4:15 PM - 4:30 PM
[MIS27-07] A historical record of frequent felt earthquakes on 1870 May and the destructive earthquake on 1870 May 13 felt in Yokohama
Keywords:historical earthquake, the 1870 Odawara earthquake, earthquake swarms off the east coast of Izu peninsula
On 1870 May 13, at about 2 AM, a destructive earthquake (M 6 ~ 6.5) occurred in the western part of Kanagawa Prefecture, causing damage to the Odawara area [Usami et al. (2013)]. Matsuura et al. (2006, 2020) defined the epicenter as the eastern part of Yamanashi Prefecture and concluded that the earthquake occurred at the seismically active area at the depth of 20 -30 km. Since early May, about 2 weeks before the earthquake, felt earthquakes had frequently occurred in the surrounding areas such as Shizuoka Prefecture [Ishibashi (1993)]. They have been interpreted as the foreshocks [Usami (2020)].
In order to investigate the frequently felt earthquakes this study examines the felt earthquake record written by a foreigner (James Gordon Kidd) who lived in Yokohama in the same period and compares it with other contemporary Japanese records. Kidd recorded the date of 131 earthquakes felt in Yokohama from 1870 May 1 to 26 including the size and duration of 9 large earthquakes. Based on this record, the diurnal variation of the number of felt earthquakes in Yokohama is shown in Fig. 1. The number of felt earthquakes per day was the largest on 1870 May 13, when the destructive earthquake of Odawara occurred. In addition, a peak of the number of felt earthquakes per day was observed on May 9.
We also examine the possibility that the frequently felt earthquake described by Mr. Kidd occurred off the east coast of Izu Peninsula. Since 1978, earthquake swarms off the east coast of Izu peninsula, caused by magma intrusions, have occurred repeatedly along and off the coast of Ito City, Shizuoka Prefecture [Earthquake Research Committee, Headquarters for Earthquake Research Promotion (2010)]. The Ito earthquake swarm, which occurred in the offshore area of Ito City in 1930, is also considered to have originated from magma intrusions [Center for Earthquake Prediction and Eruption Prediction, School of Science, Tohoku University (1990)]. Koyama (1993 , 1999), who examined earthquake swarms off the east coast of Izu peninsula before 1930, estimated that the swarm earthquake occurred either in 1868 May to July or in 1870 May to June. Our study raises the possibility that this earthquake swarm occurred not in 1868 (the former) but 1870 (the latter).
In order to investigate the frequently felt earthquakes this study examines the felt earthquake record written by a foreigner (James Gordon Kidd) who lived in Yokohama in the same period and compares it with other contemporary Japanese records. Kidd recorded the date of 131 earthquakes felt in Yokohama from 1870 May 1 to 26 including the size and duration of 9 large earthquakes. Based on this record, the diurnal variation of the number of felt earthquakes in Yokohama is shown in Fig. 1. The number of felt earthquakes per day was the largest on 1870 May 13, when the destructive earthquake of Odawara occurred. In addition, a peak of the number of felt earthquakes per day was observed on May 9.
We also examine the possibility that the frequently felt earthquake described by Mr. Kidd occurred off the east coast of Izu Peninsula. Since 1978, earthquake swarms off the east coast of Izu peninsula, caused by magma intrusions, have occurred repeatedly along and off the coast of Ito City, Shizuoka Prefecture [Earthquake Research Committee, Headquarters for Earthquake Research Promotion (2010)]. The Ito earthquake swarm, which occurred in the offshore area of Ito City in 1930, is also considered to have originated from magma intrusions [Center for Earthquake Prediction and Eruption Prediction, School of Science, Tohoku University (1990)]. Koyama (1993 , 1999), who examined earthquake swarms off the east coast of Izu peninsula before 1930, estimated that the swarm earthquake occurred either in 1868 May to July or in 1870 May to June. Our study raises the possibility that this earthquake swarm occurred not in 1868 (the former) but 1870 (the latter).