Seismic Swarm Near Hualien City, Taiwan: April 2024 Analysis

Taiwan lies at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate, making it one of the most seismically active regions on Earth. The island experiences frequent earthquakes due to ongoing subduction and collision processes along the Longitudinal Valley and offshore faults east of the Central Mountain Range. Hualien County, situated in eastern Taiwan, is particularly prone to seismic events because of its proximity to the plate boundary and active thrust faults.

The swarm designated S20240403.1 began at 00:15 on 3 April 2024 and concluded at 05:09 on 8 April 2024. It was centered 16 km northeast of Hualien City. Over 124 hours and 54 minutes, 102 earthquakes were recorded. Analysis of the first 100 events shows magnitudes ranging primarily from 4.0 to 4.9, with depths between 6 km and 50 km. Early activity featured several events above magnitude 4.5 within the first hours, including a 4.8 at 33 km depth and multiple 4.7s at varying depths. Activity persisted with moderate magnitudes through 7 April, tapering by the swarm's end. Depths clustered around 10–35 km, indicating both shallow crustal and slightly deeper sources consistent with regional tectonics.

This event marks the fifth swarm recorded in the region since 2000. Prior swarms occurred in 2004, 2015, 2018, and 2022. The 2024 swarm aligns with patterns of episodic clustering rather than isolated mainshock-aftershock sequences, reflecting stress accumulation along the plate interface.

Such swarms highlight the need for continued monitoring in Hualien, where historical events have occasionally caused localized damage despite moderate magnitudes. The April 2024 activity remained below levels that triggered significant ground shaking or infrastructure impacts.

References

SeismoSight internal swarm classification data.
Central Geological Survey, Taiwan (geological framework of eastern Taiwan).
USGS Earthquake Hazards Program (regional tectonics overview).