Seismic Swarm PS20040728.1: Analysis of Earthquake Activity Near Manokwari, Indonesia

The seismic swarm designated PS20040728.1 occurred on 28 July 2004, approximately 122 km west-northwest of Manokwari, Indonesia. The sequence began at 03:56 local time and concluded at 13:18 the same day, spanning 9 hours and 22 minutes. During this period, five earthquakes were recorded, providing a clear example of clustered seismic activity in a tectonically complex region.

The events unfolded as follows: at 03:56:28, a magnitude 6.5 earthquake struck at a depth of 13 km; this was followed at 04:01:59 by a magnitude 5.5 event at 10 km depth. Two minutes later, at 04:03:20, a magnitude 5.2 earthquake occurred at 10 km depth. Later activity included a magnitude 5.3 event at 09:26:45 with a focal depth of 23 km, and the swarm concluded with a magnitude 5.0 earthquake at 13:18:58, 12 km deep. The initial larger shock did not trigger a traditional aftershock sequence but instead formed part of a compact swarm of moderate events.

This swarm took place in western New Guinea, where the Australian and Pacific plates converge along a complex boundary involving subduction and strike-slip faulting. The Manokwari area lies within the broader Papua tectonic zone, characterized by rapid crustal deformation and frequent seismic energy release. Historical records since 1 January 2000 indicate only two swarms in the vicinity, with the first occurring in 2002, underscoring the relative infrequency of such clustered episodes compared to isolated mainshock-aftershock sequences.

Notable strong earthquakes have also affected the region. On 3 January 2009, three major events occurred nearby: a magnitude 7.4 earthquake 86 km west-northwest of Manokwari, a magnitude 7.6 event near the north coast of Papua, and a magnitude 7.7 shock 140 km west-northwest of Manokwari. These events were located between 8 km and 48 km from the 2004 swarm center, highlighting persistent strain accumulation along local fault systems.

Seismic swarms in this setting often reflect fluid migration or stress transfer along pre-existing structures rather than a single dominant rupture. The 2004 sequence, with its rapid succession of events across a narrow time window, aligns with patterns observed in convergent margin environments where multiple fault segments can activate concurrently.

References

• SeismoSight internal swarm classification database
• USGS Earthquake Catalog (global historical records since 2000)