Seismic Swarm PS20000910.1: Analysis of the September 2000 Events West of Sorong, Indonesia

The seismic swarm designated PS20000910.1 occurred on 10 September 2000, approximately 196 km west of Sorong in West Papua, Indonesia. The sequence began at 17:08 UTC and concluded at 22:18 UTC, encompassing five earthquakes within a span of five hours and nine minutes. All events were recorded at a consistent depth of 33 km, with magnitudes ranging from 4.5 to 6.1.

The recorded events unfolded as follows: a magnitude 5.4 earthquake at 17:08:18, followed closely by a 5.5 event at 17:11:55. The largest shock, magnitude 6.1, occurred at 19:06:15. Subsequent activity included a magnitude 5.0 at 20:18:15 and a final magnitude 4.5 at 22:18:03. This tight temporal clustering without a dominant mainshock-aftershock pattern is characteristic of swarm behavior, often linked to localized stress adjustments along active fault segments.

Sorong lies within a highly tectonically active zone at the boundary between the Australian and Pacific plates. The dominant structure is the Sorong Fault, a major left-lateral strike-slip system that accommodates oblique convergence and lateral extrusion. This fault zone has a well-documented history of moderate to large earthquakes, reflecting ongoing regional deformation driven by the collision and rotation of microplates in eastern Indonesia. The 33 km focal depth of the 2000 swarm places the activity within the brittle upper crust, consistent with typical seismogenic depths along the Sorong Fault.

Seismic swarms in this setting frequently arise from rapid redistribution of stress along strike-slip segments, sometimes influenced by fluid migration or aseismic slip. The 2000 sequence illustrates such a process, with the largest event (6.1) occurring midway through the swarm rather than at its onset. No significant damage or tsunami was associated with these events, owing to their offshore location and moderate magnitudes.

The Sorong region continues to exhibit elevated seismicity, underscoring the persistent tectonic forces at play. Monitoring of similar swarms contributes to refined understanding of fault mechanics and improves regional hazard assessment along this complex plate boundary.

References

SeismoSight internal swarm classification data (PS20000910.1).
Geological Survey of Indonesia tectonic maps and regional fault studies.
Peer-reviewed literature on the Sorong Fault Zone and Australia-Pacific plate interaction.