[S-IT19] Coupling of deep Earth and surface processes

[SIT19-08] Seismic Reflection Imaging and Pore-Fluid Pressure Analysis of the Japan Trench Megathrust Fault Offshore Sanriku, NE Japan

*呂 玉琪¹、野 徹雄²、Jamali Hondori Ehsan³、Mohammadigheymasi Hamzeh^1,4、朴 進午¹ (1.東京大学　大気海洋研究所　海洋底科学部門、2.国立研究開発法人海洋研究開発機構 海域地震火山部門 、3.株式会社ジオサイエンス、4.Department of Earth and Planetary Sciences, Harvard University, United States)

The Japan Trench subduction zone, located offshore northeast Japan, is distinguished by the occurrence of significant megathrust earthquakes and tsunamis. The tectonic setting and physical properties of the underthrust sediments in this region provide crucial insights into plate dynamics and seismic hazard assessment. A notable example of seismic activity in this region is the 1896 Sanriku earthquake (M 8.5), which is considered a typical tsunami earthquake, as the megathrust fault ruptured at an unusually shallow depth. This study aims to characterize the detailed crustal structures of the Japan Trench subduction zone offshore Sanriku and estimate pore-fluid pressures along the megathrust fault, which may contribute to the generation of large earthquakes and tsunamis. To achieve this, multi-channel seismic (MCS) reflection data were acquired along survey 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, were acquired by a 5800 m long streamer cable with 444 channels at 12.5 m group spacing. We carried out the MCS data processing using conventional techniques to enhance the signal-to-noise ratio prior to seismic depth imaging. We developed an interval velocity model for Kirchhoff pre-stack depth migration (KPSDM) by employing pre-processed common midpoint (CMP) gathers. This model was developed following a layer-stripping approach, starting with water velocity to achieve the optimal seafloor reflection on the KPSDM section, by minimizing residual moveouts in the horizontal semblance spectrum. Subsequently, manually interpreted layers were sequentially added to the model, adjusting the velocity and gradient for each layer, from top to bottom, until adequate horizontal semblance was achieved. Then, the velocity model was further refined using grid-based tomography. The results demonstrated the presence of horst-and-graben structures, which are attributed to normal faulting due to plate bending along the outer slope of the Japan Trench. The megathrust fault at the top of the subducting Pacific Plate is traced up to approximately 75 km landward from the trench. A lens-shaped structure is identified along the plate interface located approximately 11 km from the trench axis at a depth of around 9 km, with dimensions of roughly 6 km in length and a maximum thickness of about 500 meters. This feature may be related to basal erosion and material transfer processes in the forearc region. The interval velocity model was also employed to estimate pore-fluid pressures along the megathrust fault and will be later compared with previously published results from the Miyagi region, where the 2011 Tohoku earthquake (M 9.0) had a significant coseismic slip. This comparison will enhance understanding the role of shallow megathrust fault and underthrust sediments in generating large tsunamis.