[S-CG62] 島弧の構造・進化・変形とプレート沈み込み作用

[SCG62-P05] 能登半島北東部の群発地震活動域における稠密地震計アレイ観測

*蔵下 英司¹、酒井 慎一²、飯高 隆²、津村 紀子³ (1.東京大学地震研究所、2.東京⼤学⼤学院情報学環、3.千葉大学)

キーワード：群発地震、能登半島、稠密地震計アレイ観測

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 largest M5.4 earthquake in the swarm activity occurred on June 19, 2022. Transient displacements are observed at three permanent GNSS within 30 km from the epicentral region of earthquake swarms (Nishimura et al., 2022). Assuming a spherical inflation (Mogi) source, Nishimura et al. (2022) estimated an annual volumetric increase of ~2.5 x 10⁷ m³ at a depth of ~12 km as a source of observed displacements. Nishimura et al. (2022) speculated the volumetric increase is caused by upwelling fluids. However, little is known about the detailed geophysical structure beneath the northeastern part of the Noto Peninsula. Revealing the crustal structure of the source region of the swarm activity is important to constrain the generation mechanisms of the earthquake swarm. To investigate seismic activity and crustal structure, we conducted a dense seismic array observation in the epicentral region of earthquake swarm in Suzu City in the northeastern Noto Peninsula. Forty-three portable seismographs were deployed on ~ 8-km-long line in the north-south direction with ~ 200 m spacing during the period from September 12, 2022 to October 18, 2022. Each seismograph consisted of a 4.5 Hz 3-component seismometer and a digital data recorder (GSX-3). Waveforms were continuously recorded at a sampling rate of 250 Hz. During the seismic array observation, the Japan Meteorological Agency located 848 earthquakes (M > 1.0) in a latitude range of 37.4°–37.6°N and a longitude range of 137.1°–137.4°E. The continuously recorded data were divided into event files, starting from an origin time determined by the JMA. The seismograms of several earthquakes show clear later phases that are probably reflected from the deeper section, which are expected to give important structural constraints beneath the northeastern part of the Noto Peninsula. We estimated the reflector depth which can well explain the observed reflection travel-times, using a 3D finite difference travel-time algorithm (Hole and Zelt, 1995). We identified reflectors at ~ 15km depth. 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. This work was supported by Grant-in-Aid for Special Purposes Grant Number 22K19949, 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.