Seismic Swarm PS20001121.1: Analysis of Activity Near Kokopo, Papua New Guinea

A seismic swarm designated PS20001121.1 was recorded southeast of Kokopo, Papua New Guinea, from 17:33 on 21 November 2000 to 20:16 on 24 November 2000. Over 74 hours and 43 minutes, 25 earthquakes were detected at a fixed depth of 33 km. Magnitudes ranged from 4.3 to 6.3, with the largest event reaching 6.3 on 23 November at 18:43:15. The swarm initiated with a 5.1 foreshock followed immediately by a 6.2 mainshock, both on 21 November, and included additional events above magnitude 6.0 on 21 and 23 November. Activity showed clustering in the first two days before tapering, consistent with swarm behavior rather than a classic aftershock sequence.

The swarm location lies 145 km SSE of Kokopo in the Bismarck Sea, within the tectonically complex New Britain subduction zone. Here the Solomon Sea Plate subducts northward beneath the South Bismarck Plate at rates exceeding 10 cm per year. This convergent margin produces frequent intermediate-depth seismicity along the Wadati-Benioff zone. All events in the swarm occurred at 33 km depth, aligning with the upper portion of the subducting slab where dehydration embrittlement can trigger clustered rupture.

The swarm began five days after a magnitude 7.8 earthquake on 16 November 2000 located 37 km from the swarm centroid. This proximity suggests the larger event may have altered local stress conditions, promoting subsequent swarm activity. Since 1 January 2000, four swarms have been documented in the broader region, with PS20001121.1 representing the earliest recorded instance. Additional strong earthquakes nearby include a magnitude 7.6 event 92 km distant in September 2005, a magnitude 7.5 event 46 km distant in March 2015, and a magnitude 7.9 event 99 km distant in December 2016. These events underscore the persistent seismic hazard along the arc.

Regional geology reflects ongoing plate convergence, arc volcanism, and back-arc spreading. Kokopo lies near the Rabaul Caldera, where volcanic and tectonic processes interact. Historical records show that similar swarm sequences can precede or accompany volcanic unrest, although no direct link to surface volcanism was noted for this 2000 episode. Depths consistently near 33 km indicate intra-slab rather than crustal sources, typical for this segment of the subduction interface.

The 25 events exhibited a magnitude-frequency distribution dominated by moderate shocks, with five events exceeding magnitude 6.0. Temporal distribution showed peak rates on 21–22 November, declining sharply by 24 November. Such patterns are characteristic of fluid-driven or stress-transfer mechanisms within the subducting slab rather than purely elastic rebound.

Seismic monitoring in Papua New Guinea relies on regional networks that provide rapid detection of events above magnitude 4.0. Continued observation of swarm recurrence since 2000 highlights the value of long-term catalogs for identifying precursory patterns ahead of larger mainshocks.

References

SeismoSight internal classification data for swarm PS20001121.1.
USGS Earthquake Catalog (historical events 2000–2016).
Global CMT catalog for focal mechanisms and depths.