The 1993 Killari earthquake in central India: A new fault in Mesozoic basalt flows?

Abstract

1. Introduction In 1993 the Killari fault ruptured the surface of the Deccan Plateau in a Mw6.1/Ms6.4 earthquake that caused one of the highest death and damage tolls of any known intraplate earthquake. Mainshock and aftershock data show that the fault dips about 45 ø southwest and has a reverse mainshock slip of about 1.5 m [Baumbach et al., 1994; Seeber et al., 1996]. Before reaching the surface, the fault cuts through a =400-m stack of Deccan Traps. These-60-Myr-old basalt flows are nearly horizontal: they offer marker horizons to resolve deformation accumulated since their emplacement and suggest very low tectonic rates. Accordingly, earthquake hazard at Killari was thought to be low. The 1993 earthquake offers unique opportunities for understanding the behavior of stable continental regions (SCR) sources and for evaluating hazard mitigation strategies. A debate on this important event is likely to stimulate further research and to raise interest in hazard reduction for SCR. I welcome the comments by Rajendran and Rajendran [this issue] (hereinafter referred to as R1) and the opportunity to revisit our results and hypotheses [Seeber et al., 1993, 1996; Jain et al., 1994; Seeber, 1994]. In the following discussion, I quote from Seeber et al. [1996] (S1) and from both R1 and Rajendran et al. [1996] (R2), which R1 often refer to for evidence. R1 and R2 present paleoseismic results and other data that they argue is contrary to S l's hypotheses of a new fault in the paleoseismically sampled superficial basalt layer and of reservoir-triggered seismicity in Killari. In responding, I first point out where they misinterpreted our statements concerning the Killari seismicity and tectonics, and then I analyze critically their data and arguments. While the results presented in R1 and R2 raise some intriguing possibilities, they fail to change my views and conclusions as presented in S1. Preliminary results from ongoing borehole investigations in Killari by the National Geophysical Research Institute [Gupta and Dwivedy, 1996] show that the vertical separation accumulated during the Cenozoic on the causative fault at Killari is small, a few meters at most. This supports S1 conclusion that the source of the Killari mainshock is a small-displacement fault that had no discernable surface expression before 1993. Small displacement is a characteristic of seismogenic SCR faults that has fundamental implications for intraplate tectonic processes as well as practical consequences regarding approaches to identify potentially seismogenic faults and map earthquake hazard in SCR [e.g., Seeber and Armbruster, 1993]. Regarding the notion of a new fault, R1 state that "although the authors [of S1] are not explicit about their arguments, they conclude that the Killari earthquake was generated on a new fault in the Mesozoic basalt flows." This is incorrect on at

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