[S-SS09] Crustal Structure

[SSS09-01] Precise hypocenter distribution and crustal structure in the epicentral region of earthquake swarm in the northeastern Noto Peninsula

*Eiji Kurashimo¹, Shinichi Sakai², Masanao Shinohara¹ (1.Earthquake Research Institute, The University of Tokyo, 2.Interfaculty Initiative in Information Studies, The University of Tokyo)

Keywords:earthquake swarm, Noto Peninsula, offshore-onshore integrated seismic observation

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. After the Mj 6.5 earthquake occurred on May 5, 2023, the earthquake swarm area extended northward and considerable swarm activity has been detected in marine area. The largest M7.6 earthquake in the swarm activity occurred on January 1, 2024. Previous studies have been suggested that fluid promoted the occurrence of this swarm activity (e.g., Nakajima , 2022; Nishimura et al., 2023; Kato, 2024; Okada et al., 2024). Several active faults have been found in and around the earthquake swarm area (e.g. Inoue and Okamura, 2010). However, little is known about the detailed geophysical structure in the epicentral region of the earthquake swarm, especially in the onshore-off shore boundary area. In order to study the long-term behavior of active faults, it is important to clarify the relationship between an active fault system and hypocenter distribution. Revealing the crustal structure of the source region of the swarm activity is important to constrain the generation mechanisms of the earthquake swarm. Since the northern part of the earthquake swarm area is located below the marine area, seafloor seismic observation is also necessary to obtain precise hypocenter distribution and crustal structure. Therefore, in 2023, we conducted an offshore-onshore integrated seismic observation in and around Suzu City located in the northeastern part of the Noto Peninsula (Kurashimo et al., 2023). Five anchored-buoy OBSs (Shinohara et al., 2022) were deployed in the marine area during the period from June 26, 2023 to July 5, 2023. 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. Twelve temporary land seismic stations with a 1.0Hz three-component seismometer were also installed in the earthquake swarm area. Waveforms were continuously recorded at a sampling rate of 250 Hz. During the seismic observation, the Japan Meteorological Agency (JMA) located 205 earthquakes (M≧1.0) in a latitude range of 37.4°-37.75°N and a longitude range of 137.0°-137.6°E. The continuously recorded data were divided into event files, starting from an origin time determined by the JMA. In order to obtain a high-resolution velocity model, a well-controlled hypocenter is essential. Due to this, we combined the offshore-onshore integrated seismic data with permanent seismic station data. We used 23 telemetered seismic stations in the present study. We applied the double-difference tomography method (Zhang & Thurber, 2003) to the P- and S-wave arrival time data obtained from 205 earthquakes to investigate the earthquake locations and three dimensional seismic velocity structures in detail. The hypocentral distribution shows a concentration on a plane dipping southeastward in the vicinity of the 2023 Mj 6.5 event hypocenter. A high Vp and Vp/Vs zone is identified at a depth range of 10-15km beneath the northeastern tip of the Noto Peninsula. The swarm hypocenters appear to be distributed avoiding this high Vp and Vp/Vs zone.
In this presentation, we present precise hypocenter distribution and the crustal structure of the source region of the earthquake swarm activity in the northeastern part of the Noto Peninsula.

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 Second Earthquake and Volcano Hazards Observation and Research Program.