Refraction Seismic Survey in Suruga Bay Using OBSs and Air-gun Sources: Increased Subsurface Velocity Structure Observed at the Northern Part of the Senoumi Bank

Abstract

Suruga Bay is a tectonic bay that is traversed in the north-south direction by the Suruga Trough, which is considered to be a subduction boundary between the northern margin of the Philippine Sea Plate and the Eurasian Plate. In order to clarify the shape of the subducting plate boundary, seismic refraction surveys were conducted using 14 sets of pop-up type ocean bottom seismographs (OBSs) and air-guns as controlling seismic sources at a total of six survey lines in the years 2016, 2017, and 2018. The main survey lines are located along the western side, the axis of the Suruga Trough, and the Izu peninsula side of Suruga Bay; the three main lines run approximately north to south across Suruga Bay. This enables the subsurface velocity structure beneath the sea floor to be obtained. Features indicate the shape and subduction of the subducting plate boundary if the subsurface velocity structure beneath each survey line can be determined in detail. Details of the velocity structure at the western side of Suruga Bay investigated in 2016 are presented. The survey line 2016-NS is located at the Eurasian Plate side and runs about 36 km from the Senoumi Basin to off the Udo Hills at the west side of the Senoumi-kita Bank, almost parallel to the west coast of Suruga Bay. Survey line 2016-EW crosses Suruga Bay about 18 km from near Yaizu on the west coast of the bay to near Toi on the east coast. The subsurface velocity boundary is complex, and an unusual structure rises in the subsurface structure at the north side of Senoumi-kita Bank. The rising structure is connected to the Senoumi-minami Bank and the Senoumi-kita Bank, and is considered to be an uplift derived from the outer ridge uplift belt zone.