Temporal variation of b value associated with M ∼4 earthquakes in the reservoir-triggered seismic environment of the Koyna–Warna region, Western India

Abstract

It is generally found that the b values associated with reservoir-triggered seismicity (RTS) are higher than the regional b values in the frequency magnitude relation of earthquakes. In the present study, temporal and spatial variation of b value is investigated using a catalog of 3,000 earthquakes from August 2005 through December 2010 for the Koyna–Warna region in Western India, which is a classical site of RTS globally. It is an isolated (30 × 20 km2) zone of seismicity where earthquakes of up to M ∼5 are found to occur during phases of loading and unloading of the Koyna and Warna reservoirs situated 25 km apart. For the Warna region, it is found that low b values of 0.6–0.9 are associated with earthquakes of M ∼4 during the loading phase. The percentage correlation of the occurrence of an M ≥ 4 earthquake with a low b value outside the 1σ or 2σ level is as high as 78 %. A drastic drop in the b value of about 50 % being reported for an RTS site may be an important precursory parameter for short-term earthquake forecast in the future.