[S-SS11] 地殻構造

[SSS11-03] 2016年カイコウラ地震震源域下のS波反射面の検出およびその本震・余震活動との関連

*松本 聡¹、河村 優太¹、岡田 知己²、松野 弥愛²、飯尾 能久³、Rick Sibson⁴、Martha Savage⁵、Kenny Graham⁵、鈴木 真奈美²、Stephen Bannister⁶ (1.九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、2.東北大学大学院理学研究科、3.京都大学防災研究所、4.オタゴ大学、5.ヴィクトリア大学ウェリントン校、6.GNSサイエンス)

キーワード：カイコウラ地震、S波反射面

Geometries of the fault plane of an earthquake and S wave reflectors beneath the fault provide important information for considering earthquake genesis at the fault. Especially, fluid distribution inferred from S wave reflectors around an earthquake fault could play an important role for initiation of the earthquake rupture as a mechanism for reducing fault strength. The location and geometry of the reflector can be determined from the travel time of the reflected phases. For detecting the reflections, we need to observe at stations located close to the hypocenter, so that there is a sufficient phase separation between the direct S wave and the reflected phases. In this study, we attempted to detect reflected waves in observed seismograms at seismic stations in and around the 2016 Kaikoura, New Zealand earthquake (Mw7.6). Seismic records were obtained from the permanent GeoNet stations as well as from seismic stations deployed before the Kaikoura earthquake in the northern South Island. We applied reflection seismology techniques to the data obtained by the network. We detected several reflectors in the mid and lower crust from the sections. Strong reflected phases were observed at the southern edge of the focal area (from a reflector with depth about 20 km). Weak reflectors were detected in/beneath the aftershock area (in the mid- to lower-crust). Comparing aftershock distribution obtained from the dense seismic network data, the geometrical relation to the reflectors suggests initiation of the earthquake occurred where fluids were present.