[S-SS12] Statistical seismology and underlying physical processes

[SSS12-P11] An offshore-onshore integrated seismic observation in the westernmost focal area of the 2024 Noto-Hanto earthquake

*Eiji Kurashimo¹, Shinichi Sakai², Noriko Tsumura³, Masanao Shinohara¹ (1.Earthquake Research Institute, The University of Tokyo, 2.Interfaculty Initiative in Information Studies, The University of Tokyo, 3.Graduate School of Science, Chiba University)

Keywords:offshore-onshore integrated seismic observation, the 2024 Noto-Hanto earthquake

The earthquake swarm activity in the northeastern part of the Noto Peninsula, central Japan, has been observed since around 2018 and has become more active since December 2020. The 2024 Mj 7.6 Noto-Hanto earthquake occurred in the Noto Peninsula on January 1st and seismic activity extended to the region with approximately 150 km length in the direction of northeast to southwest. The westernmost region of seismic activity extends westward off the Noto Peninsula. Several active faults have been found in the focal area of the Noto-Hanto earthquake (e.g. Inoue and Okamura, 2010). The Integrated Research Project on Seismic and Tsunami Hazards around Japan Sea (PJS) performed to construct marine and coastal fault models along the Japan Sea. A fault model constructed by the PJS shows that several faults exist around the Noto Peninsula (Sato et al., 2020). Revealing the relationship between an active fault system and hypocenter distribution is important to constrain the process of earthquake occurrence. Since the westernmost focal area of the 2024 Noto-Hanto earthquake is located below the marine area, seafloor seismic observation is necessary to obtain precise hypocenter distribution. Therefore, we conducted an offshore-onshore integrated seismic observation in and around the Sika town located in the westernmost part of the Noto Peninsula. Four anchored-buoy OBSs (Shinohara et al., 2022) were deployed in the marine area during the period from August 3, 2024 to August 26, 2024. Each seismograph consisted of a 15 Hz 3-component seismometer, a hydrophone and an ocean bottom recorder. Waveforms were continuously recorded at a sampling rate of 500 Hz. Unfortunately, it was found that one buoy had disappeared from the sea’s surface at the deploying position on August 26. Three anchored-buoy OBSs were successfully recovered, and we obtained their data. Ten temporary land seismic stations with a 1.0Hz three-component seismometer were also installed in the westernmost part of the Noto Peninsula. Waveforms were continuously recorded at a sampling rate of 500 Hz. The data obtained by both the anchored-buoy OBSs and temporary land seismic stations had sufficient quality for picking up the arrival times of the P- and S-waves and the polarity of the first arrivals. During the offshore-onshore integrated seismic observation, the Japan Meteorological Agency (JMA) located 108 earthquakes (M≧1.0) in a latitude range of 37.05°-37.3°N and a longitude range of 136.45°-136.8°E. The continuously recorded data were divided into event files, starting from an origin time determined by the JMA.
In this presentation, we present precise hypocenter distribution in the westernmost focal area of the 2024 Noto-Hanto earthquake using the offshore-onshore integrated seismic observation data.

Acknowledgement: We used the JMA unified earthquake catalog. We thank the University of Tokyo, Kyoto University, National Research Institute for Earth Science and Disaster Prevention, and JMA for providing the waveform data. This work was supported by Grant-in-Aid for Special Purposes Grant Number 23K17482, and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its The Third Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).