Seismic Swarm PS20180225.2: February 2018 Activity Near Tari, Papua New Guinea

A seismic swarm designated PS20180225.2 occurred in the highlands of Papua New Guinea, centered 19 km south-southeast of Tari. The sequence began at 17:44 on 25 February 2018 and concluded at 05:22 on 28 February 2018, spanning 59 hours and 37 minutes. During this interval, 40 earthquakes were recorded.

The swarm initiated with a magnitude 7.5 event at 25 km depth. Subsequent activity included multiple events exceeding magnitude 5.0, with the largest aftershocks reaching 6.3 and 6.1. Depths ranged primarily between 9 km and 24 km, though isolated deeper and shallower shocks occurred. The temporal distribution showed peak frequency within the first 24 hours, followed by a gradual decline, consistent with aftershock sequences triggered by the mainshock.

Papua New Guinea lies at the complex boundary between the Australian and Pacific plates. The New Guinea Highlands, where the swarm was located, experience ongoing compression and strike-slip deformation. This tectonic setting produces frequent moderate-to-large earthquakes along thrust faults and reactivated structures within the fold-and-thrust belt. Historical records indicate recurrent seismicity in the vicinity of Tari, reflecting the region’s position within the broader collision zone.

The 25 February 2018 magnitude 7.5 mainshock ranks among the strongest events recorded in the area since 2000. Its proximity to the swarm centroid underscores the role of the initial rupture in redistributing stress and generating the subsequent sequence. Depths of most swarm events align with typical seismogenic depths in the highlands, where brittle failure occurs in the upper crust.

Analysis of event timing reveals clustering in the evening of 25 February and early hours of 26 February, followed by renewed bursts on 27 and 28 February. Magnitudes generally decreased after the mainshock, although several events above magnitude 5.5 persisted, indicating sustained stress release. The overall pattern is characteristic of a mainshock-aftershock sequence rather than a purely independent swarm.

The February 2018 activity highlights the persistent seismic hazard in the Tari region. Continued monitoring remains essential given the tectonic regime and documented history of large earthquakes along the same structural trends.

References

• SeismoSight internal swarm classification PS20180225.2
• USGS earthquake catalog (events since 2000)