Seismic Swarm PS20240422.1: Insights into Recent Activity Near Hualien, Taiwan

A seismic swarm designated PS20240422.1 occurred 28 km south of Hualien City, Taiwan, from 09:08 on 22 April 2024 to 02:20 on 23 April 2024. Over 17 hours and 12 minutes, 27 earthquakes were recorded, with magnitudes ranging from 4.1 to 6.1 and focal depths primarily between 5 km and 27 km. The sequence featured multiple events above magnitude 5.0, including peaks at 6.1 on 22 April at 18:26 and 18:32, both at shallow depths near 9–10 km. Earlier activity included a 5.8 event at 14:11 and several 5.2–5.6 shocks clustered around 10–11 km depth, while later stages showed slightly deeper occurrences up to 21 km.

This swarm reflects typical patterns in eastern Taiwan, where shallow crustal seismicity often clusters due to ongoing tectonic interactions. Most events remained below magnitude 6.0, consistent with swarm behavior rather than a single mainshock-aftershock sequence. Depths concentrated in the upper 20 km indicate activity within the brittle crust, a common feature in regions experiencing rapid plate convergence.

Taiwan lies at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate, driving frequent seismic episodes along the eastern margin. Hualien County, situated near the Longitudinal Valley fault system, experiences elevated seismicity from oblique collision and subduction processes. The area has a documented history of both isolated large earthquakes and episodic swarms. Since 2000, seven prior swarms have been identified in the broader region, occurring in 2004, 2015, 2018, 2022 (twice), and 2024 (twice).

Notable strong events include the magnitude 7.4 earthquake of 2 April 2024, located 15 km south of Hualien City and only 3 km from the PS20240422.1 swarm center. An earlier magnitude 7.1 event struck 67 km east-northeast of Hualien on 31 March 2002. These incidents underscore the persistent seismic hazard in the immediate vicinity.

The April 2024 swarm followed closely after the magnitude 7.4 mainshock, suggesting possible stress redistribution along nearby fault segments. Such sequences contribute to refined understanding of fault interactions in the collision zone, supporting improved seismic monitoring and hazard assessment for eastern Taiwan.