Presentation Information
[S23-03]Crustal deep seismic swarm in 2025 and seismic velocity structure in northern Yamaguchi Prefecture, SW Japan
*Tomomi OKADA1, Masatoshi Miyazawa2, Tomoaki Nishikawa2, Takuya Nishimura2, Airi Nagaoka2, Rintaro Miyamachi 2, Shiro Ohmi2, Satoshi Matsumoto3, Ryota Takagi1 (1. Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2. Disaster Prevention Research Institute, Kyoto University, 3. Graduate School of Science, Kyushu University)
山口県北部地域では，2025年2月ごろから，深さ25-30kmにおいて群発地震活動が見られた。気象庁一元化震源カタログによれば，地震の一部には低周波地震が含まれる。この地域は阿武火山群が分布する領域である。また，地福断層・大原湖断層・奈古断層など，M6.8-M7.5の規模の地震の発生が評価されている活断層が分布している。深さ約15kmよりも浅い地震活動が活発な領域であり，例えば，1997/06/25には深さ10km付近においてMj6.6の地震が発生した。本研究では，今回の群発地震活動やこの地域の地震活動・火山活動の原因を把握することを目的として，この地域の地震波速度構造を推定した。気象庁一元化震源カタログおよび群発地震活動域直上に京都大学防災研究所が設置した臨時観測点を用いて，Double-Difference トモグラフィ法 [Zhang and Thurber, 2003, 2006] により，2025年2月ごろから群発地震活動が発生した山口県北部地域の地震波速度構造を求めた。深部群発地震に加えて，長期の浅い地震活動と地震波速度構造の関係を確認することを目的とした。そのため，気象庁において一元化処理が開始された2000.10月以降の約25年間の地震をデータとした。解析領域は深部群発地震の付近の構造を求めるため広めに取った。解析に用いた地震は，2000.10.1-2025.8.2の期間の15791個である。活発化した地震活動域の周辺には高いVp/Vs・低いVs領域が確認できる。高いVp/Vs・低いVs領域の成因としては，阿武火山群の源となるマグマだまりやそこから放出される流体が挙げられ，この地震活動と流体との関わりが示唆される。またこれらの高いVp/Vs・低いVs領域はそれ以外の深さ約15kmより浅い地震活動の直下・周囲にも位置しており，この地域の地震活動と流体との関わりが示唆される。
In northern Yamaguchi Prefecture, SW Japan, an earthquake swarm activity occurred at a depth of 25 to 30 km from February 2025. This area is located in the Abu Volcano Group. There are some active faults, and seismic activity occurs at depths shallower than about 15 km. For example, on 25 June 1997, an earthquake with a magnitude of 6.6 occurred at a depth of approximately 10 km. In this study, we estimated the seismic velocity structure of this region to elucidate the causes of the recent swarm earthquake activity and the mechanisms underlying seismic and volcanic activity in this area.Using the Unified Earthquake Catalogue of the Japan Meteorological Agency and a temporary seismic observation installed above the swarm earthquake activity region by the Disaster Prevention Research Institute of Kyoto University, we determined the seismic velocity structure of northern Yamaguchi Prefecture, where swarm earthquake activity began around February 2025, using the double-difference tomography method [Zhang and Thurber, 2003, 2006]. The objective was to confirm the relationship between deep swarm earthquakes, long-term shallow seismic activity, and seismic velocity structure. Therefore, we used data from earthquakes that occurred about a 25-year period from October 2000, when the Japan Meteorological Agency began the unified processing. The analysis area was broadly defined to determine the structure near the deep swarm earthquakes. The earthquakes used in the analysis were 15,791 events occurring from 1 October 2000 to 2 August 2025. Areas with high Vp/Vs ratios and low Vs were identified around the active seismic activity areas. Fluids were suggested as the cause of these high Vp/Vs and low Vs, indicating a possible relation between seismic activity and fluids.
In northern Yamaguchi Prefecture, SW Japan, an earthquake swarm activity occurred at a depth of 25 to 30 km from February 2025. This area is located in the Abu Volcano Group. There are some active faults, and seismic activity occurs at depths shallower than about 15 km. For example, on 25 June 1997, an earthquake with a magnitude of 6.6 occurred at a depth of approximately 10 km. In this study, we estimated the seismic velocity structure of this region to elucidate the causes of the recent swarm earthquake activity and the mechanisms underlying seismic and volcanic activity in this area.Using the Unified Earthquake Catalogue of the Japan Meteorological Agency and a temporary seismic observation installed above the swarm earthquake activity region by the Disaster Prevention Research Institute of Kyoto University, we determined the seismic velocity structure of northern Yamaguchi Prefecture, where swarm earthquake activity began around February 2025, using the double-difference tomography method [Zhang and Thurber, 2003, 2006]. The objective was to confirm the relationship between deep swarm earthquakes, long-term shallow seismic activity, and seismic velocity structure. Therefore, we used data from earthquakes that occurred about a 25-year period from October 2000, when the Japan Meteorological Agency began the unified processing. The analysis area was broadly defined to determine the structure near the deep swarm earthquakes. The earthquakes used in the analysis were 15,791 events occurring from 1 October 2000 to 2 August 2025. Areas with high Vp/Vs ratios and low Vs were identified around the active seismic activity areas. Fluids were suggested as the cause of these high Vp/Vs and low Vs, indicating a possible relation between seismic activity and fluids.