[S-SS07] Crustal Structure

[SSS07-P08] Seismic imaging of the subducting Kyushu-Palau ridge in relation to the distribution of tectonic tremors

*Yanxue Ma¹, Kimihiro Mochizuki¹, Rie Nakata¹, Seiichi Miura², Yasuyuki Nakamura², Ryuta Arai², Gou Fujie², Yuka Kaiho², Shuichi Kodaira² (1.Earthquake Research Institute, University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:Kyushu-Palau ridge, Nankai Trough, ocean bottom seismometers, multi-channel seismic, reflection profile

Various types of slow earthquakes including slow slip events (SSEs), very low frequency earthquakes (VLFEs) and tectonic tremors occur around the boundary in the region off the Bungo-Channel located between the Kyushu and Shikoku Islands and their distributions appear to vary along depth of the subduction of the Philippine Sea Plate. Such variations are considered to reflect the frictional properties of the interface between the subducting and overriding plates. The morphology and the physical properties of the materials along the plate interface are the major controlling factors of the frictional properties. A subducted seamount has been identified from existing seismic reflection profiles and the magnetic anomaly map as a part of the Kyushu Palau Ridge. The VLFE and tremor activities have been found to occur around this subducted seamount (Yamashita et al., 2015; Tonegawa et al., 2020; Nakata et al., 2020). We conducted a marine active-source seismic survey in August-September, 2020 along four survey lines over the tremor activity region, NE-SW (HYU01), WNW-ESE (HYU02), and two NW-SE (HYU18 and 22) directions, using ocean bottom seismometers (OBSs) and a multi-channel seismic (MCS) hydrophone streamer. The HYU01 profile runs along the strike of the Nankai Trough, and the HYU02 runs over the subducting Kyushu-Palau Ridge in the along-dip direction of the Philippine Sea plate. We analyze and image seismic structures of the along-dip profile (HYU02) as well as the previously acquired KR0114-8 profile (Park et al., 2008) that intersects with HYU02 over the tremor activity region.

For wide-angle reflection / refraction analyses, we deployed 50 OBSs at a 2 km interval, and the 10,600-cubic-inch air gun array was fired at 50 and 200 m intervals along the line HYU02 during the KM20-05 research cruise using R/V Kaimei operated by Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The data quality of the OBS records is good. We can trace the refraction first arrivals up to ~70 km offset, and some wide-angle reflections are identified. We will first apply the travel time inversion to reveal a 2D P-wave velocity structure along the subduction of the Kyushu-Palau Ridge. The reflection data were recorded by using 444-channel streamer (5.6 km long) with 12.5 m interval for HYU02 and a 156-channel streamer (4 km long) with 25 m interval for KR0114-8. The HYU02 reflection data are being processed using broadband techniques and pre-stack time/depth migration to generate detailed reflectivity images as deep as the plate interface (Nakata et al., 2021). We reprocess the KR0114-8 line using the same modern methodology and aim to derive the high-resolution image with the focus on the subducted seamount and associated deformation within the overriding plate. By combining the OBS velocity, reflection data and magnetic data, we discuss the relationships between the distribution of the tremor activity, the morphology and physical properties along the plate interface.