Simultaneous estimation of site response and source parameters of reservoir-triggered earthquakes using data from the borehole seismic network in the Koyna–Warna seismic zone, Maharashtra, India

The horizontal components of S-wave data from 25 events (Mw 2.9–4.2) recorded at five borehole seismograph sites in the Koyna–Warna seismic zone, Maharashtra, India, were analysed to estimate the site response (SR) and source parameters. In the 0.2–1.8 Hz frequency band, the maximum SR value (~ 1.2) is modelled at FBH site, lying in the region west of Warna reservoir. In the 3–7 Hz frequency band, the average SR estimates suggest a maximum value of 1.6 at UBH site (near the Warna reservoir) and a minimum value of 0.8 at PBH site (showing localized de-amplification) on the eastern side of the Donachiwada fault. While at RBH site (near the Koyna reservoir), SR at 3–7 Hz shows a value of 1.3. At FBH site, average site response values at both frequency bands reveal higher values of 1.2–1.5, lying in the NNE-SSW trending Koyna seismic zone where the 1967 Mw6.3 Koyna earthquake took place. The modelled seismic moment (Mo) and source radius (r) of earthquakes are in between 3.0 × 1013 and 2.5 × 1015 N-m and 175 to 364 m, respectively, while stress drops (Δσ) vary from 1.9 to 34.2 MPa. The estimated relation Mofc2.9 = 5.82 × 1016 Nm/s3 between the moment and corner frequency agrees well with the relation for other seismically active areas in the world. The modelled stress drops and seismic moment are found to satisfy the following relation: $$\log_{10} \Delta \sigma \left( {MPa} \right) \, = 0.62 \, \log_{10} M_{o} \left( {N\! - \!m} \right) \, {-} \, 8.03$$ log 10 Δ σ MPa = 0.62 log 10 M o N - m - 8.03 . Our modelling suggests that most of the Koyna–Warna earthquakes (21 out of 25) follow the frictional overshoot rupture model while a few (4 out of 25) Koyna–Warna events satisfy the partial stress-drop rupture model.