Seismic Swarm PS20050929.1: Analysis of Activity Near Kokopo, Papua New Guinea

The seismic swarm designated PS20050929.1 occurred approximately 134 km south-southwest of Kokopo, Papua New Guinea, in a tectonically active segment of the western Pacific. The sequence initiated at 15:50 on 29 September 2005 and concluded at 12:54 on 30 September 2005, spanning 21 hours and 3 minutes. During this interval, seven earthquakes were recorded, beginning with a magnitude 6.6 event at 25 km depth. Subsequent events included a magnitude 6.2 quake at 28 km depth, followed by five additional shocks ranging from magnitude 5.7 to 5.0, all at 25 km depth, and a final magnitude 5.1 event.

This swarm reflects clustered seismicity typical of the region’s subduction-related stress release. The initial larger events likely triggered aftershocks through static stress transfer along nearby fault segments. All events occurred within a compact spatial footprint, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Papua New Guinea lies at the complex junction of the Pacific, Australian, and several microplates, including the South Bismarck and Solomon Sea plates. The Kokopo area sits near the New Britain subduction zone, where oceanic lithosphere descends beneath the Bismarck arc, generating intermediate-depth seismicity and arc volcanism. Historical records document repeated episodes of both isolated large earthquakes and swarm activity driven by slab dehydration and plate coupling variations.

Since 1 January 2000, nine swarms have been identified in the immediate vicinity. Earlier swarms occurred in 2000 (six events), 2002 (one event), and 2003 (one event), with the 2005 sequence representing the fourth documented instance in that five-year window. These recurrent swarms indicate persistent zones of elevated crustal permeability or fluid migration that facilitate episodic slip without producing a single dominant rupture.

Notable larger earthquakes have also struck nearby. A magnitude 7.8 event occurred 7 km from the swarm centroid on 17 November 2000. A magnitude 7.5 shock took place 18 km away on 5 May 2015, and a magnitude 7.0 earthquake was recorded 61 km distant on 10 October 2018. These events underscore the region’s capacity for great subduction-zone ruptures alongside smaller swarm sequences.

The 2005 swarm did not produce documented surface rupture or significant tsunami, owing to its moderate magnitudes and depths. Nevertheless, the proximity of prior magnitude 7+ events highlights ongoing seismic hazard for coastal communities in East New Britain Province. Continued monitoring of swarm recurrence may improve short-term forecasting of elevated seismicity periods in this segment of the arc.

References

USGS Earthquake Catalog
Global CMT Project
SeismoSight internal swarm classification database