Seismic Swarm PS20180206.1: Analysis of February 2018 Activity Near Hualien, Taiwan

Taiwan lies at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate, where ongoing collision and subduction drive one of the world's highest rates of seismic activity. The eastern region around Hualien City experiences particularly intense deformation due to the northward motion of the Philippine Sea Plate and the presence of the Longitudinal Valley Fault system. This tectonic setting produces frequent moderate-to-large earthquakes, with historical records documenting destructive events that have shaped regional hazard assessments.

The seismic swarm designated PS20180206.1 began at 15:50 on 6 February 2018 and concluded at 15:21 on 7 February 2018, lasting 23 hours and 30 minutes. During this interval, seven earthquakes were recorded in close succession. The sequence initiated with a magnitude 6.4 event at 17 km depth, followed by six aftershocks ranging from magnitude 4.0 to 5.7 at depths between 4 km and 15 km. The temporal clustering and spatial proximity of these events classify the sequence as a swarm rather than a typical mainshock-aftershock series.

The individual events unfolded as follows: the initial magnitude 6.4 shock at 15:50:43 on 6 February, a magnitude 5.1 event at 16:17:57, two magnitude 5.2 shocks at 18:00:12 and 18:07:38, a magnitude 4.0 event at 18:09:23, a magnitude 5.4 earthquake at 19:15:29, and a final magnitude 5.7 shock at 15:21:31 on 7 February. Depths remained consistently shallow, consistent with crustal faulting in the Hualien area.

Since 1 January 2000, only two prior swarms have been documented in the same region: one in 2004 and another in 2015. This low frequency underscores the episodic nature of swarm activity amid Taiwan's otherwise continuous background seismicity. In contrast, strong independent earthquakes have occurred nearby, including a magnitude 7.4 event 15 km south of Hualien City on 2 April 2024, located 27 km from the 2018 swarm center, and a magnitude 7.1 earthquake 67 km east-northeast of Hualien City on 31 March 2002, situated 52 km from the swarm epicenter.

Such swarms contribute to refined understanding of fault interactions and stress transfer in Taiwan's collisional orogen. Continued monitoring supports improved probabilistic seismic hazard models for the eastern Taiwan coastal zone, where population centers remain exposed to shallow crustal events.