Seismic Swarm PS20161217.2: Analysis of Activity Near Kokopo, Papua New Guinea

A notable seismic swarm, designated PS20161217.2, occurred east of Kokopo in Papua New Guinea between 10:51 on 17 December 2016 and 06:14 on 20 December 2016. Over 67 hours and 23 minutes, the sequence produced 14 earthquakes. The swarm initiated with a magnitude 7.9 event at 94 km depth, followed by events ranging from magnitude 4.3 to 5.6, with depths varying between 10 km and 103 km. Subsequent notable shocks included a magnitude 5.6 at 82 km depth shortly after the mainshock, several magnitude 5.0–5.3 events at shallower depths on 18–19 December, and a final magnitude 5.6 at 10 km depth closing the sequence.

This activity unfolded in a tectonically complex zone 173 km east of Kokopo on the island of New Britain. The region lies at the convergent boundary between the South Bismarck and Pacific plates, where subduction along the New Britain Trench drives frequent seismicity. The crust here experiences both deep subduction-related events and shallower crustal adjustments, consistent with the observed depth range of the swarm.

Papua New Guinea ranks among the most seismically active nations due to its position on the Pacific Ring of Fire. Multiple microplates interact in the Bismarck Sea, producing both isolated large earthquakes and episodic swarm sequences. Historical records since 2000 show 18 swarms in the broader area, with clusters in 2000 (seven events), 2014 (four), and 2015 (four). Earlier swarms occurred in 2007 and 2013, underscoring the recurring nature of clustered activity.

A magnitude 7.8 earthquake on 16 November 2000, located 135 km southeast of Kokopo and 77 km from the 2016 swarm center, illustrates the potential for strong events nearby. Such quakes often result from thrust faulting on the subduction interface, releasing accumulated strain over decades.

Swarm sequences like PS20161217.2 typically reflect fluid migration or stress triggering rather than a single mainshock-aftershock pattern. The rapid succession of moderate events following the initial large shock suggests distributed fault slip across multiple segments. Depths spanning from near-surface to over 100 km indicate involvement of both the overriding plate and the subducting slab.

Ongoing monitoring by regional networks remains essential for understanding recurrence patterns in this high-hazard setting. The combination of plate-boundary tectonics and historical clustering indicates that similar swarms will continue to occur.

References

USGS Earthquake Catalog (historical events and regional seismicity).
Global CMT Catalog (focal mechanisms and plate interactions).
SeismoSight internal classification (swarm parameters and statistics).