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

Study of Historical Earthquakes in Japan 2815. Basic Study on Historical Earthquakes Due to New MaterialsOn 9 April 1858, the so-called Hi-etsu earthquake (M = 7.0) took place in centralJapan. As shown in the first paper (Usami, 1979), this earthquake was caused bythe activity of the Atotsugawa-fault which trends ENEWSW. Near the epicenter, inthe mountainous areas on the west foot of Mt. Tateyama (3,015 m), there occurredmany landslides and mudflows which blocked the flow of rivers at various places.One of the natural dams gave way suddenly at about 11 a.m. of 23 April 1858 and aflood containing mud, rock and big trees ran down to the Toyama plain, causing lossof lives and damaging houses and rice paddies. At about 8 a.m. of the same day, anearthquake took place near Matsushiro, about 50 km east of Mt. Tateyama, causingslight damages. Owing to the 50 km distance, this event has not been considered asthe cause of failure of the natural dam. New materials indicate that the earthquaketook place near Omati-city, about 20 km east of Mt. Tateyama, and caused mediumdamage to villages near Omati-city. Moreover, the occurrence time of this event isabout 11 a.m. of 23 April 1858. This event will be called the Omati earthquake.The similar occurrence time of the Omati earthquake and the natural dam failuresuggests to us that the Omati event may have triggered the dam failure.The damaged area of the Omati earthquake is shown in Figure 4, by the hatchedarea indicated by the occurrence year. The intersection of the elongated part ofAtotsugawa-fault and the Itoigawa-Shizuoka tectonic line running N-S near Omaticityis found within the damaged area of the Omati earthquake. This fact may havesome geophysical meaning which is a theme for future study.Stimulated by this study, damaged areas due to medium-size earthquakes alongthe Itoigawa-Shizuoka line and along the median tectonic line south of Matsumotocity are plotted in Figures 3 and 4. Figure 3 is drawn employing old materials andFigure 4 is drawn by adding new materials. Two earthquakes in the year 715 tookplace within two days and their damaged areas are shown by rough estimation.From Figure 3, we can not deduce definite conclusions about the seismic activityalong the considered tectonic lines. However, Figure 4 shows that in the northernportion, medium-size earthquakes move from north to south, and in the southernportion, from south to north, and they meet near Omati and Matsumoto, that is,the junction of the Itoigawa-Shizuoka line and the median tectonic line.In the northern portion, damaged areas contact each other, indicating that inthese 300 years, seismic energy in this area has been released. On the other hand, forthe southern portion there still remain intermittent areas where no seismic activitywas recorded during historical time. However, it is not certain that earthquakeshave not occurred in the intermittent areas, because the old documents are notcompletely collected.From fault studies in Japan, it is said that inland faults give rise to earthquakesevery several hundred to several thousand years. It is not clear whether or not theseconsiderations and the serial occurrence of earthquakes in Figure 4 in these 1000years are concordant.The Kansei earthquake of 17 February 1793 (7 January of the 5th year of Kanseiera) occurred off the Pacific coast of northeastern Japan. Characteristic features ofthis earthquake according to old materials are:(1) The few documents available have descriptions at several coastal places between39' and 40' N.