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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. 113-123




 An important site of hydration and alteration of the incoming oceanic plate prior to subduction, the trench-outer rise region determines the distribution and amount of water transported into the Earth's mantle by subduction of the oceanic plate. Recent seismic surveys conducted in the northwest Pacific Ocean indicate reductions of seismic velocities within the oceanic crust and the uppermost mantle of the incoming Pacific plate toward the trench. These velocity reductions probably reflect hydration and alteration of the Pacific plate prior to subduction, although there are regional variations in the crustal structures of the oceanic lithosphere. Besides structural change, the stress regime within the incoming Pacific plate, which is derived from seismicity observations in the trench-outer rise region of the Japan Trench after the 2011 Tohoku-Oki earthquake, suggests that extensional stress of the incoming Pacific plate extends down to a depth of about 40 km. However, normal-faulting earthquake activity within the incoming Pacific plate was limited at depths shallower than about 20 km before the 2011 Tohoku-Oki earthquake. The change of the depth extent of normal-faulting earthquake activity suggests that the 2011 Tohoku-Oki earthquake changed the stress regime within the incoming Pacific plate. Distribution and focal mechanisms of shallow seismicity within the oceanic crust indicate that seismicity is affected by the pre-existing structure, which probably formed at the mid-ocean ridges. Heterogeneity and pre-existing structure of the incoming oceanic lithosphere should be taken into account when considering hydration and alteration of the oceanic lithosphere subducting into the trench.

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