SOURCE PARAMETERS OF THE ANJAR EARTHQUAKE OF JULY 21, 1956, INDIA, AND ITS SEISMOTECTONIC IMPLICATIONS FOR THE KUTCH RIFT BASIN

The Kutch rift basin is a major seismic region in India characterized by a low earthquake occurrence rate but a very high maximum intensity. The largest event in the region is the great Kutch earthquake of 1819 (estimated M(w) = 7.8). However, lacking instrumental data, its exact source parameters are not well known. The Anjar (or Rann of Kutch) earthquake of July 21, 1956 (M(w) = 6.0) took place very close to the 1819 Kutch event and caused considerable damage and casualties. Our modeled waveforms of teleseismic P- and SH-waves and P-wave first motions determine a reverse faulting mechanism (strike = 235 degrees, dip = 46.5 degrees, and rake = 85 degrees) as well as source parameters for the Anjar earthquake. A previous strike-slip mechanism for this event has been eliminated on the basis of waveform constraints. The two nodal planes of our solution both strike northeast, sub-parallel to the trend of the Kutch rift, very close to the trend of the isoseismals. The seismic moment is 1.19 x 10(25) dyne-cm. The focal depth and the stress drop are 15 km and 162 bar, respectively. The Anjar reverse fault may represent a reactivated normal fault undergoing tectonic inversion in the Kutch rift zone under present-day compressional stress. Two larger earthquakes adjacent to Anjar but outside of the Kutch rift are also found to be mainly reverse faulting with a minor strike-slip component. Judging from available information, the great Kutch event of 1819 is most likely a reverse faulting event.