2025-05-26 静岡県立大学グローバル地域センター,統計数理研究所,伊豆半島ジオパーク
図1. 伊豆半島東部のLFEが地下深くで起きていることを示す図。a, 黒枠はbの領域を示す。b, 研究領域を黒枠で示す。気象庁が2005年1月〜2020年12月に観測したLFE(マグニチュードM≥0.1)とLFE以外の通常の地震(M≥1)をそれぞれ赤印と黒印で示す。2006年と2009年の群発地震を起こしたマグマ貫入と関連する変動があった位置を、それぞれ橙色と青色で示す。三角は手石海丘(1989年の海底噴火により形成された火山)の位置を示す。c, bの断面図。
＜関連情報＞
- https://www.u-shizuoka-ken.ac.jp/news/20250526/
- https://www.u-shizuoka-ken.ac.jp/media/20250526_press.pdf
- https://www.sciencedirect.com/science/article/pii/S0377027325000915
伊豆半島の火山活動域における地震活動の変化 Changes in seismicity in a volcanically active region of the Izu Peninsula, Japan
K.Z. Nanjo, Y. Yukutake, T. Kumazawa
Journal of Volcanology and Geothermal Research Available online: 8 May 2025
DOI:https://doi.org/10.1016/j.jvolgeores.2025.108355
Highlights
- Deep low-frequency earthquakes in Izu Peninsula, Japan, showed seismic quiescence.
- Shallow ordinary earthquakes in the same region showed similar quiescence.
- The start of quiescence was earlier for shallow earthquakes than for deep ones.
- Seismic quiescence occurred with no significant uplift during the study period.
- We propose a hypothesis of magma behavior related to quiescence and no uplift.
Abstract
The eastern Izu Peninsula in Japan is volcanically and seismically active. Ordinary earthquakes frequently occurred at shallow depths in 2006 and 2009, when they clustered as swarms. Beneath ordinary earthquakes, low-frequency earthquakes (LFEs) were infrequently observed. To better understand the characteristics of those LFEs, we produced a LFE catalog for 2005–2020, using the matched-filter method. Timeseries analyses based on the Epidemic-Type Aftershock Sequence model showed quiescence, i.e., a change in occurrence rate, which became quiet. For comparison, the same analysis was conducted using the Japan Meteorological Agency catalog of ordinary earthquakes, and similar results were shown. The change points for both types of earthquakes fell during and after each of the swarms, revealing an earlier start of quiescence for shallow ordinary earthquakes than for deep LFEs. Surface displacement data obtained from the Geospatial Information Authority of Japan showed that the uplift was in abatement or unobservable during the study period. Our seismicity and geodetic results are interpretatively hypothesized as being associated with magmatic activity, as follows. When the absence of a supply of magma from a depth toward the crustal magma reservoir caused no significant uplift, magma intruded from this reservoir into the shallow crust, resulting in the 2006 and 2009 swarms. Due to this intrusion, stresses decreased in and around shallow parts of the reservoir and then quiescence for ordinary earthquakes began during or after each swarm. Due to this decrease, stresses in and around deep parts of it late decreased, delaying the start of quiescence of deep LFEs.
