[J08-P] Poster

[J08-P-14] Geometry of the frontal thrust at the trench axis around the Hyuga-nada region revealed by high-resolution seismic reflection imaging

^○Mikiya Yamashita¹, Ayako Nakanishi¹, Ryuta Arai¹, Shuichi Kodaira¹, Yasuyuki Nakamura¹, Seiichi Miura¹, Yoshiyuki Kaneda^1,2 (1.Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, 2.Kagawa University, Japan)

Great earthquakes with tsunamis with recurrence intervals of 100–200 years have occurred along the Nankai Trough near central Japan. Hyuga-nada is located in the western part of Nankai Trough, subducting the Kyushu-Palau ridge which is remnant arc constructed by backarc spreading in Shikoku Basin. New tsunami source model of historical tsunamigenic earthquake (e.g. 1707 Hoei) is extended to Hyuga-nada region revised by inundated deposits at coastal lake in Kyushu. Recent ocean-bottom monitoring results have also yielded presence the episodic low-frequency tremor associated with the activity of very-low-frequency earthquake at shallow plate interface at the northern end of subducting Kyushu-Palau ridge.
In order to understand the geometry of the frontal thrust around the Kyushu-Palau ridge, a dense high-resolution seismic reflection survey was conducted by Japan Agency for Marine-Earth Science and Technology over 600 km of line length from Sep. to Oct., 2016 using R/V Yokosuka.
Clear seismic reflection images of frontal thrust in the accretionary prism, trench-fill deposits are shown around the Kyushu-Palau ridge from preliminary profiles. The distribution of trench-fill sediments is limited to only the east side of Kyushu-Palau ridge. There is no deformation structure in the trench-fill sediments from the trench axis to the seaward region. In the west side of Kyushu-Palau ridge, there are few sediments on the subducting oceanic crust. These characteristics suggests that the crustal structure at the trench axis around the Kyushu-Palau ridge is not “typical" Nankai subduction zone. Although our surveys covered a part of Nankai seismogenic zone, it is important to understand the deformation process of the western part of Nankai Trough.
This study is part of "Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai Trough region" funded by the Ministry of Education, Culture, Sports, Science, and Technology of Japan.