Geological investigations at Killari and Ter, central India and implications for palaeoseismicity in the shield region

The 1993 Killari earthquake occurred in the central part of the Indian shield, an area generally believed to be of low seismogenic potential. This rare event provided an opportunity to understand the seismogenesis within the shield regions. Trench excavations in the rupture zone as well as other natural exposures in the vicinity of the epicentral region of the Killari earthquake indicate episodic activity separated by long periods. Deep drilling in the epicentral area recorded a maximum of ∼6 m of slip in the deeper strata ( Gupta et al., 1998a). Assuming an average of about 1 m slip for a moderate earthquake, as revealed by the 1993 event, accumulated slip indicates at least six moderate earthquakes at Killari. While the timing of the penultimate event may work out to be hundreds of thousands of years or more, evidence for at least one moderate earthquake ∼1500 yr ago was obtained from Ter, about 40 km northwest of Killari. Observations at Killari and Ter indicate the localization of deformation along a preexisting shear zone that are being reactivated in the current stress field. Because of factors like low background seismicity, long interseismic intervals and smaller ruptures compounded by low preservation of surface exposures, behaviour of seismogenic structures in the shield regions may appear to be random in space and time. Observations presented in this paper suggest that the perceived randomness probably holds only for individual patches in the current time window. Other parts of the seismogenic fault may behave independently, showing a different temporal pattern. Comprehensive investigations of the entire structure are needed to understand the long-term behaviour of such faults, and thereby improve the seismic hazard assessment in the cratonic interiors.