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

3 44 H. N. Snvastava and S. K. Das5. IASPEI Historical Seismogram Filming ProjectSeismograms of earthquakes recorded on photographic paper have a tendency todeteriorate with time under changing weather conditions. They require a large storageplace, greater care in handling, and special precautions to prevent damage fromwhite ants. Due to the rapid developments in modern seismological instrumentationof digital as well as analog type, a tendency has been noticed among some seismologiststo undermine the utility of old records which were obtained through low-gaininstruments like Omori-Ewing, Wiechert, Milne-Shaw, and Wood- Anderson. Therecent development of new techniques of digitization and modern methods of analysishave increased the utility of old records to supplement results pertaining tosource mechanism studies, crustal and upper mantle structure, etc. The IASPEIHistorical Seismogram Filming Project will ensure the preservation and availabilityof old seismic records for a long time in the future. Among the Indian seismologicalstations, Bombay has been selected as one of the stations for participation in theHistorical Seismogram Filming Project, and more than 20,000 seismograms havebeen microfilmed so far.Table 1 lists the instruments and the seismological stations that were in operationduring 1962. Because of storage of some seismograms at damp places like Calcuttaand Bombay, some records have been lost due to white ants.During the last few years, there has been a rapid advancement in understandingthe nature of stress and source parameters of earthquakes in India and elsewhere(Ichikawa et al., 1972; Tandon and Srivastava, 1975; Srivastava and Chaudhury,1979; and many others). The Historical Seismogram Filming Project will enableseismologists to study these aspects in detail from near as well as far field anglebased on similar records in Europe, India, Japan, and many other countries.6. Significant Seismological Problems in the Indian RegionThe following seismological problems are of immediate interest for the Indianregion for which the Historical Seismogram Filming Project may provide additionaldata:(I) The catalogue of earthquakes (Tandon and Srivastava, 1974) gives a range ofmagnitudes for many earthquakes in the Indian region. Refinement of methodsof magnitude determination from historical seismograms will enable us to assign amore realistic value in different Indian regions from the point of view of risk analysis.(2) A number of reliable fault-plane solutions of earthquakes in and around theIndian plate (Ichikawa et al., 1972; Tandon and Srivastava, 1975) suggest refinementof regional plate tectonics models. A few studies have been made for someearthquakes in the Indian region prior to 1962 using body waves (Ichikawa et al.,1972, and others). However, there is a need for detailed studies to evaluate stressdrop, seismic moment and related parameters which may enable us to explain thedifference of energy released between earthquakes near plate boundaries and withinthe plate, prior to 1962.