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

IMPORTANCE OF HISTORICAL SEISMOGRAMSFOR GEOPHYSICAL RESEARCHHiroo KanamoriSeismological Laboratory, California Institute of TechnologyPasadena, California 911.25ABSTRACTAmong the most important data in geophysics are the seismogramswhich have produced most of the basic and quantitative information concerningthe seismic source and the Earth’s interior. Modern seismographicinstruments provide high quality seismograms with a wide dynamic rangeand frequency band. The analysis of these seismograms, together with therecent developments in theory and methodology, has resulted in an orderof magnitude increase, both in quantity and quality, of our knowledge ofthe Earth’s interior and physics of earthquakes. However, the earthquakecycle is a long-term process so that study over a long period of time isessential for a thorough understanding of the earthquake phenomenon.Furthermore, since earthquakes may not repeat in exactly the same way,detailed analyses of earthquakes in the past are important.For more than a century, a large number of seismograms have beenrecorded at many stations in the world, but many of them have notbeen fully utilized mainly because theories and methods had not beendeveloped well enough to fully interpret the seismograms at the timewhen the earthquakes occurred. In many cases, methods developed inlater years enabled seismologists to investigate existing records of earlierevents, searching for further information. In this paper, we illustrate theimportance of historical Seismograms for various geophysical studies.1. Global SeismicityDuring the past 80 years, about 10 earthquakes larger than surface-wave magnitude7 (Ms 2 7) occurred every year in the world. Table 1 lists the earthquakeswith Ms 2 8 (from Abe, 1981). Recently Abe and Noguchi (1983) revised Ms forthe events for the period 1897 to 1912. The revised values are given in the parenthesesin Table 1. The magnitude-frequency relation is shown in Table 2 and Figure 1.In terms of the energy released in seismic waves, this level of activity correspondsto about 4.5 x ergs/year. The spatial distribution of these great earthquakes isshown in Figure 2. The temporal variation of earthquake energy release (Figure 3)shows a pronounced peak during the period from 1952 to 1965. The data for the19th century shown in Figure 3 are obtained from tsunami data (Abe, 1979) andare somewhat incomplete; however, it is probably true that during the period from1835 to 1900 there is no peak comparable to that for the period 1952 to 1965. It isclear that the global seismic activity is very non-unitorm in time, at least on a timescale of 100 years or so. This is one of the reasons why seismicity data over a longperiod of time are essential.Historical Seismogranisand Earthquakes of the World16Copyright 8 1988 by Academic Prrs, Inc.All rights of reproduction in any form rererved.