M 7.5; 53 km SE of Kokopo, Papua New Guinea; (29 Mar 2015) (98km from the swarm center)
M 7.8; 138 km SSW of Kokopo, Papua New Guinea; (17 Nov 2000) (35km from the swarm center)
Seismic Swarm PS20181221.1 South of Kokopo, Papua New Guinea: Geological Context and Event Analysis
The seismic swarm PS20181221.1 occurred approximately 127 km south of Kokopo on the island of New Britain in Papua New Guinea. Registered between 05:23 and 12:26 on 21 December 2018, the sequence included five earthquakes within a span of seven hours and two minutes. This event reflects the persistent tectonic stresses that characterize the region.
New Britain lies within the Bismarck Volcanic Arc, where the Solomon Sea Plate subducts beneath the South Bismarck Plate along the New Britain Trench. This convergent boundary generates frequent seismic activity, including both isolated large-magnitude events and clustered swarms. The arc also hosts active volcanoes such as Rabaul caldera, underscoring the interplay between subduction-driven seismicity and magmatism. Depths of the recorded events ranged from 10 km to 23 km, consistent with shallow crustal deformation above the subducting slab.
The five earthquakes of the swarm exhibited the following parameters:
21 Dec 2018 05:23:42, magnitude 5.3, depth 16 km
21 Dec 2018 06:33:52, magnitude 5.9, depth 10 km
21 Dec 2018 07:28:33, magnitude 5.0, depth 10 km
21 Dec 2018 08:30:15, magnitude 5.7, depth 10 km
21 Dec 2018 12:26:05, magnitude 5.3, depth 23 km
The sequence began with a moderate event and peaked at magnitude 5.9 before tapering, a pattern typical of fluid-related or stress-transfer swarms in subduction forearc settings.
Since 2000, the area has experienced 29 documented swarms, with notable concentrations in 2000 (seven events), 2015 (five events), and 2016 (three events). These episodes demonstrate recurring episodic release of strain. Three strong earthquakes have also occurred nearby since 2000: a magnitude 7.8 event on 17 November 2000 located 35 km from the swarm center, a magnitude 7.5 event on 29 March 2015 located 98 km away, and another magnitude 7.5 event on 5 May 2015 located 25 km from the swarm center. Such proximity indicates that swarm activity often occurs within zones already stressed by prior large ruptures.
Collectively, the 2018 swarm and historical record illustrate the dynamic nature of the New Britain subduction zone, where repeated moderate sequences help accommodate ongoing plate convergence while occasionally preceding or following major earthquakes.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification PS20181221.1