Geological structure of the forearc slope in the Japan Trench: Preliminary results of the KH-18-1 MCS survey

The forearc slope of the Japan Trench is a typical subsidence region associated with the subduction erosion in the Japan Trench. Upon the forearc slope, the existence of the isolated basins with widths of up to several tens of kilometers are observed based on Multi-channel seismic (MCS) profiles that acquired before the 2011 Tohoku earthquake (Mw 9.0) (Arai et al., 2014). The formation of the isolated basins is considered to result from subduction erosion along the plate boundary and regional subsidence along with activities of normal fault systems, including the most recent mega-earthquake events. For example, seafloor on the landward slope of the Japan Trench moved 50 m east-southeast toward trench (Fujiwara et al., 2011). In addition, aftershock activities after the 2011 tohoku earthquake are predominated by the normal faulting. Therefore, there have a possibility that the newly isolated basin is formed after the 2011 tohoku earthquake in the forearc slope of the Japan Trench.

In this study, we conducted two marine geological and geophysical surveys in the offshore Tohoku region using R/V Shinsei-Maru of JAMSTEC (KS-17-8 cruise) in August 2017 and R/V Hakuho-maru of JAMSTEC (KH-18-1 cruise). To observe the small-scale structural deformation in the shallow part of the isolated basins, our survey lines are designed to follow the published MCS survey lines. In addition to MCS data, swath bathymetric, subbottom profiler, geomagnetic, and gravity surveys are also acquired. This MCS survey used two 355 cubic inch GI guns and 600 m 48ch streamer cable. In our new MCS profiles, we could obtain clear images down to the maximum of 3.0 sec (Two-Way Travel Time) below the seafloor in the forearc region. The isolate basin that was recognized in the MCS profiles acquired before the earthquake was generally able to be confirmed. Moreover, it was able to confirm the existence of several isolated basins in the N-S direction MCS profile passing through the forearc region. In this presentation, we will show the preliminary results of the KH-18-1 MCS survey in the offshore Tohoku region.