[S-SS03] Seismological advances in the ocean

[SSS03-P03] Ocean bottom seismological observation at the boundary between slow earthquakes and ordinary earthquakes in Hyuga-nada, western part of Nankai Trough

*Yusuke Yamashita¹, Shukei Ohyanagi², Yasunori Sawaki², Yukihiro Nakatani³, Hiroshi Yakiwara³, Yoshihiro Ito¹, Masanao Shinohara⁴ (1.Disaster Prevention Research Institute, Kyoto University, 2. Graduate School of Science, Kyoto University, 3.Nansei-Toko Observatory for Earthquakes and Volcanoes, Kagoshima University, 4.Earthquake Research Institute, the University of Tokyo)

Keywords:Ocean bottom seismological observation, shallow tectonic tremor, Hyuga-nada

The Hyuga-nada is a region where shallow slow earthquake activity is particularly high along the Nankai Trough, and recent seafloor observations are revealing details of shallow tectonic tremor activity (Yamashita et al., 2015; 2021) and shallow very-low-frequency earthquake activity (Tonegawa et al., 2020). Ordinary earthquake activity is also high, and it has been pointed out that small repeating earthquakes and slow earthquakes were occurring synchronously accompanied with long-distance migration (Uchida et al., 2020). The down-dip limit of the shallow tremor activity in Hyuga-nada is about 15 km, which is almost the same as the up-dip limit of the interplate earthquakes including the small repeating earthquakes. On the other hand, there are large errors in the source locations of both, which raises the question of whether the regions of both activities are separated or overlap. This problem is an important key to understanding the slip properties at plate boundaries and is also important for further discussion of the interaction between ordinary and slow earthquakes.
To discuss these issues, it is necessary to improve the accuracy of source determination for both types of earthquakes to clarify the detailed source distribution. Therefore, we are constructing a 10-15 km-span seafloor seismic observation network using long-term ocean bottom seismometers (OBSs) including a mini-dense array with an aperture of 1 km, around the boundary of the active area of shallow tremor and interplate earthquake in Hyuga-nada. The dense array observation using five OBSs was started in November 2020, and one additional OBS was added in March 2021, making a total of six OBSs. In addition, nine OBSs, including five compact broadband ocean bottom seismometers (CBBOBSs; Shinohara et al., 2021), were installed around the array. The OBSs that make the array were replaced in August 2021 and are still under observation. All the OBSs installed around the array were recovered in January 2022. Unfortunately, due to a critical bug in the recording system, data could only be recorded intermittently from mid-December 2021.
During this observation period, we have confirmed that shallow tremor activity occurred in Hyuga-nada in January 2021 and from May to June 2021 by monitoring from the land-based seismic observation network. In June 2021, the ordinary earthquake activity around the observation network became active. This activity is considered to have occurred at the plate boundary based on the analysis of the focal mechanism and probably related to the shallow slow earthquake activity.
In this presentation, we will report and discuss the initial analysis results of these activities based on the data obtained by the OBSs.

Acknowledgments:
The OBSs were deployed and retrieved by the T/S Nagasaki Maru, Faculty of Fisheries, Nagasaki University and the R/V Kaiyo Maru No. 3, Kaiyo Engineering Co., Ltd.. We would like to thank all the crew members. We are grateful to the technical staff of the Earthquake Research Institute, the University of Tokyo who worked on the preparation and operation of the OBSs. We would also like to thank the all students, researchers, and staffs who participated in the observation voyage.
This study was supported by the Research Project for Disaster Prevention on the great Earthquakes along the Nankai trough, the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and under its Earthquake and Volcano Hazards Observation and Research Program.