Seismic Swarm PS20231007.2 Near Madang, Papua New Guinea

A seismic swarm designated PS20231007.2 occurred 43 km southeast of Madang, Papua New Guinea, on 7 October 2023. The sequence began at 08:34 local time and concluded at 22:39 the same day, spanning 14 hours and 4 minutes. During this interval, nine earthquakes were recorded, with magnitudes ranging from 5.1 to 6.9 and focal depths between 31 km and 78 km.

The two largest events reached magnitudes 6.7 at 08:34:26 (55 km depth) and 6.9 at 08:40:11 (52 km depth). Subsequent shocks included a magnitude 5.5 event at 09:18:50 (71 km depth), followed by a pair of events near 13:10 with magnitudes 5.6 (31 km) and 5.7 (78 km). Additional activity comprised magnitudes 5.3, 5.2, 5.7, and a final 5.1 event at 22:39:23 (62 km depth). All events clustered tightly in both space and time, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

Papua New Guinea lies at the complex tectonic junction where the Australian Plate converges with the Pacific Plate and several smaller microplates. This setting produces the New Guinea Trench and associated thrust systems, driving both shallow crustal seismicity and intermediate-depth events within subducting slabs. The Madang region sits near the western termination of the Solomon Sea Plate, where oblique convergence generates frequent moderate-to-large earthquakes.

Historical records since 2000 indicate only one prior swarm in the immediate vicinity, occurring in 2001. A notable regional event was the magnitude 7.6 earthquake of 10 September 2022, located 70 km east of Kainantu and approximately 94 km from the 2023 swarm center. That quake highlighted ongoing strain accumulation along regional thrust faults.

Swarm activity of this type often reflects fluid migration or aseismic slip along fault networks rather than progressive failure from a single large rupture. The 2023 sequence displayed a rapid onset followed by a steady decline in event rate, consistent with such mechanisms. Depths spanning the upper 30–80 km align with both crustal faulting and the upper portion of the subducting slab beneath the region.

Continued monitoring remains essential given Papua New Guinea’s high seismic hazard. The 2023 swarm underscores the value of dense networks for capturing short-lived clusters that may otherwise go unnoticed between larger events.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geological Survey of Papua New Guinea reports