Seismic Swarm PS20110626.1: Analysis of Activity Near Biak, Indonesia

A seismic swarm designated PS20110626.1 occurred 138 km south-southeast of Biak, Indonesia, in a tectonically active zone shaped by interactions between the Australian and Pacific plates. The swarm began at 12:16 on 26 June 2011 and concluded at 04:57 on 27 June 2011, spanning 16 hours and 40 minutes. During this period, eight earthquakes were recorded, with magnitudes ranging from 5.0 to 6.3 and focal depths between 2 km and 30 km.

The sequence opened with a magnitude 6.3 event at 17 km depth, followed rapidly by additional shocks of 5.6, 5.3, and two magnitude 5.4 events within the first ten minutes. Subsequent activity included a magnitude 5.0 quake at 11 km depth, another 5.4 event at only 2 km depth, and a final magnitude 5.0 shock at 10 km depth. This pattern of clustered, moderate-magnitude events without a single dominant mainshock is characteristic of swarm behavior, often linked to localized stress redistribution or fluid migration along fault networks in subduction-related settings.

Biak lies within the complex collision zone of New Guinea, where oblique convergence produces both thrust and strike-slip faulting. The region has experienced recurrent seismic swarms since 2000, with five documented episodes in total. Earlier swarms occurred in 2004 (four events) and 2010 (one event). A notable magnitude 7.0 earthquake struck 121 km south-southeast of Biak on 16 June 2010, approximately 22 km from the 2011 swarm center, underscoring persistent strain accumulation along nearby structures.

Such swarms contribute to ongoing seismic hazard assessment in Papua, where shallow crustal events can produce significant ground shaking despite moderate magnitudes. Monitoring of depth variations and temporal clustering helps refine models of fault interaction in this high-strain environment.

References
SeismoSight internal swarm catalog (PS20110626.1 parameters)
USGS Earthquake Catalog (regional events since 2000)
Tectonic framework summaries from the Geological Survey of Indonesia and Pacific Ring of Fire studies.