Stress drop variations of triggered earthquakes at Koyna-Warna:
The spectral ratio technique (SRT) is implemented for the estimation of
source parameters of local earthquakes (ML 0.5-4.0) of
the Koyna–Warna region, western India. The SRT uses the concept of
empirical Green’s function and accounts for the path and site effects;
thus, it contributes to the optimal estimates of the source parameters.
Here, the corner frequencies and stress drops of a new earthquake
sequence that occurred during 1 May - 25 June 2017 were calculated and
compared with the stress drop behavior of the existing seismicity
cluster in the region. The dependency of stress drop with increasing
seismic moment of these earthquakes was also tested. A total of 689
P-wave spectra of earthquakes recorded by a short-period borehole and a
broadband surface seismic network are utilized for this purpose. It is
found that the corner frequency varies from 1 to 25 Hz and stress drops
vary from 0.01 to 14 MPa for earthquakes of both the clusters (Mahato
and Shashidhar, 2022).
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 (\(\Delta \sigma\) in MPa) estimates is obtained for the Koyna earthquakes
as: \(\log_{10}\Delta \sigma = 0.62 \log_{10}M_{o}-8.03\).
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 (\(\Delta \sigma\) and \(\sigma_{a}\) reveals a seismogenic zone
between 2.2 and 5.6 km depth, where most of the earthquakes (with large \(\Delta\sigma\) 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