[S-CG49] Structure, evolution and deformation of island arcs associated with subduction processes

[SCG49-P05] Upper mantle structure beneath the Japan Sea revealed by repeating long-term seafloor seismic observations

*Kazuo Nakahigashi¹, Takeshi Akuhara², Tomoaki Yamada³, Kimihiro Mochizuki², Yusuke Yamashita⁴, Hajime Shiobara², Masanao Shinohara² (1.Tokyo University of Marine Science and Technology, 2.Earthquake Research Institute, 3.Japan meteorological Agency, 4.Disaster Prevention Research Institute, Kyoto University)

The Japan Sea, which has located between the Asian continent and the Japan Islands, is a back-arc basin in the northwestern Pacific Ocean. The Japan Sea is believed to have been formed by back-arc opening and numerous and various models have been proposed by many geophysical and geological studies to explain the formation and evolution. Many crustal structure surveys with ocean bottom seismometers (OBSs) have been performed to better understand the formation of the Japan Sea. Different types of crust have been found. The Japan Basin has an oceanic crust in the northeastern area. The Tsushima Basin has the crustal thickness approximately 15 km. The Kita-Yamato Trough, which splits the Yamato Rise into two parts, has the crustal thickness of about 12 km without high velocity layer (7.1 km/s) in the lower crust. In the Yamato Rise area, seismic crustal survey with OBSs has not been conducted, however, gravity data suggest that depth of Moho is about 24 km. To understand the formation of the crust and the evolution of the Japan Sea, it is necessary to determine the deep seismic structure. Regional tomography using the land station data did not have a sufficient resolution to image a deep structure beneath the sea area. To obtain the deep structure, observations of natural earthquakes within the sea area are essential. Therefore, we started the repeating long-term seismic observations using OBSs from 2013 to 2019. We apply travel-time tomography method to the regional earthquake arrival-data recorded by OBSs and land stations. In this presentation, we will report the P and S wave tomographic images beneath the Japan Sea.
This study was supported by “Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan” conducted by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.