Seismic Swarm PS20090106.1 Near Sorong, Indonesia: Geological Context and Event Analysis

The seismic swarm designated PS20090106.1 occurred approximately 146 km east-northeast of Sorong in Indonesia’s West Papua region. Registered between 00:34 on 6 January 2009 and 15:02 on 7 January 2009, the sequence comprised six earthquakes over 38 hours and 28 minutes. Magnitudes ranged from 4.6 to 5.3, with focal depths between 18 km and 100 km.

This swarm unfolded in a tectonically complex zone where the Australian Plate converges with the Pacific Plate. The Sorong Fault system, a major left-lateral strike-slip structure, accommodates significant lateral motion and contributes to the region’s elevated seismicity. Historical records indicate that only four swarms have been documented in the area since 1 January 2000, occurring in 2002 (two events), 2004 (one event), and 2009 (this sequence). Such swarms remain infrequent compared with the more common mainshock-aftershock patterns observed along the same fault corridor.

Three large earthquakes preceded the swarm by only three days. On 3 January 2009, events of magnitudes 7.4, 7.6, and 7.7 struck near Manokwari at distances of 64 km, 9 km, and 13 km from the swarm centroid, respectively. These events released substantial strain along adjacent segments of the plate boundary and likely influenced local stress conditions that facilitated the subsequent swarm activity.

The temporal distribution of the six swarm events shows clustering within the first 24 hours, followed by a single late event on 7 January. Depths transitioned from an initial 100 km event to predominantly shallower foci between 18 km and 35 km, consistent with activity along both deeper crustal structures and shallower fault splays. Magnitudes remained moderate, with the largest shock reaching 5.3, underscoring the swarm’s non-destructive character relative to the preceding mainshocks.

Seismic swarms in this setting often reflect fluid migration or aseismic slip transients along the Sorong Fault and its subsidiary structures. The 2009 sequence fits this pattern, occurring shortly after major strain release and exhibiting a rapid onset followed by quick decay without a dominant mainshock.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog (events 2009)
Regional tectonic summaries, Papua Province, Indonesia