Repeating long-term ocean bottom seismic observations in the southern part of Japan Sea

Abstract:

Subducting hydrous oceanic plates carry water by hydrous minerals into the Earth and contribute to melt generation. Aqueous fluids that are dehydrated from a subducting oceanic plate play an important role in magma generation. To understand water circulation, magmatism, huge numbers of geophysical studies have been conducted in subduction zones. The Japanese Islands are located at subduction zones where the Philippine Sea (PHS) plate and the Pacific plate, and have a high density of seismic stations. Many seismic tomography studies using land seismic station data were conducted to reveal the seismic structure such as subducting slabs and the existence of a low-velocity zone in the mantle beneath the Japanese Islands. These studies discussed the relationship between heterogeneous structures and the release of fluids from the subducting slab, magma generation and movement in the subduction zone. However, regional tomography using the land station data did not have a sufficient resolution to image such a deep seismic structure because the deeper part of the subducting slab is located beneath the Japan Sea. This information regarding the deep mantle wedge structure is important to understand the transportation and circulation of water and melt generation in subduction zones. To obtain the deep structure with high spatial resolution, observations of natural earthquakes such as deep earthquakes and teleseismic events within the Japan Sea are essential. Therefore, we started the repeating long-term seismic observations using ocean bottom seismometers (OBS) in the southern part of the Japan Sea. In 2013, we started the observation consisted of 3 broadband OBSs and 3 LT-OBSs. In 2014, all OBSs were recovered and the data from one of broadband OBS were not retrieved due to recorder trouble. And we also started second observation period. In August 2015, we will finish second observation period. In this presentation, we will present a three-dimensional P and S wave tomography model across the margin in the southern part of the Japan Sea using arrival-time data obtained by the long-term ocean bottom seismometer observations and the land seismic stations.

This study was supported by “Integrated Research Project on Seismic and Tsunami Hazards Around the Sea of Japan” conducted by the MEXT of Japan.