Disaster Mitigation vis-á-vis Time of Occurrence and Magnitude of Earthquakes in India

Earthquakes occurring during the night or early morning hours cause a heavy loss of life. Also, an earthquake occurring in the late evening hours poses serious handicap for disaster mitigation efforts due to failure of electricity and blocking of roads due to fall of debris. The larger aftershocks may cause further damage depending upon the magnitude of the main earthquakes of magnitude 5.5 and more in the Indian region. Out of 7 great earthquakes (M >7.8) in the Indian region during the last 200 years, five (1819, 1897, 1905, 1941 and 1950) have occurred during the evening or early morning hours. About 67% of all the earthquakes of magnitude >7.0 show similar result. It is found that in general, the percentage of earthquakes occurring during evening/early morning is larger than that during the daytime. However, the difference in time of occurrence is not significant at 95% level of confidence using χ<Superscript>2</Superscript> test. Keeping in view that most of the earthquakes in India of magnitude more than 6 have caused significant damage in the last decade (Uttarkashi, 1991; Latur, 1993; Jabalpur, 1997; Chamoli, 1999) and have occurred in the night/ early morning, disaster management plans need to be designed for awareness and education separately for the night and day times. The limitations of the seismic zoning map with reference to the earthquakes in Koyna (1967) and Latur (1993) have suggested to safeguard the life and property of the Indian population from the effects of future damaging earthquakes which should be failsafe instead of following the code and then introduce further changes in the code. It is suggested that residential houses (including tall structures) should be built for a design earthquake of magnitude 6.0 in all the three seismic zones namely I, II and III demarcated by the Bureau of Indian Standards giving proper weight age to site response. However, for Zones IV and V usual code recommendations may be followed through micro zoning of important, densely populated or most vulnerable areas. Copyright Kluwer Academic Publishers 2004