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Vol. 126 (2017) No. 2 特集号:沈み込む海洋プレート科学の最前線-アウターライズ海洋掘削に向けて- p. 207-221




 Seismicity accompanying subduction of one plate under the other is active in subduction zones. This seismicity is not uniformly distributed along strike; aseismic regions may be identified in a band of active seismicity. Such an uneven distribution may be attributed to the heterogeneity of frictional properties along the plate interface. Variations in the physical properties of the top-most layer over the subducting plate and rough topography along the plate interface are considered to be factors determining frictional properties. Recent active-source seismic surveys have revealed characteristics of the plate interface in detail and provide an understanding of how regional seismicity and slip distributions of major earthquakes are controlled. Seismicity around the northern limit of the 2011 Tohoku-oki earthquake shows a good correlation with seismic reflectivity along the plate interface, and suggests that the water content and/or the amount of clay minerals in a thin layer over the top of the subducting plate is a major factor. A subducting seamount was identified from a seismic survey around the southern limit of the 2011 Tohoku-oki earthquake off Ibaraki Prefecture. Repeating M 7 earthquakes share the same source at the subduction front of the seamount, while the source area of the largest aftershock of the 2011 Tohoku-oki earthquake lies next to it. The fault slip of these earthquakes, once initiated at the base of the seamount, did not propagate over the seamount. The same relationship between fault slip and subducted seamount is identified for a large slow slip at the Hikurangi margin. Physical or structural properties controlling seismicity or fault slip along the plate interface appear to be characteristics of the subducting plate. The importance of geophysical surveys of the incoming plate can now be further emphasized to provide a better understanding of seismicity along the plate interface.

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