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Activity and focal mechanism of deep low-frequency earthquakes beneath Mt. Fuji revealed through waveform classification

Takuma Ikegaya 1, Yohei Yukutake2, Kazuya Yamakawa1, Ryo Honda1, Mitsuhiro Yoshimoto1

  • Affiliations: 1Mount Fuji Research Institute, Yamanashi Prefectural Government, Fujiyoshida, Japan; 2Earthquake Research Institute, The University of Tokyo, Bunkyo-ku, Japan

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 111

  • Programme No: 2.1.21

  • Theme 2 > Session 1

Abstract

Deep low-frequency earthquakes (DLFs) are the earliest precursors of volcanic eruptions, and their monitoring is crucial for early disaster preparedness. Mt. Fuji exhibits the most frequent occurrence of DLFs in Japan, including relatively large events (M1--2). The hypocenters were located at 10--20 km depths, just above the high Vp/Vs zone. At the base of the hypocenter distribution, the inflation event occurred in 2008--2010. Following the 2011 Shizuoka Earthquake (Mw 5.9) near the northeastern edge of the hypocenter region of DLFs, the number of DLFs increased. This study systematically classified the waveforms of DLFs recorded between 2006 and 2022 to investigate their activity and focal mechanisms. Hierarchical clustering using correlation coefficients was applied to seismic data of DLFs. The analysis identified five groups (G1--G5): G1, G2, and G4 showed low-frequency (1--2 Hz) waveforms, while G3 and G5 included both low- and mid-frequency (3--8 Hz) waveforms. Hypocenter locations were slightly different between groups. G3 exhibited relatively steady activity and a high (~2) b-value. In contrast, G4 and G5, became more active after 2009 and 2011, respectively, and showed lower b-values (~1.5), indicating higher differential stress in their source regions. Focal mechanisms, estimated using S/P spectral ratios, had isotropic components possibly associated with fluid oscillation. These findings may indicate that increased differential stress, driven by magma supply linked to the inflation event and the 2011 Shizuoka Earthquake, triggered DLFs. Further studies are needed to refine waveform classification and focal mechanism estimation.