Seismic Swarm PS20041202.1: Analysis of Activity Near Nabire, Indonesia

Seismic swarm PS20041202.1 occurred in a tectonically complex region of western Papua, Indonesia, approximately 27 km west-northwest of Nabire. The sequence began at 23:17 on 1 December 2004 and concluded at 03:54 on 2 December 2004, spanning 4 hours and 37 minutes. During this interval, five earthquakes were recorded, all at a focal depth of 10 km.

The events unfolded as follows: a magnitude 5.6 earthquake at 23:17:21 on 1 December; a magnitude 5.6 event at 03:32:21 on 2 December; a magnitude 5.7 shock at 03:45:33; a magnitude 5.3 event at 03:48:57; and a final magnitude 4.9 earthquake at 03:54:38. The clustering of similar magnitudes within a narrow time window and consistent shallow depth characterizes this episode as a classic swarm, distinct from typical mainshock-aftershock sequences.

Nabire lies within the collision zone between the Australian and Pacific plates, where oblique convergence drives active deformation along the New Guinea fold-and-thrust belt and associated strike-slip structures. This tectonic setting produces frequent moderate-to-large earthquakes and occasional swarms linked to fluid migration or stress transfer along fault networks. Historical records since 1 January 2000 indicate four swarms in the immediate area, with PS20041202.1 representing the earliest documented cluster.

The swarm followed two magnitude 7.1 earthquakes on 26 November 2004—one 47 km from the swarm centroid and another 21 km SSW of Nabire—by only six days. Additional large events in early 2004, including a magnitude 7.3 on 7 February and a magnitude 7.0 on 5 February, occurred 74 km and 29 km from the swarm center, respectively. These preceding shocks likely altered local stress conditions, contributing to the subsequent swarm activity.

Such sequences underscore the elevated seismic hazard in the Nabire region, where shallow crustal faulting can generate both damaging ground shaking and secondary effects such as landslides in the rugged terrain. Continued monitoring remains essential for understanding swarm recurrence and potential links to larger tectonic events.

References

SeismoSight internal swarm classification PS20041202.1
USGS Earthquake Catalog (events since 2000)