Magnitude calibration of north Indian earthquakes

This article is concerned primarily with the evaluation of the size and location of northern Indian and southern Tibetan earthquakes during the last 200 yr. It draws attention to the problems of assessing intensity of early and more recent earthquakes in a built environment, which is different from that for which the intensity scale has been constructed and to the way in which isoseismals are drawn. Through a re-evaluation of intensities and a reassessment of isoseismals, a formula for the estimation of surface wave magnitude using isoseismal radii is derived. This formula is used to estimate the surface wave magnitudes of 16 earthquakes that occurred in the region between 1803 and 1900. This study shows that it is possible to calculate accurate surface wave magnitudes for earthquakes that occurred before the advent of the scale and that there is no need to resort to empirical formulae for the assessment of the size and seismic moment release of pre-20th-century earthquakes. Also derived are formulae for the conversion of M_s to M₀. In total, locations, surface wave magnitudes and M₀ estimates are presented for 43 important events that occurred in the region between 1803 and 1974, eight of which were in the lower crust or were subcrustal. We find that the M₀-M_s scaling for India yields smaller M_s than the global relation and that the methodology used can help to evaluate more realistic slip rates as well as to address other issues related to earthquake hazard in northern India.