Seismic Swarm Activity Near Hualien City, Taiwan: The March 2022 Event

Taiwan lies at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate, resulting in frequent seismic activity across the island. The region southwest of Hualien City, where the March 2022 swarm occurred, sits within a tectonically active zone characterized by thrust faulting and subduction-related processes. This area has a documented history of both isolated strong earthquakes and clustered swarm sequences.

The swarm designated PS20220322.1 began at 17:41 on 22 March 2022 and concluded at 20:29 the same day, lasting two hours and forty-eight minutes. It was centered 68 km SSW of Hualien City. During this period, seven earthquakes were recorded. The sequence opened with a magnitude 6.7 event at 24 km depth, followed within two minutes by a magnitude 5.9 shock at 16 km depth. Subsequent events included magnitudes 4.6 at 23 km, 5.1 at 11 km, 5.0 at 26 km, 5.5 at 24 km, and a final magnitude 5.8 at 18 km depth. Depths remained relatively shallow throughout, consistent with activity along upper-crustal structures in the region.

Earthquake swarms in this part of Taiwan are infrequent but recurrent. Since 2000, only four such swarms have been identified in the vicinity: one each in 2004, 2006, 2015, and 2018. These episodes typically involve multiple events of moderate magnitude occurring over short time windows without a single dominant mainshock-aftershock pattern. The 2022 swarm fits this established pattern of clustered seismicity.

The broader Hualien area has also experienced significant individual earthquakes. On 2 April 2024, a magnitude 7.4 event struck 15 km south of Hualien City, approximately 45 km from the 2022 swarm center. This underscores the persistent seismic hazard along the eastern Taiwan collision zone, where strain accumulation and release occur regularly.

Analysis of the 2022 swarm suggests a possible role for fluid migration or localized stress transfer along pre-existing faults, common mechanisms in swarm sequences worldwide. The rapid succession of events, with magnitudes ranging from 4.6 to 6.7 and depths between 11 and 26 km, indicates activation of a compact fault segment rather than widespread rupture propagation. No surface rupture was reported, aligning with the moderate magnitudes and depths involved.

Continued monitoring remains essential given Taiwan’s tectonic setting. Historical swarm recurrence and proximity to larger events such as the 2024 magnitude 7.4 earthquake highlight the value of real-time seismic networks for distinguishing swarm behavior from foreshock sequences.

References

SeismoSight internal swarm classification records (PS20220322.1 parameters).
Taiwan Central Weather Administration seismic catalogs (2000–2024 regional events).
USGS Earthquake Catalog (Hualien area strong events).