Seismic Swarm Near Hualien City, Taiwan: April 2024 Event Analysis

A seismic swarm was recorded approximately 25 km north-northeast of Hualien City, Taiwan, beginning at 23:58 on 2 April 2024 and concluding at 02:28 on 3 April 2024. Over this 2.5-hour period, 21 earthquakes were registered, with magnitudes ranging from 5.0 to 7.4 and focal depths between 6 km and 41 km. The sequence opened with a magnitude 7.4 event at 40 km depth, followed by multiple aftershocks, including a magnitude 6.4 shock at 13 km depth shortly after midnight.

Taiwan lies at the active collision zone between the Philippine Sea Plate and the Eurasian Plate. Eastern Taiwan, including the Hualien region, experiences frequent seismicity due to rapid convergence and complex fault interactions along the Longitudinal Valley and offshore thrust systems. The area's geology features steep topography, active faults, and sedimentary basins that amplify ground motion during events.

Historical records since 2000 indicate only five prior swarms in the vicinity: one each in 2004, 2015, and 2018, plus two in 2022. Strong earthquakes have also occurred nearby, including a magnitude 7.1 event 67 km east-northeast of Hualien in March 2002 and the recent magnitude 7.4 shock 15 km south of the city on 2 April 2024, located 39 km from the swarm center. These patterns reflect the region's persistent tectonic stress accumulation.

Swarm sequences such as this typically involve clustered events without a single dominant mainshock-aftershock decay, often linked to fluid migration or stress triggering along fault networks. Depths in the current swarm spanned shallow crustal levels to around 40 km, consistent with the plate boundary structure beneath eastern Taiwan. The rapid succession of moderate-to-large events underscores the potential for cascading activity in this high-strain environment.

Continued monitoring remains essential given the proximity to populated areas and infrastructure. Such swarms provide valuable data for refining seismic hazard models in one of the world's most tectonically active zones.

References

SeismoSight internal classification data for swarm PS20240403.1
USGS Earthquake Catalog for regional historical events and plate tectonics context