Koyna earthqrake of December 11, 1967

Abstract

The earthquake at koyna, india, on december 11, 1967, occurred in an area which had no previous recorded seismic activity. Because the epicenter was under the newly filled koyna dam and reservoir, the possibility of a causal relation was investigated. Because small shocks were felt early in the filling of the reservoir, 2 seismographs, 2 accelerographs, and 2 tiltgraphs were installed in the dam, and an array of seismographs 20-40 miles from the dam. Bores near the dam revealed no major near-surface faults in the nearly horizontal deccan basalts and volcanic rocks of the area. The early tremors were mild, shallow, centered upstream, and are thought to be evidence of crustal adjustments caused by filling the reservoir. The major shocks are deeper and stronger. The body of the dam was accelerated at 0.42 g along its axis, 0.39 g across the long axis, and 0.34 g vertically. The epicenter was 30 km deep, just downstream from the dam. Slippage probably occurred along the malabar fault on india's west coast. It is concluded that the major shocks are of tectonic origin and that major structural design for the deccan plateau should take the possibility of future earthquakes of large magnitudes into account.