| 講演要旨(和文) | | 1999年に取得された反射法地震探査データと、地震前（2008年）と地震後（2011年）に実施された海底調査から、2011年3月に発生した東北地方太平洋沖地震の震源断層、特に巨大津波を発生させた断層メカニズムを調べた。今回の地震では、海溝周辺の（浅い部分の）プレート境界断層が大きく滑り、その上盤側の東西方向への引張によって、正断層が発達したと考えられる。水深3500ｍの正断層を挟んで、水平変位が大きく変化していることも分かった。この正断層は反射法地震探査データからも、明瞭に確認することができる。また、これらの引張に伴う亀裂は、海底露頭探査でも確認することができた。 |
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| | 講演要旨(英文) | | Faults related to the 2011 Tohoku earthquake (Mw 9.0) were investigated by using seismic reflection data acquired in 1999 and submersible seafloor observations before the earthquake (in 2008) and after the earthquake (in 2011). Because the surveyed area includes the region where the largest vertical displacement is predicted to have occurred, the shallow faults here are likely to be directly related to the tsunami characteristics. On the seismic profile off Miyagi, we identified three predominant faults branching from the plate interface. Normal faults are further observed within the continental crust. The normal fault may be generated by tensile stress due to large displacement along the plate boundary fault near the trench. The seafloor deformation revealed by position of Ocean Bottom Seismometer (OBS) demonstrated that dynamic horizontal seafloor displacement across the branch normal fault is much changed, suggesting that the normal fault could be ruptured associated with the earthquake. We further reveal dynamic change of seafloor geometry and environment associated with the 2011 Tohoku earthquake by comparing the seafloor fault traces before and after the earthquake. The observation after the earthquake demonstrates that several fissures are developed along the reverse fault which could not be observed before the earthquake. |
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