Earthquake Swarm PS20161217.1: Seismic Insights from Bougainville, Papua New Guinea

The Bougainville region of Papua New Guinea experienced a notable earthquake swarm designated PS20161217.1, which began at 11:25 on 17 December 2016 and concluded at 22:07 on 18 December 2016. Over 34 hours and 42 minutes, 29 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.3 and focal depths varying between 8 km and 93 km. This swarm reflects the intense tectonic stresses characteristic of the area, where shallow to intermediate-depth events often cluster due to plate boundary interactions.

Geologically, Bougainville forms part of the Solomon Islands arc within the Pacific Ring of Fire. The region sits at the convergent boundary between the Australian and Pacific plates, involving subduction along the New Britain Trench and associated fault systems. This setting produces frequent seismic activity, including both isolated large events and swarm sequences driven by stress transfer and fluid migration in the crust. Depths in the provided swarm data indicate a mix of shallow crustal ruptures and deeper slab-related seismicity, consistent with the complex subduction dynamics of the Solomon Sea.

Historical patterns underscore the region's persistent activity. Since 1 January 2000, 13 swarms have occurred in the Bougainville area, with notable clusters in 2014 (five swarms) and additional episodes in 2000, 2001, 2007, 2015, and 2016. These swarms typically involve multiple moderate-magnitude events over short periods, often preceding or accompanying larger tectonic adjustments.

Significant earthquakes since 2000 further illustrate the hazard. A magnitude 7.9 event struck 35 km west-northwest of Panguna on 22 January 2017, approximately 88 km from the swarm center. Earlier, a magnitude 7.5 quake occurred 70 km southwest of Panguna on 19 April 2014 (90 km from the center), followed closely by a magnitude 7.1 event 56 km west-southwest of Panguna on 11 April 2014 (83 km from the center). Such events highlight the potential for swarm activity to occur near major ruptures along the plate interface.

Analysis of the December 2016 swarm reveals a rapid onset with the largest event (magnitude 6.3 at 8 km depth) occurring early, followed by a sequence of magnitude 5.0–5.4 aftershocks distributed across shallow and deeper levels. This distribution suggests possible triggering along multiple fault segments within the subducting slab and overriding crust. The swarm's duration and event count align with patterns observed in prior Bougainville sequences, where moderate-magnitude clustering serves as an indicator of ongoing strain accumulation.

Overall, the PS20161217.1 swarm provides valuable data on the short-term seismic behavior in this high-risk zone. Continued monitoring remains essential given the region's history of both swarms and great earthquakes.

References SeismoSight internal swarm classification records USGS Earthquake Catalog (historical events since 2000)