Seismic Swarm S20041127.1: Analysis of Activity near Nabire, Indonesia

Nabire lies in Central Papua, Indonesia, within a tectonically complex zone where the Australian Plate converges with the Pacific Plate. This interaction occurs along the New Guinea Trench and associated strike-slip faults, producing frequent shallow crustal earthquakes. The region experiences ongoing deformation, with historical records indicating elevated seismicity since at least the early 20th century. Updated geological assessments confirm that Nabire sits near active fault systems capable of generating moderate-magnitude events at depths of 10 km or less.

The seismic swarm designated S20041127.1 began at 03:01 on 26 November 2004 and concluded at 16:17 on 28 November 2004. It was centered 19 km southwest of Nabire and comprised 33 earthquakes recorded over 61 hours and 16 minutes. All events occurred at a uniform depth of 10 km, with magnitudes ranging from 3.8 to 4.8. The sequence lacked a single dominant mainshock, instead featuring repeated events of comparable size distributed throughout the period.

Activity initiated with two events above magnitude 4.7 within the first five minutes, followed by a sustained series of 4.0–4.8 magnitude shocks on 26 November. The following day saw continued clustering, including three events of magnitude 4.5–4.8. The final recorded event on 28 November measured magnitude 4.1. This temporal pattern reflects a classic swarm signature, in which energy release occurs through numerous similar-magnitude events rather than a foreshock–mainshock–aftershock progression.

Such swarms provide insight into localized stress release along pre-existing fault planes. The tight clustering in both time and space, combined with consistent focal depths, suggests activation of a small fault segment under uniform tectonic loading. No larger triggering event preceded the swarm, supporting an interpretation of spontaneous rupture driven by regional plate motion.

Since 1 January 2000, three swarms have been identified in the Nabire area, with S20041127.1 representing the earliest. Subsequent swarms followed similar characteristics of shallow depth and moderate magnitudes, underscoring the region’s persistent low-to-moderate seismic productivity without escalation to great earthquakes in the immediate vicinity.

This swarm illustrates how short-term clusters contribute to the long-term accommodation of convergence along the Papua margin. Continued monitoring remains essential given the proximity of population centers and infrastructure to active structures.

References

USGS Earthquake Catalog (events 2000–2024)
Global CMT Project focal mechanism database
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) regional reports