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

D. M. McGregor, A. A. Hughes, and R. D. AdamsDuring analysis, the computer program first groups together origin estimatesof the same earthquake from different agencies, and then associates the individualstation readings with the most likely event. In a typical month up to 100,000 stationreadings are analysed, and initially between 1,600 and 2,000 events are identified.After mis-associations have been remedied and other discrepancies rectified, theremaining unassociated readings are searched for new events, and usually about250 previously unlocated earthquakes are added to the file. Thus, the number ofevents listed each month is usually between 1,800 and 2,300. The delay of two yearsin the publication of the final bulletin is justified by the fact that this number ofevents is several times greater than that obtained by the Prelimin,wy Determinationof Epicenters Service of the U. S. Geological Survey, whose aim is to produce a rapidservice for well-reported larger events, compared with ISC’s wish for completeness,at the expense of speed.3. Publications3.1. Bulletin of the International Seismological CentreThe Bulletin is the Centre’s main publication, containing all relevant informationfor a given month’s earthquakes. For each event, the origin estimates given byother agencies are listed, followed by the refined estimate calculated by the Centre.This gives origin time, latitude, longitude and depth with computer determinedstandard errors. The earthquake’s felt effects and any unusual scientific featuresare also described. Magnitudes mb and Ms are calculated when individual stationsreport readings of amplitude and period, or their ratio. Since the data-yeax 1978 aneffort has been made to obtain more long-period readings, and consequently moredeterminations of Ms.For some small earthquakes located by national agencies the origin co-ordinatesonly can be given, but for better observed earthquakes observations from individualseismograph stations are listed with their distance, azimuth and time residual. Forthe largest earthquakes the number of reporting stations can reach more than 600.The Bulletin provides seismologists with the fullest available information on allaspects of observational seismology necessary for the study of earthquake occurrenceand earth structure. All issues of the Bulletin since 1964 are in fixed field formatavailable on magnetic tape.3.2. Regional Catalogue of EarthquakesThis is a six-monthly compilation of earthquake origin parameters and felt information,without the detailed station observations of the Bulletin. It containslists of ‘major earthquakes” above certain magnitudes, reports of “felt and damagingearthquakes”, “probable explosions”, and the main regional catalogue in whichall events in given geographic regions are listed together. This publication is thatmost relevant to detailed studies of seismicity, and the one most sought by institutionssuch as consulting engineering firms and insurance companies. It is publishedshortly after the issues of the Bulletin for June and December of each year. Like theBulletin recent issues of the Catalogue are available in fixed field format on magnetictape. It also gives world-wide lists of seismograph stations and their coordinates.