Deep crustal structure off Akita, eastern margin of the Japan Sea, deduced from ocean bottom seismographic measurements

Abstract

A seismic exploration was carried out to detect the heterogeneity of the seismic wavespeed structure associated with the plate convergence at the eastern margin of the Japan Sea. Two airgun–OBS (ocean bottom seismograph) profiles were shot off Akita, Japan, where a seismic gap seems to exist but the location of the plate boundary has not been confirmed. One of the profiles was 60 km long, trending NNE–SSW, named Line OBS-9, at the northeastern end of the Yamato Basin and the other was 170 km long, trending WNW–ESE, Line NT95-1, parallel to the direction of the supposed plate convergence. The crustal structure beneath Line OBS-9 consists of six layers. The uppermost layer is sediment. Three layers are identified beneath the top sedimentary layer and their P wavespeeds are estimated to be 3.3–3.4 km/s, 5.1–5.4 km/s and 5.8–6.3 km/s, which corresponds to the upper crust. Underneath these layers, a layer with P wavespeed ranges of 6.3–7.2 km/s comprises the middle and lower crust. The depth of Moho is inferred to be 19 km. These wavespeed values are comparable with those of the present Japanese island arc, while the thickness of the crust is one-half of that of the Japanese arc. The crustal model supports the scenario that the Yamato Basin is formed by extension of the island arc. The crustal model for Line NT95-1 shows a transition from the extended island arc structure beneath the Yamato Basin to a thicker crust similar to the Japanese arc. P wavespeed heterogeneity related to the plate boundary is not detected. However, a significant change in the structural model along the profile is found around the region where the largest change in the seafloor topography exists. In that region, the wavespeeds in the middle crust have lower values than those of the neighboring area and the Moho begins to deepen towards the Japanese island arc. From comparison with the relationship between P wavespeed structure and aftershock distribution of the 1993 Hokkaido-Nansei-Oki earthquake (southwest of Hokkaido, M JMA 7.8), the region may correspond to a possible fault position of the pending earthquake.