13:45 〜 14:00
[SSS31-13] 糸魚川—静岡構造線北部セグメントの神城断層における古地震研究
キーワード：神城断層、古地震、2014年Mw6.2長野県北部地震、糸魚川ー静岡構造線活断層系
2014年11月22日に発生したMj 6.8（Mw 6.2）長野県北部地震は、糸魚川－静岡構造線北部神城断層沿いに最大1.5mの鉛直変位を持つ、長さ9.3kmの地表地震断層が現れた（林ほか、2015a）。京大調査チームは地震直後に地表地震断層の性状の調査とともに、今回の地震を引き起こした神城断層の古地震の初歩調査研究により、神城断層沿いに過去千年間に少なくとも二回の大地震が発生したことが明らかにされた（林ほか、2015b）。その後の野外調査と二箇所のトレンチの追加掘削調査および４０個以上の年代測定を行った。その結果、神城断層沿いに過去約6000年間に７回の大地震(E1~E7)が発生したことが明らかになった。そのうちの３回は過去約1200年間に起きており、それぞれ、1918 M 6.5 (E1)、1791 M 6.8 (E2)、AD 841 M 6.5 (E3)の歴史地震と対応できると推測される。これらのことから、神城断層沿いに過去約1200年間の大地震の繰り返し周期は約300~500年であると推定される。今回の調査はこれまでの先行研究により推定された大地震の再来周期と最新活動の時期と異なる結果を示した。今後、これらのことを明らかにするためにはさらなる詳細な調査が必要であると思われる。
Field investigations and trench excavations reveal that seven morphogenic earthquakes (E1~E7) prior to the 2014 Mw 6.2 Nagano earthquake have occurred on the Kamishiro Fault during the past ~6000 years, in which the timings of three recent events (E1~E3) corresponding to historical-recorded earthquakes occurred in the past ~1200 years are well constrained, suggesting an average recurrence interval of ~300–500 years on the seismogenic fault of the 2014 Kamishiro earthquake. The most recent event (E1) prior to the 2014 earthquake occurred within the past 200 yr, and corresponds to the 1918 M 6.5. The penultimate faulting event (E2) occurred in the period between AD1800 and AD 1400 and is probably associated with the 1791 M 6.8 earthquake. The antepenultimate faulting event (E3) is inferred to have occurred in the period between AD ~700 and AD ~1000, corresponding to the AD 841 M 6.5 earthquake. The oldest faulting event (E7) is identified to be occurred in the period during ~5600–6000 yr BP in this study area. The vertical slip rate during the early Holocene is estimated to be 1.2–3.3 mm/yr with an average of 2.2 mm/yr. When compared with the active intraplate faults of Honshu Island, Japan, the relatively high slip rates and short recurrence intervals for morphogenic earthquakes within the Kamishiro Fault developed along the ISTL indicate that the present activity of this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.
Keywords: 2014 Mw 6.2 Nagano earthquake, paleoseismicity, Kamishiro Fault, recurrence interval, morphogenic earthquake, plate boundary
References
Lin, A., Mikako, S., Yan, B., Wang, M., 2015a. Co-seismic surface ruptures produced by the 2014 Mw 6.2 Nagano earthquake, along the Itoigawa-Shizuoka Tectonic Line, central Japan. Tectonophysics, 656, 142-153.
Lin, A., Mikako, S., Yan, B., Wang, M., 2015b. Preliminary study of paleoseismicity on the Kamishiro Fault that triggered the 2014 Mw 6.2 Nagano earthquake. Abstract, No.: 01341, 2015 Annual Meeting of Japan Earth and Planetary Science Union.
Field investigations and trench excavations reveal that seven morphogenic earthquakes (E1~E7) prior to the 2014 Mw 6.2 Nagano earthquake have occurred on the Kamishiro Fault during the past ~6000 years, in which the timings of three recent events (E1~E3) corresponding to historical-recorded earthquakes occurred in the past ~1200 years are well constrained, suggesting an average recurrence interval of ~300–500 years on the seismogenic fault of the 2014 Kamishiro earthquake. The most recent event (E1) prior to the 2014 earthquake occurred within the past 200 yr, and corresponds to the 1918 M 6.5. The penultimate faulting event (E2) occurred in the period between AD1800 and AD 1400 and is probably associated with the 1791 M 6.8 earthquake. The antepenultimate faulting event (E3) is inferred to have occurred in the period between AD ~700 and AD ~1000, corresponding to the AD 841 M 6.5 earthquake. The oldest faulting event (E7) is identified to be occurred in the period during ~5600–6000 yr BP in this study area. The vertical slip rate during the early Holocene is estimated to be 1.2–3.3 mm/yr with an average of 2.2 mm/yr. When compared with the active intraplate faults of Honshu Island, Japan, the relatively high slip rates and short recurrence intervals for morphogenic earthquakes within the Kamishiro Fault developed along the ISTL indicate that the present activity of this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.
Keywords: 2014 Mw 6.2 Nagano earthquake, paleoseismicity, Kamishiro Fault, recurrence interval, morphogenic earthquake, plate boundary
References
Lin, A., Mikako, S., Yan, B., Wang, M., 2015a. Co-seismic surface ruptures produced by the 2014 Mw 6.2 Nagano earthquake, along the Itoigawa-Shizuoka Tectonic Line, central Japan. Tectonophysics, 656, 142-153.
Lin, A., Mikako, S., Yan, B., Wang, M., 2015b. Preliminary study of paleoseismicity on the Kamishiro Fault that triggered the 2014 Mw 6.2 Nagano earthquake. Abstract, No.: 01341, 2015 Annual Meeting of Japan Earth and Planetary Science Union.