Seismic Swarm PS20061009.2 Offshore Southern Taiwan

On 9 October 2006, a seismic swarm designated PS20061009.2 was recorded 161 km south-southwest of Hengchun, Taiwan. The sequence began at 10:01 local time and concluded at 11:43, encompassing nine earthquakes within 1 hour and 41 minutes. Magnitudes ranged from 6.3 to 5.0, with focal depths predominantly near 10 km and isolated events at 14 km and 33 km.

The initial event at 10:01:47 registered magnitude 6.3 at 14 km depth. Subsequent shocks followed rapidly: a magnitude 5.0 at 10:28:32 (10 km), another 5.0 at 11:00:23 (10 km), magnitude 5.1 at 11:02:03 (10 km), magnitude 5.2 at 11:04:26 (10 km), magnitude 5.4 at 11:04:28 (33 km), magnitude 5.9 at 11:08:28 (10 km), magnitude 5.0 at 11:29:48 (10 km), and a final magnitude 5.0 at 11:43:35 (10 km). This tight temporal clustering without a dominant mainshock typifies swarm behavior, where energy release occurs through distributed, moderate events rather than a single large rupture.

Southern Taiwan lies within the active collision zone between the Eurasian and Philippine Sea plates. Hengchun Peninsula marks the southern tip of the island's Central Range, where oblique convergence produces frequent seismicity. The offshore region southwest of Hengchun forms part of the northern Manila Trench system, characterized by subduction-related deformation and back-arc spreading influences from the South China Sea. Historical records document recurrent moderate-to-large earthquakes in this corridor, including events in 2006 and earlier episodes linked to plate-boundary adjustments.

The 2006 swarm occurred amid this tectonic framework, highlighting localized stress redistribution along shallow crustal faults. Depths clustered around 10 km suggest activation within the upper seismogenic zone, consistent with the region's young, fractured lithosphere. No significant surface rupture or tsunami generation accompanied the sequence, aligning with the moderate magnitudes and submarine setting.

Long-term monitoring indicates that such swarms contribute to the cumulative strain release in southern Taiwan's seismic cycle. Updated regional hazard assessments continue to emphasize preparedness for both isolated large events and clustered activity in this plate-boundary domain.

References

United States Geological Survey Earthquake Catalog
Central Weather Administration, Taiwan Seismic Network Reports