- 著者
- 小松 正直 竹中 博士 岡元 太郎 中村 武史
- 出版者
- 公益社団法人 日本地震学会
- 雑誌
- 地震 第2輯 (ISSN:00371114)
- 巻号頁・発行日
- vol.76, pp.17-30, 2023-05-24 (Released:2023-07-27)
- 参考文献数
- 57
In this study, we conduct First-motion Augmented Moment Tensor (FAMT) analysis to estimate the source parameters of small-to-moderate suboceanic earthquakes (MJMA 4.5-5.8) occurring off the Okinawa island, Japan, from October 2007 to April 2020. We calculate Green’s functions for a land-ocean unified 3D structure model that includes non-flat complicated land and seafloor topographies, the sediment layers, the oceanic plate and Moho interfaces obtained by compiling with recent seismic survey results. For the inversion, we use velocity waveform data in two period ranges of 4-40 s and 10-40 s and reproduce both the body and surface waves at K-NET and F-net stations. The results indicate that two of ten events are located in the oceanic mantle of the subducting Philippine Sea (PHS) slab, and eight ones are located in the oceanic crust of the PHS slab. The focal mechanisms of the former two events are primarily strike-slip and/or normal-fault type. The latter crustal events have mechanisms of reverse-fault type with the extension axis of NW-SE direction, which is consistent with the subducting direction of the slab. It is found that the depth difference between the JMA unified hypocenter catalog in travel time data and our estimated FAMT solution is large for events near the Ryukyu trench. Our results show that the offshore events in the trench side are located around the submarine active faults and are the same strike direction as the orientation of the faults.
1 0 0 0 OA 海底地動水圧同時観測による超低周波地震に伴う海底地殻変動の検出
海溝近傍で発生する超低周波地震と呼ばれる破壊過程が異常に遅い地震は、古くから存在は指摘されていたものの(和達,1928)、陸上からの遠地観測では実態を把握するには限界があった。先行研究で実施した南海トラフでの広帯域海底地震観測では、超低周波地震を直近で捉え、観測地震波の近地項から地震に伴い生じた海底変動を初めて推定した(Sugioka et al., 2012)。本研究では、この上下変位を直接測定することを目的とし、高分解能水圧計である差圧計と広帯域地震計とを組み合わせた地動・水圧同時観測システムを開発した。本システムは固体・海洋の境界面である海底観測に有用な幅広い帯域をカバーするものである。