8. Site response and source parameters of RTS earthquakes using borehole seismic data:
A linear scaling between seismic moment (\(M_{o}\), in N-m) and stress drop (\(\sigma\) in MPa) estimates is obtained for the Koyna earthquakes as: \(\operatorname{}{\sigma}=0.62\ \operatorname{}{M_{o}-8.03}\). The maximum \(\sigma\) of 34.2 MPa is modeled at 0.1 km depth for an event of Mw 4.09, while the largest apparent stress (\(\sigma_{a}\)) of 43.3 MPa is modeled at 4 km for an event of Mw 3.16. The hypocentral depth plot of \(\sigma\) and \(\sigma_{a}\) reveals a seismogenic zone between 2.2 and 5.6 km depth, where most of the earthquakes (with large\(\sigma\) and \(\sigma_{a}\)) have occurred. This could be attributed to the large stress/stain (associated with brittle, competent rocks) and high pore-fluid pressure (water saturated zone at hypocentral depths) beneath the Koyna seismic zone. The results suggest that most of the small Koyna events (Mw 2.9–4.2) satisfy the frictional overshoot stress-drop rupture model while only a few events follow the partial stress-drop rupture model. The reservoir-triggered Koyna earthquakes are associated with smaller stress drops in comparison with the rift-associated earthquakes in Kachchh, Gujarat and NSL (Mandal et al., 2021a).

8. Environmental Seismology and Earthquake Precursory Studies

8.1 Environmental Seismology