Source parameters of two rift-associated intraplate earthquakes in peninsular India: the Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970

Abstract

The Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970 are two major intraplate earthquakes associated with paleorift zones in the stable continental region of peninsular India. These two events shook very large portions of the peninsula, and casualties were reported for the Broach earthquake. Ancient rifts have recently been identified as a major seismogenic tectonic feature for intraplate seismicity in stable continental regions. P-wave first motions and waveforms of long-period teleseismic P and SH waves have been modeled to constrain the focal mechanisms and to determine the source parameters for the earthquakes. Previous investigators proposed multiple fault-plane solutions for each of the events. In this paper, some of the early solutions are eliminated and the best source mechanism is derived for each earthquake. For the Bhadrachalam earthquake, the focal mechanism determined is predominantly strike-slip (strike = 245°, dip = 72°, rake = 358°). The strike direction of the NE-SW-trending nodal plane is in good agreement with the trend of the observed isoseismals and is probably the fault plane. The seismic moment is 4.59 × 10 24 dyne-cm and the moment magnitude M w is determined to be 5.7. The focal depth and the static stress drop are estimated to be 10 km and 17 bar, respectively. The strike-slip motion represents a reactivated fault motion under the present-day stress field. For the Broach earthquake, the focal mechanism determined is reverse faulting with a strike-slip component (strike = 273°, dip = 58°, rake = 130°). The direction of the E-W-striking nodal plane is in good agreement with the mapped Narmada fault, dislocation direction of fissures, and the trend of the isoseismals. The seismic moment and moment magnitude are determined to be 1.39 × 10 24 dyne-cm and 5.4, respectivety. The focal depth and the stress drop are estimated to be 11 km and 9 bar, respectively. The reverse motion of this event represents an inverted slip or tectonic inversion within a rift zone under the present-day compressional stress field. Some rift-associated earthquakes are characterized by large multiple shocks with significant strike-slip motions. The two Indian earthquakes, however, occurred as regular main shock-aftershock sequences-and represent a different mode of strain release. These two earthquakes also represent events with low stress drops and relatively long recurrence intervals. The interpretation of this last observation is uncertain and needs more study in the future.