11:00 〜 13:00
[MIS22-P03] Discovery of Historical Records of the Earthquake Swarms off the east coast of the Izu Peninsula — Cases of 1816 – 1817, 1870 and 1880
キーワード：群発地震、伊豆半島東方沖
On the east coast of the Izu Peninsula, Shizuoka Prefecture, Japan, earthquake swarms caused by magma intrusion have occurred repeatedly since 1978. Among them, the 1980 earthquakes included the damaging earthquake (29, June 1980), and the 1989 earthquakes produced a submarine eruption that resulted in the formation of a submarine volcano “Teishi Knoll”. The duration of active seismic activity (the period during which local residents feel often earthquakes) is about less than a week and in some cases, such as the July 1988 and April 1998 swarm earthquakes had a longer duration for multiple magma intrusions.
The 1930 Ito earthquake swarm that occurred off the coast of Ito City of the eastern part of the Izu Peninsula was also thought to be caused by magma intrusion and lasted for about three months from February to May 1930.
Prior to 1930, the earthquakes that are certain or highly probable to be swarm earthquakes off the east coast of the Izu Peninsula occurred in 1816-1817 and 1870 (or 1868). However, due to the scarcity of historical records on these earthquakes, the information has been not clear. In this study, we examine and introduce new historical documents on three earthquake swarms on 1816-1817, 1870 and 1880 in the eastern part of the Izu Peninsula.
The 1816-1817 case is seen in a local document of Numazu (Shizuoka prefecture), the western part of the Izu Peninsula. According to the document, frequent earthquakes have occurred in the eastern part of the Izu Peninsula for about two weeks from December 1816 to January 1817.
The 1870 case is seen in one of “Koubunroku” (Dajokan, 1870). Kobunroku are official documents exchanged by Dajokan (the Grand Council of State) with the ministries of the Meiji government during the early Meiji period (1868-1885). The documents are compiled by year and agency and preserved by the National Archives of Japan. Nirayama Prefecture, which ruled the Izu Peninsula at that time, reported to Dajyokan that the frequent earthquakes occurred in the Izu regions recently (May 1870).
The 1880 case has been newly found in the study. Its description is included in an article by John Milne, a British engineer, and its information was obtained from the county office of Izu which controls the eastern part of the Izu Peninsula, with the help of Toshio Nakano of Imperial College of Engineering. From the end of April to the beginning of May in 1880, a swarm of earthquakes occurred on the eastern coast of the Izu Peninsula, and 25 or 26 earthquakes were felt in a day in the vicinity of Ito. Its duration is shorter than in 1816 – 1817, 1868 (or 1870), or 1930, and is similar to the cases after 1978.
The 1930 Ito earthquake swarm that occurred off the coast of Ito City of the eastern part of the Izu Peninsula was also thought to be caused by magma intrusion and lasted for about three months from February to May 1930.
Prior to 1930, the earthquakes that are certain or highly probable to be swarm earthquakes off the east coast of the Izu Peninsula occurred in 1816-1817 and 1870 (or 1868). However, due to the scarcity of historical records on these earthquakes, the information has been not clear. In this study, we examine and introduce new historical documents on three earthquake swarms on 1816-1817, 1870 and 1880 in the eastern part of the Izu Peninsula.
The 1816-1817 case is seen in a local document of Numazu (Shizuoka prefecture), the western part of the Izu Peninsula. According to the document, frequent earthquakes have occurred in the eastern part of the Izu Peninsula for about two weeks from December 1816 to January 1817.
The 1870 case is seen in one of “Koubunroku” (Dajokan, 1870). Kobunroku are official documents exchanged by Dajokan (the Grand Council of State) with the ministries of the Meiji government during the early Meiji period (1868-1885). The documents are compiled by year and agency and preserved by the National Archives of Japan. Nirayama Prefecture, which ruled the Izu Peninsula at that time, reported to Dajyokan that the frequent earthquakes occurred in the Izu regions recently (May 1870).
The 1880 case has been newly found in the study. Its description is included in an article by John Milne, a British engineer, and its information was obtained from the county office of Izu which controls the eastern part of the Izu Peninsula, with the help of Toshio Nakano of Imperial College of Engineering. From the end of April to the beginning of May in 1880, a swarm of earthquakes occurred on the eastern coast of the Izu Peninsula, and 25 or 26 earthquakes were felt in a day in the vicinity of Ito. Its duration is shorter than in 1816 – 1817, 1868 (or 1870), or 1930, and is similar to the cases after 1978.