2015 年 68 巻 2 号 p. 17-29
To determine crustal structure of the Neodani Fault zone, where the largest inland Nobi earthquake occurred in 1891, an active-source seismic experiment was carried out in the northern Mino region, central Japan. The shots were recorded by seismic array stations settled across the Neodani Fault Zone and we applied seismic reflection analysis to the data and obtained reflection profiles of the crust. Then we estimated crustal velocity by forward modeling using travel times of first break and reflected waves. We found significant reflections with duration of 2 s around 10 s two way travel time. Based on comparisons with other reflection profiles, we interpreted the events to come from the laminated lower crust within the overriding plate. The depth of the laminated crust varies across the Neodani Fault Zone, which is shallower in the southwest than in the northeast, implying that displacement along the Neodani faults extends to the deep crust. In the southwestern part of study area, the top of the Philippine Sea plate (PSP) estimated from travel time tomography is shallower than the lower limit of the laminated lower crust. We thus suggest that the subducting PSP may contact with the bottom of the overriding crust beneath the northern Mino district.