Variations in the frequency–magnitude distribution of earthquakes with depth in the Koyna–Warna region, India

We map the earthquake frequency–magnitude distribution (b value) as a function of depth in the Reservoir Triggered Seismicity (RTS) region in Koyna–Warna region, in Western India. About 1600 well-located events recorded during 1996–2005 in the Koyna region are selected for the analysis. The earthquake data sets are separated in 2 km zones from 0 to 12 km depth. The result shows a systematic decrease in b value up to 8 km followed by an increase. The increase in b value is interpreted to be caused by the presence of fluids. We also investigate the spatial distribution of b value with depth. Significant variations in the b value are detected, with b ranging from b ≈ 0.7 to 1.8. Low b value areas at 5 km depth indicate localized high stresses which are favorable for future rupture.