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

3 36 H. N. Srivastava and S. K. Das2. IMD Seismological BulletinsPrior to 1938, seismological data from the Indian seismological stations werebeing published in the Annual Summary (Part D) of the Indian Weather Review.Beginning January 1938, a quarterly seismological bulletin was printed. This waslater changed to a monthly seismological bulletin. It contains phase data fromall of the permanent stations maintained by the India Meteorological Department(IMD). The epicentres as given by the U. S. Geological Survey (USGS) monthlylistings generally are included. Occasionally, due to late receipt of USGS monthlylistings, epicentres of earthquakes from Moscow Seismological Bulletins have alsobeen used. For local events, epicentres are being determined by the HYPO 71programme through IBM 360/44 (memory, 512K) using an appropriate velocitymodel (Lee and Lahr, 1975). For other earthquakes occurring in the Indian region,epicentres are reported using the programme given by Shaikh e t al. (1982) based onthe Jefferys-Bullen model. IMD Seismological Bulletins also contain felt earthquakereports and the microseismic data recorded by seismograph stations in India.Although most of the data contained in IMD Seismological Bulletins are reportedthrough the International Seismological Centre (ISC), U.K., the utility of the bulletincontinues to remain for local seismicity studies. Even if the data is not sufficientto determine the epicentres for many events, it can still be used to study the trendof seismic activity from a single station. For example, Chaudhury and Srivastava(1976) reported 2,500 earthquakes within 4 degrees epicentral radius around ShillongObservatory for the period 1970 to 1973, highlighting the active seismicityof the region where the great Shillong earthquake of 1897 occurred. If we confineour attention to the epicentral map of the region (Figure l), where earthquakes ofmagnitude 4.5 or more have been plotted, a high level of seismic activity is noticednear the Manipur-Burma border but not around Shillong as revealed from the localearthquakes recorded at one station. This is attributed to the sparsity of theseismological network. Of late, attempts have been made to lower the threshold ofepicentral determination in northeast India with six permanent stations and a fewothers maintained by the India Meteorological Department, National GeophysicalResearch Institute, and Geological Survey of India (in collaboration with Universityof Roorkee). This has enabled us to report epicentres of many more events inShillong plateau (Figure 2).It may be mentioned that a number of “river valley project” seismological stationsare being run in the country by several agencies. It has been noticed that their data,in combination with IMD Seimological Bulletins, have often enabled us to determineepicentres of many events which otherwise would have gone unrecorded.Tandon and Srivastava (1974) have published a catalogue of earthquakes in theIndian region based on the following sources: (1) ISC, U.K.; (2) BCIS, Strasbourg(fiance); (3) Oldham (1883); (4) Gutenberg and Richter (1954); (5) IMD SeismologicalBulletins and Catalogue; and (6) USGS Epicentral Cards.Epicentral data has been divided into the following Indian regions: (a) Kashmirand Western Himalayas; (b) Nepal Himalayas; (c) Northeast India, extending upto Andaman-Nicobar Islands; (d) Indo-Gangetic plains and Rajasthan; (e) Cambayand Rann of Kutch; and (f) peninsular India.This catalogue is fairly complete for earthquakes of magnitude 2 5 in the region.After 1964, data taken from the ISC bulletin includes earthquakes of magnitudeA4 < 5 as well, up to the year 1970. In this catalogue however, H. M. Chaudhury