of earthquakes and the reduction of the b-value of the Gutenberg -Richter magnitude distribution [Smirnov et al, 2013[Smirnov et al, , 2022Zhukova et al, 2023], which reflects an increase in the proportion of strong earthquakes.…”