Seismic Swarm PS20220918.1: Geological Context and Event Analysis in Taiwan

Taiwan occupies a tectonically active zone at the convergent boundary between the Eurasian Plate and the Philippine Sea Plate. This collision drives rapid crustal deformation, frequent earthquakes, and the formation of the island's Central Mountain Range. The region east of Yujing lies within this compressional regime, where shallow crustal faults accommodate ongoing convergence at rates exceeding 7 centimeters per year. Historical records document persistent seismic activity, with multiple earthquake swarms reflecting episodic strain release along distributed fault networks.

The swarm designated PS20220918.1 began at 13:41 on 17 September 2022 and concluded at 02:07 on 19 September 2022. Centered 81 kilometers east of Yujing, the sequence lasted 36 hours and 26 minutes and included 11 events. Magnitudes ranged from 4.6 to 6.9, with all but one hypocenter located at depths of 9–11 kilometers. The initial event registered magnitude 6.5, followed within hours by magnitude 5.1 and 5.5 shocks. Activity intensified on 18 September with a magnitude 5.6 event at 05:19, a magnitude 6.9 mainshock at 06:44, and subsequent events reaching magnitude 5.9. The final magnitude 5.5 earthquake occurred early on 19 September.

This swarm exhibited classic characteristics of clustered seismicity without a single dominant mainshock-aftershock decay pattern. Multiple events above magnitude 5.5 occurred within a narrow temporal window, consistent with fluid migration or stress triggering along interconnected fault segments. Depths remained uniformly shallow, indicating rupture within the brittle upper crust typical of Taiwan's active collision zone.

Since 1 January 2000, six swarms have been recorded in the broader region. These occurred in 2004, 2006, 2015, 2018, and twice in 2022. The recurrence underscores the area's capacity for episodic swarm activity superimposed on background seismicity. On 2 April 2024, a magnitude 7.4 earthquake struck 15 kilometers south of Hualien City, approximately 69 kilometers from the 2022 swarm center. This event further illustrates the persistent seismic hazard along Taiwan's eastern plate boundary.

Analysis of the 2022 sequence provides insight into short-term clustering behavior. The rapid succession of moderate-to-strong events within 36 hours highlights the potential for accelerated moment release in this segment of the collision zone. Such swarms contribute to long-term strain accommodation while posing challenges for real-time hazard assessment due to their abrupt onset and variable duration. Continued monitoring remains essential for refining models of fault interaction in this geologically dynamic setting.