[S-SS10] 地震活動とその物理

[SSS10-03] The 2018 Hokkaido Eastern Iburi earthquake (M_JMA = 6.7) was triggered by a strike-slip faulting in a stepover segment: Insights from the aftershock distribution and the focal mechanism solution of the mainshock

*勝俣 啓¹、大園 真子¹、青山 裕¹、田中 良¹、高田 真秀¹、一柳 昌義¹、山口 照寛¹、岡田 和見¹、高橋 浩晃¹、酒井 慎一²、松本 聡³、岡田 知己⁴、松澤 暢⁴、平野 舟一郎⁵、寺川 寿子⁶、堀川 信一郎⁶、小菅 正裕⁷、片尾 浩⁸、飯尾 能久⁸、長岡 愛理⁸、津村 紀子⁹、上野 友岳¹⁰、2018年北海道胆振東部地震 合同余震観測グループ (1.北海道大学大学院理学研究院附属地震火山研究観測センター、2.東京大学地震研究所、3.九州大学大学院理学研究院附属地震火山観測研究センター、4.東北大学大学院理学研究科附属地震・噴火予知研究観測センター、5.鹿児島大学地震火山地域防災センター、6.名古屋大学大学院環境学研究科附属地震火山研究センター、7.弘前大学理工学研究科、8.京都大学防災研究所地震予知研究センター、9.千葉大学大学院理学研究科、10.防災科学技術研究所)

キーワード：北海道胆振東部地震、余震分布、震源メカニズム解

The Hokkaido Eastern Iburi earthquake (M_JMA = 6.7) occurred on 6 September 2018 in the Hokkaido-corner region where the Kurile and the northeastern Japan island arcs meet. We relocated aftershocks of this intraplate event immediately after the mainshock by using data from a permanent local seismic network and found that the aftershocks concentrated in depths from 20 to 40 km, which is extraordinarily deep when comparing with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Moreover, we found that the aftershock area consists of three segments. The first one is located in the northern part of the aftershock area, the second one is located in the southern part, and the third one is located in a stepover segment between the first one and the second one. The hypocenter of the mainshock, which is a rupture initiation point, is located in the stepover segment. The centroid moment tensor solution for the mainshock indicates a reverse faulting, whereas the focal mechanism solution determined by using a first motion polarity of P wave indicates a strike-slip faulting. To explain this mismatch qualitatively, we present a model that the rupture started as a small strike-slip faulting in the stepover segment of the aftershock area and it is followed by the large rupture of two reverse faulting in the northern and the southern segments.