Abstract We present the first sub-basalt 3D local earthquake tomography of the Koyna seismic zone, western India, using travel-time data from 5325 local earthquakes recorded by a network of eight deep borehole seismometers. These seismometers were installed below the Deccan basalts and within the underlying granite basement. The resulting VP and VP/VS models reveal a well-defined image of the Donachiwada fault (DF), the plausible source of the 10 December 1967 (M 6.3) earthquake—the largest reservoir-triggered event documented globally. These models show clear variations in VP and VP/VS values, from 1 to ∼9 km depth, across the DF. At depths (5–9 km), the deeper anomalies spatially correlate with the main zone of seismicity along the DF. These results provide one of the first seismic image of the DF beneath the Deccan basalt and highlights the value of borehole instrumentation for resolving fault-zone structure in basalt-covered regions. The depth extent and seismic signature of the fault zone may have implications for understanding intraplate faulting processes and the long-term persistence of reservoir-triggered seismicity in the Koyna region.
Shashidhar et al. (Fri,) studied this question.