Seismic Swarm S20240422.1: Analysis of Recent Activity Near Hualien City, Taiwan

A seismic swarm designated S20240422.1 was recorded 24 km south of Hualien City, Taiwan, beginning at 09:17 on 22 April 2024 and concluding at 09:03 on 25 April 2024. Over 71 hours and 46 minutes, the sequence produced 84 earthquakes, with magnitudes ranging from 4.0 to 4.9 and focal depths primarily between 4 km and 37 km. The majority of events clustered at depths of 6–20 km, consistent with shallow crustal activity in the region.

The swarm exhibited a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock. Peak activity occurred on 22 April, with multiple events exceeding magnitude 4.8 within the first 12 hours. Subsequent days showed a gradual decline in both frequency and intensity, though isolated events above magnitude 4.3 persisted until the swarm’s termination. This temporal distribution aligns with fluid-driven or stress-transfer mechanisms often observed in Taiwanese swarms.

Taiwan lies at the tectonic boundary between the Philippine Sea Plate and the Eurasian Plate, where oblique collision produces high seismicity along the Longitudinal Valley and adjacent fault systems. Hualien County occupies a particularly active segment of this margin, characterized by thrust and strike-slip faulting. The swarm’s location places it near the southern extension of the Milun Fault and within the broader collision zone that has generated repeated moderate-to-large earthquakes.

Since 2000, seven swarms have been documented in the area, occurring in 2004, 2015, 2018, 2022 (two episodes), and 2024 (two episodes including the present sequence). These recurrent swarms reflect ongoing strain accumulation and release along the plate boundary. A notable preceding event was the magnitude 7.4 earthquake of 2 April 2024, centered 15 km south of Hualien City and approximately 12 km from the swarm centroid. That mainshock likely altered local stress conditions, potentially contributing to the subsequent swarm.

The shallow focal depths recorded during S20240422.1 indicate activity within the upper crust, where brittle failure predominates. Such depths increase the potential for felt ground motion in nearby populated areas, although the moderate magnitudes limited damage. Continued monitoring remains essential given Taiwan’s tectonic setting and the documented history of swarm recurrence.

References

• SeismoSight internal swarm classification data (S20240422.1 parameters and historical swarm statistics)
• Tectonic framework of Taiwan derived from standard geological literature on Philippine Sea–Eurasian plate collision
• Location and magnitude details of the 2 April 2024 M7.4 Hualien earthquake as provided in prompt data