Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Historical Seismic it y and Earthquake Catalogues for Indian Region 337Figure 1. Earthquake Epicentres in Northeast India (1897-1980).(personal communication) has pointed out one omission of a damaging earthquakeon 18 October 1934 of estimated intensity VIII (M.M.), near Gorakhpur (UttarPradesh), which was reported in Indian Weather Review. For detailed studiespertaining to risk analysis of critical facilities to be made, there is a need to includeearthquakes of M < 5 for the period prior to 1964.The magnitudes of earthquakes in the above catalogue prior to the instrumentalperiod were assigned on the basis of intensity-magnitude conversion formulae usingGutenberg and Richter's (1956) relation:2M = 1 + -I,,3where M is magnitude, and .', is epicentral intensity.In the case of some past earthquakes, focal depth (h) of an event could be estimatedfrom the relation given by KQrnik (1969):DnI, - I, = v log -,hwhere I, is the maximum intensity, I, is a particular intensity, V is a constantwhich may take typical values of 3.0 or 4.5., and D, is the distance for a particularintensity.For earthquakes occurring during the instrumental era, magnitudes were assignedon the basis of distance up to which they have been recorded. At times, we havegiven a magnitude range of 5 to 6 instead of a single value for an earthquake, due(1)(2)