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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. 247-262




 It is generally accepted that hydration due to plate bending-induced normal faults (bend-faults) occurs in the region between a trench and an outer rise. Hydration of the oceanic plate has played a major role in global deep water circulation, as well as co-seismic megathrust slip at subduction zones. It is, however, emphasized that little is known yet about the degree and mode of hydration in the oceanic plate at outer rises. Investigating several subduction zones with various conditions is crucial to expand our knowledge of bend-fault hydration processes. The northwest Pacific (NW Pacific) region is one of the oldest, thus coldest, and most studied oceanic plates. Water circulation and hydration through bend-faults in the NW Pacific region are supported by the results of extensive recent geophysical surveys: (1) best-developed horst and graben structures, (2) high VP/VS ratio beneath the outer rise region, and (3) anomalously high heat flow. It is noted that tensional stress in the incoming plate is assumed to have extended to depths of 40 km during and after the 2011 Tohoku Earthquake, whereas it is shallower than 40 km for previous earthquakes. The projections of epicenters of microearthquakes after the 2011 Tohoku Earthquake are aligned with topographic lineation of horst and graben structures in the outer rise region. These lines of evidence suggest that hydration has progressed extensively to deeper sections of the oceanic plate since the 2011 Tohoku Earthquake. To address (a) hydration processes and their extents along the bend-fault and (b) physical properties of the bend-fault, in-situ physical properties and lithofacies are best obtained by ocean drilling in the NW Pacific region.

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