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