Spatial variation in the crustal deformation pattern in the Koyna-Warna region, western India using shear wave splitting

Abstract

SUMMARY This study investigates crustal anisotropy beneath the Koyna-Warna region (KWR) using waveforms of local earthquakes of magnitude ML ≥ 1.0, recorded at 24 surface and 6 borehole seismic stations during September 2005–February 2020. Waveforms of 1402 local earthquakes that fulfil the splitting criteria (ic < 45°) are used for shear wave splitting analysis, yielding 738 measurements. The study examines the spatial, temporal and depth variations in the fast polarization azimuths (FPAs) and delay times (δt). The average delay times in the Koyna and Warna regions are 0.035 and 0.050 s, respectively, which did not show any significant correlation with depth. The FPAs are predominantly oriented in the NNW–SSE direction in the KWR, commensurate with the in situ maximum horizontal stress (SHmax) obtained using data from a 3-km-deep scientific borehole and individual focal mechanism solutions. The anisotropy beneath KWR is due to the alignment of fluid-filled cracks and pore throats along the SHmax, and is attributed to stress-induced anisotropy. A considerable variation in the orientation of FPAs from NW to NNW–SSE beneath KWR could be associated with heterogeneity in the lithological properties due to variation in the fracture orientation under regional stress. The non-parallel orientation of FPAs with SHmax or faults at some stations is attributed to structure-induced anisotropy, which seems to be associated with surface topography. Measurements at stations, even near the reservoirs, did not show any significant temporal variation in the splitting parameters due to the fluctuation in the water levels.