[SCG62-07] Spatial variations in incoming oceanic plate and its implications for subduction zone processes along the Japan Trench
Keywords:incoming oceanic plate, seismic structure, crustal thickness
In 2017 and 2018, we conducted additional seismic experiment at the trench-outer rise areas of the southern Japan Trench off Fukushima. Unlike the northern part of the Japan trench, a large seamount chain exists on the incoming plate off Fukushima, but the water depth other than the seamounts are basically deeper than the northern area. The maximum water depth at the trench axis is significantly deeper in the southern Japan Trench than in the northern Japan Trench and the western Kuril Trench. We applied traveltime inversion to the OBS-airgun data and obtained a P-wave velocity structure model along the survey line. The Vp model clearly shows the reduction in Vp near the trench axis which can be explained by bend faulting. The crust is remarkably thick beneath a large seamount, but except for the seamount, the crustal thickness seems to be a little thinner than the northern Japan Trench area, which is consistent with the gravity anomalies. These observation implies that the thickness of the crust show regional variations along the Japan Trench and the Kuril Trench, which is well correlated with the seafloor topographic high called the Hokkaido rise. As the plate bending prior to subduction cannot explain the thickening of the oceanic crust, we infer that the crustal thickness beneath the Hokkaido rise is originally thicker than the other areas, suggesting the nature of the incoming oceanic plate, the thickness and the weight, is different between the northern and the southern Japan Trench. In this presentation, we will show Vp models on the oceanic Pacific plate in the western part of the Kuril Trench and northern and southern Japan Trench, and discuss the variations in the crustal thickness and implication for the subduction zone.