[S06P-03] Seismic reflection imaging and pore fluid pressures of the Japan Trench megathrust fault offshore Sanriku, NE Japan
The Japan Trench subduction zone offshore Sanriku region of northeastern Japan, is characterized by significant megathrust earthquakes and tsunamis. This region's tectonic and physical properties provide critical insights into plate dynamics and seismic hazard assessment. Notably, the 1896 Sanriku earthquake (M 8.5) is recognized as a typical tsunami earthquake, where the megathrust fault ruptured unusually at a shallow depth. This study aims to elucidate the detailed crustal structures of the Japan Trench subduction zone and estimate the pore-fluid pressures of the megathrust fault that potentially generating large earthquakes and tsunamis. To achieve this, multi-channel seismic (MCS) reflection data were collected along line KJ2204 by the R/V Kaimei during the KM22-07 cruise in 2022, using a large-volume (~174 liters) air gun array for deep-penetration seismic imaging. The MCS data, with a total record length of 16 seconds acquired using a 5800 m long streamer cable with 444 channels at 12.5 m group spacing, were processed using a series of conventional techniques to enhance signal to noise ratio prior to seismic depth imaging. An interval velocity model was constructed for Kirchhoff pre-stack depth migration (KPSDM) using pre-processed common midpoint (CMP) gathers. The velocity model was constructed following a layer stripping strategy, starting with a water velocity to obtain the optimal seafloor reflection on the KPSDM section with minimum residual moveouts on a horizontal semblance spectrum. Subsequently, manually interpreted layers were sequentially incorporated into the model by adjusting the velocity at the top of each layer and its increase rate (gradient), from top to bottom. Each layer is added through a loop of velocity adjustment and KPSDM application until a reasonable horizontal semblance is achieved. We identified horst-and-graben structures caused by normal faulting due to plate bending on the Japan Trench's outer slope. The megathrust fault at the top of the subducting Pacific plate was traced up to ~75 km landward from the trench. Lens-shaped structures were identified along the plate interface: a smaller lens and a megalens. The smaller lens is located approximately 11 km from the trench axis at a depth of around 8 km, with dimensions of roughly 6 km in length and a maximum thickness of about 500 meters. The megalens, positioned approximately 28 km from the trench axis at a depth of around 10 km, extends over a length of approximately 28 km and has a maximum thickness of about 1 km. These features may be associated with basal erosion mechanisms and material transfer processes of the forearc region. The interval velocity model was further utilized to estimate pore-fluid pressures along the megathrust fault, which will be compared with the 2011 Tohoku earthquake (M 9.0) to better understand the behavior of the shallow megathrust fault in generating large tsunamis.