[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-27] Active source seismic experiment investigating structural characteristics associated with slow earthquake activities in the Hyuga-nada, southwestern Japan

*Seiichi Miura¹, Kimihiro Mochizuki², Ryuta Arai¹, Yasuyuki Nakamura¹, Yuka Kaiho¹, Gou Fujie¹, Shuichi Kodaira¹, Rie Nakata², Yanxue Ma², Masataka Kinoshita², Yoshitaka Hashimoto³, Yohei Hamada¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Earthquake Research Institute, University of Tokyo, 3.Kochi University)

Keywords:Slow earthquake, Hyuga-nada, MCS, OBS

The Hyuga-nada is located in the westernmost part of the Nankai subduction zone, connecting to the Ryukyu Trench toward southwest. The Kyushu-Palau Ridge in the Philippine Sea Plate is now subducting in the Hyuga-nada region where the seafloor topography is very rough (e.g., Yamamoto et al. 2013). Magnitude (M) seven class large earthquakes have occurred in the Hyuga-nada such as 1961 (M7.0), 1968 (M7.5), and 1984 (M7.1), whereas no earthquake greater than M8 has occurred. Frequent slow slip events (SSE) are detected in the shallow part of the Hyuga-nada as well as low frequency tremors and very low-frequency earthquakes that exhibit spatial migration (e.g. Yamashita et al., 2015; Tonegawa et al., 2020; Yokota and Ishikawa, 2020).

To investigate the structural characteristics associated with the slow earthquake activity in the Hyuga-nada, we conducted two seismic cruises using R/Vs Kaimei and Kairei of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from August to September, 2020. These cruises are partly supported by KAKENHI “Science of Slow Earthquakes”, and also a part of site survey for IODP drilling proposal in Hyuga-nada. The seismic source was an airgun array with total volume of 10600 cu. in. of R/V Kaimei. Multi-channel seismic (MCS) hydrophone streamer cable with 6-km length was also towed by the R/V Kaimei to acquire seismic reflection profiles along the four seismic lines crossing tremor area (Yamashita et al, 2015): NE-SW (HYU01), WNW-ESE (HYU02), and two NW-SE (HYU18 and 22) directions. One hundred ocean bottom seismometers (OBS) were deployed by R/V Kaimei along two seismic lines (HYU01 and HYU02) with 2-km spacing to record wide-angle reflection and refraction signals and retrieved by R/V Kairei .

In our presentation, we will show the detail of the active source experiments, acquired MCS and OBS data, and preliminary processing results.