[S-SS09] Crustal Structure

[SSS09-P07] Pre-stack depth imaging and pore-fluid pressure estimation for the Japan Trench megathrust fault off Sanriku, NE Japan

*Yuqi Lyu¹, Tetsuo No², Jin-Oh Park¹ (1.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Keywords:Japan Trench, seismic reflectioin, prestack depth migration

The Japan Trench subduction zone offshore Sanriku region of northeastern Japan is marked by large megathrust earthquakes and tsunamis. The tectonic and physical characteristics of this subduction zone offer valuable insights into plate dynamics and seismic hazard assessment. Particularly, the 1896 Sanriku earthquake (M 8.5) is known as one of the typical tsunami earthquakes that megathrust earthquakes unusually nucleate at shallow depth. The primary objective of this study is to figure out detailed crustal structures of the Japan Trench subduction zone offshore the Sanriku region and estimate pore-fluid pressures of the megathrust fault potentially generating large earthquakes and tsunamis. For this objective, multi-channel seismic (MCS) reflection data were acquired on a line of KJ2204 by R/V Kaimei of JAMSTEC during the KM22-07 cruise in 2022. For deep-penetration seismic imaging, a large-volume (~174 liters) air gun array was used as the controlled sound source. The MCS data were recorded with a ~5800 m, 444-channel streamer with 12.5 m group spacing. A total of 16 seconds of data were recorded. Conventional MCS data processing techniques were applied to the MCS data, including trace editing, pre-filtering, spherical divergence correction, de-bubble, de-ghost, predictive deconvolution, multiple suppressions by surface-related multiple attenuation and parabolic radon transform, common midpoint (CMP) sort, normal moveout correction, CMP stack, and Kirchhoff post-stack time migration (PoTM). An interval velocity model is built using the pre-conditioned CMP gather data for pre-stack depth migration. We identify many horst-and-graben structures caused by normal faults due to plate bending in the outer slope of the Japan Trench. We can trace a topmost oceanic crust (i.e., megathrust fault) of the subducting Pacific plate up to ~75 km landward from the trench. The interval velocity model will also be used to estimate pore-fluid pressures of the megathrust fault. Then, we will compare the pore-fluid pressures of the 1896 Sanriku earthquake rupture area with those of the 2011 Tohoku (M 9.0) coseismic rupture area to understand the shallow megathrust fault behavior generating large tsunamis.