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

COMPILATION AND PROCESSING OF HISTORICAL DATA:SUMMARY REMARKSby V. Ka'rnikGeophysical Institute, Czechoslovak Academy of SciencesPrague, CzechoslovakiaEvery study on earthquake hazard and seismic risk started with the inventory ofinformation available on past seismic events and on their geological and geophysicalsetting. As a matter of fact, the results of any study depend on the quality andcompleteness of input information.Earthquake generating processes are rather slow; and for their understanding adata sample covering at least several centuries is required, if we have to answerthe basic questions about the location of potential seismogenic zones and about thefrequency of occurrence of events at different magnitude levels.Statistical models employed in earthquake hazard and seismic risk analyses mustintroduce various simplifying assumptions, for instance, a constant rate in earthquakeactivity, independency of events, etc. Observations, however, show that theactivity varies with time at different 'periodsn, that the centres of activity migratewithin an active belt, that events cannot be independent, etc. Consequently, thetime window of a data sample must be as long as possible to allow for detection ofpossible time variations. Thus, there is a need for searching in historical recordsand even in geological deposits for any trace of past earthquake activity.Another requirement is for as many earthquake parameters as possible definingthe earthquake generating process. In this respect we depend on the capabilitiesof the observation techniques as well as on the development of theoretical andlaboratory investigations.Basic parameters defining a seismic event are: date, origin time, coordinates ofthe focus (the point from which the initial P waves propagate; usually specified bylongitude, latitude and focal depth), and size of the event in terms of seismic energy(log E), Magnitude (M) or macroseismic intensity in the epicenter (Io). Thisis the minimum information; naturally, it is desirable to determine other quantitiesdescribing the event, such as the stress drop (Aa), or seismic moment (Mo), d' imensionsand orientation of the focus (fault plane), direction and size of the movementalong the fault, shaken area, damage assessment, accompanying geological and geophysicalphenomena, etc. Additional parameters are, however, available only for arather limited number of events in a particular area and cannot be as yet processedst at istically.For seismic events prior to 1900, even the determination of basic parameters isa problem because of the decreasing completeness and accuracy of information ifwe go back in history. Earthquake information is dispersed in chronicles, in varioustreaties on natural disasters, in administrative records, in old newspapers, etc. Thebasic problem is the reliability or accuracy of the sources, and every investigatormust check it by consulting historians and other experts. The tendency of somechroniclers to attract at tention by exaggerating or manufacturing information isknown. The ideal situation is when we can confront several sources of information,which is the case only for some large events. Often seismologists have to considerHistorical Seismogramsand Earthquakes of the World139 Copyright 6 1988 bq Acadeniic Press, Inc.All rights of reproduction in any form reserved.