[S-SS09] Crustal Structure

[SSS09-P09] Seismic Velocity Structure off the Kumano Basin ,Nankai Trough by Seismic Waveform Inversion of OBS Data

*Tatsuya Fujinaka^1,2, Shuichi Kodaira², Ryuta Arai², Ayako Nakanishi², Yuka Kaiho², Yanfang Qin², Ryo Miura², Seiichi Miura², Yasuyuki Nakamura², Gou Fujie² (1.Yokohama National University, 2.JAMSTEC)

Keywords:Nankai trough, seismic survey, Slow Earthquake, travel-time tomography

Various types of slow earthquakes occur in the Nankai Trough. Recent seafloor observations have revealed that very low-frequency earthquakes (VLFEs) and low-frequency tremors (LFTs) occur not only along plate boundaries deeper than the seismogenic zone, but also in shallow areas near the subduction zone.The distribution of shallow VLFEs off the Kumano Basin is not uniform, and they are concentrated in the central to western part of the region, with only a few occurring in the eastern part. (Takemura et al.,2023).To investigeate the relationship between the VLFEs inactive zone and the structural features off the Kumano Basin,a refraction and wide-angle seismic reflection survey using ocean bottom seismometers (OBS) was conducted in 2022 by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) .We analyzed the OBS data by ray-tracing travel time tomography (Fujie et al, 2006, 2013). The initial model for travel-time tomography was constructed by referring a study in the central Nankai Trough (Qin et al., 2020).So far, we identified a low-velocity region on the upper plate (P-wave velocity of 4 km/s) ,extending approximately 50 km landward of the trough and reaching a depth of about 9 km. Additionally,a landward-sloping P-wave velocity region of 8 km/s indicates a subducting ocean mantle.The root-mean-square of the traveltime residuals between the picked and calculated were 230.3 (ms) for the initial model and 86.6 (ms) for the final model. We will construct a more detailed velocity structure model using wide-angle seismic reflection data and compare it with seismic reflection data to obtain the shape of the surface of the subducting oceanic plate and the heterogeneous structure of the upper plate,and discuss the relationship between the subsurface structure and slow events in this study area.