[S-VC30] International Volcanology

[SVC30-05] Time-dependent activity of low-frequency earthquakes and ordinary earthquakes in the eastern side of the Izu Peninsula, Japan

*楠城 一嘉^1,2,3,4、行竹 洋平⁵、熊澤 貴雄³ (1.静岡県立大学、2.静岡大学、3.統計数理研究所、4.海洋研究開発機構、5.東京大学)

キーワード：低周波地震、ETAS、地表変位、伊豆半島

The Izu Peninsula in central Japan is in the northern most part of the Izu Bonin Mariana Arc and is located in the collision zone with Honshu Island, where crustal deformation is active. The eastern side of the Izu Peninsula is volcanically and seismically active (e.g., Aramaki and Hamuno, 1977). In this region, earthquake swarms of ordinary earthquakes frequently occur at shallow depths, and this is considered to be associated with magma intrusion. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) are infrequently observed. To better understand the characteristics of LFEs, we produced an LFE catalog for 2005-2020, using the matched-filter method (e.g., Yukutake et al., 2019). We then conducted a timeseries analysis based on the Epidemic-Type Aftershock Sequence model (Kumazawa et al., 2019; Nanjo et al., 2023), showing relative quiescence (change in occurrence rate, which became quiet). For comparison, the same analysis was conducted by using the Japan Meteorological Agency (JMA) catalog of ordinary earthquakes, showing a similar result. The change points for both types of earthquakes fell sometime between late 2009 and mid-2013, during which two out of three swarms of ordinary earthquakes occurred. We associated the quiescence with a decrease in background rate, where background rate, by removing the triggering effect of aftershocks, is known to be caused directly by the magma source, which can vary with time. We further discussed surface displacement data obtained from a leveling survey (Geospatial Information Authority of Japan; 2016) and the GEONET (Muramatsu et al., 2021; Takamatsu et al., 2023), showing that the uplift due to magma intrusion was significant during the 1970s-1990s, although it was in abatement or unobservable during the study period (2005-2020). We interpreted the quiescence with no significant uplift as an indication that magma source, which caused magma intrusion into the study region, is in a transition phase, becoming less active, compared with the magma source during the 1970s-1990s.
Preprint associated partially with this presentation is available at https://doi.org/10.48550/arXiv.2407.19648.
This study was partially supported by the MEXT, under The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research) (K.Z.N., Y.Y.) and under STAR-E (Seismology TowArd Research innovation with data of Earthquake) Program Grant Number JPJ010217 (K.Z.N., T.K.), JSPS KAKENHI Grant Numbers JP 22K03752 (Y.Y.) and 20K11704 (T.K.), and a Research Grant of the Izu Peninsula UNESCO Global Geopark (K.Z.N., Y.Y.). The authors thank Y. Noda for help with implementing the MF method. The waveform records were obtained from the permanent stations of the National Research Institute for Earth Science and Disaster Resilience, Earthquake Research Institute at the University of Tokyo, JMA, and the Hot Springs Research Institute of the Kanagawa Prefectural Government.