Seismic Swarm Analysis: PS20061226.1 in Southern Taiwan

The seismic swarm designated PS20061226.1 occurred in the waters southwest of Hengchun, Taiwan, beginning at 12:26 on 26 December 2006 and concluding at 02:30 on 27 December 2006. Over this 14-hour period, seven earthquakes were recorded, with the sequence centered approximately 38 km southwest of Hengchun. This event unfolded within one of Taiwan’s most tectonically active zones, where the Philippine Sea Plate converges with the Eurasian Plate along a complex boundary marked by subduction and collision processes.

Taiwan experiences frequent seismic activity due to its position at the junction of major tectonic plates. The southern offshore region near Hengchun lies adjacent to the Manila Trench, where rapid plate motion generates both shallow crustal events and deeper Wadati-Benioff zone seismicity. Depths recorded during the swarm ranged from 10 km to 30 km, consistent with regional patterns of upper-plate and interface earthquakes. Historical records document repeated moderate-to-large events in this corridor, reflecting ongoing strain accumulation from oblique convergence rates exceeding 7 cm per year.

The swarm initiated with a magnitude 7.1 earthquake at 12:26:21 on 26 December at 10 km depth. Within minutes, a magnitude 6.9 event followed at the same depth, succeeded by a magnitude 5.5 shock eight minutes later. Subsequent activity included a magnitude 5.4 event at 15:41:44 (22 km depth), a smaller magnitude 3.2 at 15:56:32 (10 km), a magnitude 5.3 at 17:35:13 (30 km), and a final magnitude 5.6 at 02:30:36 on 27 December (10 km). These events illustrate a rapid decay in frequency after the initial pair of large shocks, a pattern often observed in sequences where mainshock-aftershock dynamics dominate.

Since 1 January 2000, only two swarms have been identified in the broader Hengchun offshore area, with this 2006 sequence representing the first. The magnitude 7.1 mainshock, located 30 km southwest of Hengchun and 11 km from the swarm centroid, remains the strongest event recorded in the region during this timeframe. Such sequences underscore the potential for clustered seismicity to affect submarine infrastructure, as demonstrated by cable disruptions linked to the 2006 activity.

Geological studies of the Hengchun Peninsula and adjacent seafloor reveal a mix of accreted sedimentary units and volcanic basement rocks shaped by long-term plate interaction. Ongoing monitoring by regional networks continues to track microseismicity that delineates active fault segments capable of producing similar swarms in the future.

References

SeismoSight internal swarm classification PS20061226.1
USGS Earthquake Catalog (2006 Hengchun sequence parameters)
Central Geological Survey of Taiwan tectonic framework reports