Seismic Swarm PS20180225.1: Analysis of Activity Near Mendi, Papua New Guinea

On 25 February 2018, a seismic swarm designated PS20180225.1 was recorded approximately 51 km west-southwest of Mendi in Papua New Guinea’s Southern Highlands Province. The sequence began at 18:07 UTC and concluded at 19:40 UTC, lasting one hour and 32 minutes during which eight earthquakes were detected. Magnitudes ranged from 4.6 to 5.4, with all events occurring at a focal depth of 10 km. The largest shocks reached 5.4, occurring at 18:16 and 18:27 UTC.

This swarm exhibited a tight temporal clustering and relatively uniform magnitudes, consistent with swarm-type seismicity rather than a classic mainshock-aftershock pattern. Events were distributed across a short interval, with the majority occurring within the first 35 minutes. Such patterns often reflect fluid migration or localized stress adjustments along fault networks without a dominant rupture.

Papua New Guinea lies within one of the world’s most tectonically active zones at the convergence of the Australian and Pacific plates. The region experiences oblique collision and subduction along the New Guinea Trench and associated thrust systems. The Southern Highlands Province sits near the Papuan Fold and Thrust Belt, where sedimentary sequences overlie crystalline basement and are deformed by ongoing compression. Historical records document frequent moderate-to-large earthquakes, with the area also influenced by volcanic arcs farther east.

Since 2000, only two swarms have been identified in the broader catalog for this locale, including a single swarm in 2012 and the present 2018 sequence. This low recurrence highlights the episodic nature of swarm activity amid the region’s background of distributed seismicity. Shallow focal depths around 10 km place events within the upper crust, where sedimentary and volcanic rocks are prone to brittle failure.

The 2018 swarm underscores the value of dense seismic monitoring for distinguishing swarm behavior from foreshock sequences that might precede larger events. Continued observation supports improved hazard assessment in a province where population centers and infrastructure remain vulnerable to ground shaking.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geoscience Australia – Papua New Guinea Tectonic Framework reports