[S-SS09] Crustal Structure

[SSS09-P05] Deep structure of the Hidaka collision zone, Hokkaido Japan inferred from offshore-onshore integrated seismic survey (1)

*Noriko Tsumura¹, Yoshio Murai², Hisatoshi Baba³, Toshinori Sato¹, Eiji Kurashimo⁴, Masanao Shinohara⁴, Shintaro Abe⁵, Yasuhito Uchida⁶, Takuya Ishii⁷, Nagisa Nakao⁸, Kazuya Matsumoto³, Takumi Asada⁹, Takashi OGAMI¹⁰, Miwako Ando⁴, Shinichi Tanaka⁴, Aina Idoguchi³, Keitaro Takamoto⁸, Tomoya Hiramatsu⁷, Naoki Momomoto⁷, Yusuke Yamanaka², Tatsuya Nakagaki¹¹, Hiroaki Yamahana¹², Yojiro Sasagawa⁷, Hayato Onuki¹³, Saki Shibata¹³, Issei Yasuhara¹³ (1.Graduate School of Science, Chiba University, 2.Institute of Seismology and Volcanology, Hokkaido University, 3.Department of Marine and Ocean Engineering, Tokai University, 4.Earthquake Research Institute, The University of Tokyo, 5.Association for the Development of Earthquake Prediction, 6.Hokkaido Research Organization, 7.Graduate school of Science and Engineering, Chiba University, 8.Graduate school of Science and Technology, Tokai University, 9.Muroran Institute of Technology, 10.National Institute of Advanced Industrial Science and Technology, 11.Graduate school of Science, Hokkaido University, 12.Graduate school of Science, The University of Tokyo, 13.Faculty of Science, Chiba University)

Keywords:offshore-onshore seismic survey, Hidaka collision zone, reflection survey

The 2018 Hokkaido Eastern Iburi earthquake is a unique earthquake that occurred at an unusually deep depth compared to normal crustal earthquakes. The occurrence of this earthquake was estimated to be related to the complex subsurface structure of the Hidaka collision zone(HCZ), which exists to the east of the epicenter and was formed by the collision of the Northeast Japan Arc and the Kuril Arc [e.g. Iwasaki et al., 2019]. Although several deep structures of the HCZ have been inferred from seismic reflection surveys and analysis of natural seismic waveforms, no unified interpretation has been obtained for depths greater than 40 km [e.g. Tsumura et al. 1999; Kita et al. 2012]. However, to understand the relationship between arc-arc collision and the occurrence of earthquakes deep in the crust, such as the Iburi earthquake, it is necessary to clarify the shape of the collision boundary between two island arcs, which defines the deformation to resolve the forces and mass excess acting in the collision.
In this study, we test the working hypothesis proposed by Tsumura et al. based mainly on reflection survey results that the upper part of the colliding Kuril Arc crust thrust up on the Northeast Japan Arc while a portion of the lower crust has been delaminated and subducted into the deeper part. This study aims to clarify the boundary geometry of the inter-island arc collision to a depth of about 50 km by conducting an integrated sea-land survey, in which airgun shootings off Urakawa, which is the frontal margin of the Hidaka collision zone, are received on land, and find weak reflections from the deep part of the delaminated structure that could not be followed continuously in the previous reflection profiles.
From October 16 to November 14, 2023, 210 land receiver points were set up along national roads and forest roads from Urakawa Town to Taiki Town, Hokkaido, at intervals of about 250 m, and the ground motions were continuously recorded in the field. The sampling frequency was 250Hz.
The marine survey was conducted by the joint research cruise of the Hakuho Maru (KH-23-8: November 8-12, 2023) at the Atmosphere and Ocean Research Institute of the University of Tokyo. The marine survey line was set on a northeast-southwest alignment offshore of Urakawa, with a total length of 50 km, and the northeast end of the survey line was located approximately 30 km from the southwest end of the land station. Eight ocean bottom seismometers were settled at intervals of 2.5 to 10 km below the survey line. The shootings were made on November 9-10 by four 1500 cuin. air guns at 4 knots at 75-second intervals (approximately 150 m interval). The total shots were 1039.
The waveforms observed on land were arranged as receiver gather, and at the southwestern end of the land station, the initial motion of the shot was found in the two way travel time of 10s to 22s. At the Obihiro side's station over the Hidaka Mountains, we can see the first motion of the shot atwhich was shoot a distance of about 20 km from the land side of the oceanic seismic line. We applied reflection analysis to these waveforms and clarify it if there are reflections from the target depth.
This study was supported by ERI JURP 2023-B-04 in Earthquake Research Institute, the University of Tokyo.