[S-CG49] Structure, evolution and deformation of island arcs associated with subduction processes

[SCG49-04] Seismic structure from the forearc region off Miyagi to the central part of Northern Honshu arc, Japan, revealed by onshore-offshore seismic experiment

*Eiji Kurashimo¹, Hiroshi Sato¹, Takaya Iwasaki², Takashi Iidaka¹, Tatsuya Ishiyama¹, Masanao Shinohara¹, Hirokazu Ishige³, Hidehiko Shimizu³, Shinji Kawasaki³, Susumu Abe³, Naoshi Hirata⁴ (1.Earthquake Research Institute, the University of Tokyo, 2.Association for the Development of Earthquake Prediction, 3.JGI, Inc., 4.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Northern Honshu, onshore-offshore seismic experiment, seismic tomography

Northern Honshu is a typical island arc with a trench-arc-backarc basin. The 2011 Tohoku-Oki Earthquake (Mw9.0), that occurred on the Japan Trench off the eastern shore of northern Honshu, Japan, generated enormous crustal deformations. Detailed crustal structure and geometry of the fault is important to understand the crustal deformation and the process of earthquake occurrence. In the summer of 2019, an onshore-offshore integrated seismic survey was carried out along a 850 km long profile crossing the central part of Northern Honshu from Japan trench to the Yamato bank (Sato et al., 2020). Sato et al. (2020) presented seismic reflection image and tomography-derived velocity model along the onshore part of the survey line, on which 24 large energy shots and 1,667 receivers were set. Previous studies with use of active sources have revealed the velocity structure beneath the forearc region off Miyagi (e.g., Watanabe et al., 2006). However, seismic structure from the forearc region to Northern Honshu arc has not been resolved, even though this would provide key for understanding the crustal deformation and the process of earthquake occurrence. Onshore recording of the air gun shots is a useful to obtain a structure of the ocean–land transition zone. Twenty-four temporary seismic stations equipped with a 1.0-Hz three-component seismometer were deployed along our onshore survey line. The air gun shots in the forearc side were recorded by these temporary seismic stations. Refraction from the uppermost mantle beneath the forearc region of the Northern Honshu is recognized. Clear reflections from the Moho boundary are also recognized. To obtain the seismic structure from the forearc region to the central part of Northern Honshu, first arrival refraction tomography (Zelt and Barton, 1998) was applied. We picked first arrivals from the air gun shots in the forearc side into temporary seismic stations deployed along our onshore survey line. These arrival data were combined with first arrivals of 24 large energy shots obtained at 1,667 receivers. We also applied the seismic reflection technique to air-gun data recorded by onshore stations to obtain the reflection image beneath the forearc region. The tomography-derived P-wave velocity structure shows that low-velocity zones exist along the estimated deeper extension of the active faults. We obtained a seismic velocity model revealing good correlations with the surface geology along the profile. High-velocity zone (>7.0km/s) can be recognized deeper than ~25km beneath the forearc region of the Northern Honshu. Reflection image shows the reflector interpreted as the Moho beneath the forearc region at ~25 km depth.