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

Study of Historical Earthquakes in Japan 279replaced by the Earthquake Research Institute in order to promote more intensiveearthquake studies. In 1928, K. Suyehiro, the first director of the EarthquakeResearch Institute, asked K. Musha to revise and enlarge the work by Tayama.After thirteen years of efforts, Musha (1942) published the first volume of the revisededition of Historical Data on Japanese Earthquakes. The second and thirdvolumes were published in 1943 (Musha, 1942-1943). These three volumes weremimeographed. The fourth volume (Musha, 1951) was published in 1951 in type.The four volumes amount to 4,000 pages in all, and include 6,000 earthquakes (excludingaftershocks) from the years 416 to 1867. The revised edition by Musha istoo voluminous to read through. He picked up large disastrous earthquakes, gaveappropriate summaries for each event, and arranged them in a report (Musha, 1950-1953). This report was also mimeographed. Since then the importance of such achronological table has been acknowledged among seismologists. The first seismologistwho recognized the importance of this table was the late Dr. H. Kawasumi. Heestimated magnitude and epicenter location for destructive historical earthquakesand his efforts resulted in two works: (1) sixty-nine year periodicity of strong earthquakesin southern Kanto district (Kawasumi, 1963); and (2) the so-called Kawasumi’smap (Kawasumi, 1952) showing maximum acceleration anticipated in every75, 100, and 200 years.3. Short Summary of the Study of Historical Change of SeismicityThe present author (Usami, 1977) made a descriptive table of destructive earthquakesin Japan. The number of such earthquakes is 693, during the period 416to 1973. Of these, 458 earthquakes occurred in historical time. Figure 2 showsthe number of destructive earthquakes in ten-year intervals for Kyoto, Kamakura,and Tokyo. Kamakura became the capital of Japan at the beginning of the 13thcentury and Tokyo at the beginning of the 17th century. Figure 2 shows that disastrousearthquakes in the vicinity of the capital increase suddenly at the time ofthe opening of a new government.Comparison of disastrous earthquakes according to old and new materials isshown in Table 1. Here, ‘old” means materials collected by Tayama and Musha.It should be noted that new materials are four times more voluminous than oldones, and the more we search, the more new materials will be found. There seemsno limit for finding additional materials. Total number of disastrous earthquakestabulated from old and new materials is 282 and 350 respectively. Table 1, showsthat the addition of new materials changes the history of earthquakes and opens anew viewpoint relevant to modern seismology.4. Recent Work on the Collection of Old DocumentsSince the publication of Historical Data on Japanese Earthquakes by Musha in1951, no extensive collection of old documents has been done by any Japanese until1971, when the present author began collecting them. Since then, interest in historicalearthquakes has increased among seismologists, especially in relation to thelong-range prediction of earthquakes and mitigation of earthquake disasters. At theend of 1976, by the strong desire of the Ministry of Education, the HistoriographicalInstitute of Tokyo University agreed to cooperate with the Earthquake ResearchInstitute in the collection and reading of old documents. Historians know about old