次世代の史料地震学 １９２３年関東地震の本震直後の２つの大規模余震 強震動と震源位置

Abstract

Immediately after the 1923 Kanto Earthquake (M=7.9), two large aftershocks of M=7.2 and 7.3 occurred in succession somewhere in the southern Kanto district. The first aftershock occurred about 3 minutes after the main shock and the second about 4.5 minutes after the main shock. Strong ground motions from these events and locations of their epicenters were examined mainly from data of 548 descriptions of personal experiences. It was deduced from them that shaking due to the first aftershock was severe in the Tokyo Metropolis and eastern Kanagawa prefecture. In consequence, there were many descriptions in the Tokyo Metropolis that the shaking was as strong as that due to the main shock and caused extensive damage.On the other hand, there were few descriptions for the first aftershock in the western area of the southern Kanto district, which is western Kanagawa prefecture, Yamanashi prefecture, and eastern Shizuoka prefecture. However, shaking due to the second aftershock was strong in this area. Some people living near the boundary of the three prefectures described that its shaking was as strong as that due to the main shock. Comparing the facts described in these personal experiences with the distributions of seismic intensities from other M=7 class earthquakes occurring in the southern Kanto district after the 1855 Ansei Edo Earthquake, it was concluded that the epicenter of the first aftershock was located in and around northern Tokyo Bay and that the epicenter of the second aftershock was in eastern Yamanashi prefecture. The result for the second aftershock was consistent with the epicenter determined from the seismic records by the Kumagaya Meteorological Observatory and with the epicentral distance estimated from data of S-P time at the Gifu Meteorological Observatory. The characteristics of the sequence of strong shakings within 5 minutes after the 1923 Kanto Earthquake could be elucidated in the southern Kanto district from the results of the present study.