Structure of the Koyna‐Warna Seismic Zone, Maharashtra, India: A possible model for large induced earthquakes elsewhere

Abstract

AbstractThe Koyna‐Warna area of India is one of the best worldwide examples of reservoir‐induced seismicity, with the distinction of having generated the largest known induced earthquake (M6.3 on 10 December 1967) and persistent moderate‐magnitude (>M5) events for nearly 50 years. Yet, the fault structure and tectonic setting that has accommodated the induced seismicity is poorly known, in part because the seismic events occur beneath a thick sequence of basalt layers. On the basis of the alignment of earthquake epicenters over an ~50 year period, lateral variations in focal mechanisms, upper‐crustal tomographic velocity images, geophysical data (aeromagnetic, gravity, and magnetotelluric), geomorphic data, and correlation with similar structures elsewhere, we suggest that the Koyna‐Warna area lies within a right step between northwest trending, right‐lateral faults. The sub‐basalt basement may form a local structural depression (pull‐apart basin) caused by extension within the step‐over zone between the right‐lateral faults. Our postulated model accounts for the observed pattern of normal faulting in a region that is dominated by north‐south directed compression. The right‐lateral faults extend well beyond the immediate Koyna‐Warna area, possibly suggesting a more extensive zone of seismic hazards for the central India area. Induced seismic events have been observed many places worldwide, but relatively large‐magnitude induced events are less common because critically stressed, preexisting structures are a necessary component. We suggest that releasing bends and fault step‐overs like those we postulate for the Koyna‐Warna area may serve as an ideal tectonic environment for generating moderate‐ to large‐ magnitude induced (reservoir, injection, etc.) earthquakes.