Recurrent earthquake swarms and transient crustal deformation off the Tokara Islands, southe, rn Japan: Insights from the 2025 swarm sequence

OHZONO Mako

Institute of Seismology and Volcanology

Jul. 07, 2026

図1. (a)解析対象地域周辺の広域地図。赤色の三角は活火山を示す。(b) 2025年6月1日から7月31日までの地震活動（桃色円）とGNSS観測局分布。ダイヤモンドは国土地理院の運用するGEONET観測局を、四角はソフトバンクの独自基準点をそれぞれ示す。(c) 宝島（BQ1I）、小宝島（BQ1J）、悪石島（BQ1K）の観測局の GNSS 時系列。背景の色は地震活動と地殻変動に基づき同定した「Phase」を示す。(d) (b)に示した地震の時間－経度図。黒い破線は宝島、小宝島、悪石島のおおよその位置を示す。(e) (b)に示した地震の時間規模図。

Joint press release (in Japanese) by Tohoku University, Hokkaido University and Kagoshima University

Abstract
Transient crustal deformation is often observed in conjunction with earthquake swarms and is attributed to aseismic processes that drive swarm activity, such as magma intrusion, fluid migration, or slow slip events. Off the Tokara Islands, along the volcanic front of the Ryukyu subduction zone in the southern Japanese Archipelago, earthquake swarms have repeatedly occurred; however, the driving mechanism of this anomalous seismicity remains unclear. Here, we investigate this mechanism through integrative analyses of Global Navigation Satellite Systems (GNSS) data from the Geospatial Information Authority of Japan and SoftBank Corp. The daily GNSS position time series at multiple stations exhibits clear transient signals that coincide with recurrent earthquake swarms. Notably, during the June–July 2025 episode, both the temporal trend and spatial pattern of the displacement changed abruptly at the onset of the secondary swarm. Motivated by these distinctive characteristics, we estimated….

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Article Information:
Recurrent earthquake swarms and transient crustal deformation off the Tokara Islands, southe, rn Japan: Insights from the 2025 swarm sequence, Yutaro Okada, Yusaku Ohta, Miku Ohtate, Yoshiaki Ito, Mako Ohzono, Hiroshi Yakiwara, and Shigeru Nakao, Earth, Planets and Space, 78, 130 (2026)
DOI: 10.1186/s40623-026-02489-6