SEISMIC HAZARD ESTIMTION FROM THE ISOSEISMALS OF THREEGREAT INDIAN EARTHQUAKES

摘要

This study is devoted to the construction of anomalous residual intensity maps from isoseismals of three great Indian earthquakes namely, the Kangra earthquake of April 4, 1905 (Ms =8.0), the Bihar-Nepal earthquake of January 15, 1934 (Ms=8.3) and the Assam earthquake of August 15, 1950 (Ms =8.6) for the purpose of delineating areas of anomalous intensities. Computed intensities (Ic) at various localities have been estimated by fitting a simplified model, IC = A + BΔ + C log Δ, into the observed intensity data,where, A is the average outer radius for each intensity level and A, B and C are constants estimated using regression analysis. The residual intensities (IR) are calculated from the difference between the observed intensity (IOB) and the computed intensity (IC). The anomalous areas of low and high residual intensities have been correlated with geology, tectonics, subsurface topography and Bouguer gravity anomalies.Four prominent areas of anomalous residual intensities (Ic>2) have been delineated. These areas fall in the Sub Himalaya and the Lesser Himalaya near Dehradun, around Sitamarhi town and Monger-Saharsa ridge in Bihar, and Mikir hills in Assam. These areas are characterized by undulating basement topography and subsurface massif and uplifts in the form of ridges and generally exhibit high Bouguer gravity anomalies. It seems that the basement topography influences the observed anomalous intensities. The expected peak accelerations computed at bed rock level should be modified in these areas of anomalous intensities while making seismic hazard estimation.