Seismic Swarm PS20000604.1: Tectonic Context and Event Analysis Near Bengkulu

The seismic swarm designated PS20000604.1 occurred approximately 129 km south of Bengkulu, Indonesia, within the tectonically active Sunda subduction zone. This region marks the convergent boundary where the Indo-Australian Plate subducts beneath the Eurasian Plate along the Sunda Trench, generating frequent seismicity through megathrust rupture and associated crustal faulting. The swarm initiated at 20:02 on 3 June 2000 and concluded at 09:58 on 6 June 2000, spanning 61 hours and 56 minutes during which 26 earthquakes were recorded.

The sequence featured a prominent mainshock of magnitude 7.9 at 16:28 on 4 June 2000, located at a focal depth of 33 km. Subsequent events clustered at similar depths, with magnitudes ranging from 5.0 to 6.7, including a magnitude 6.7 aftershock minutes later and a magnitude 6.2 event near the swarm’s close. One outlier registered at 83 km depth early in the sequence. These parameters align with typical intermediate-depth activity along the subduction interface, where slab dehydration and stress transfer can trigger clustered seismicity.

Bengkulu lies within Sumatra’s forearc setting, where oblique convergence produces both trench-parallel strike-slip faults, such as the Great Sumatran Fault, and trench-perpendicular thrusting. Historical records document recurrent large events in this segment, reflecting strain accumulation and release cycles on the order of decades to centuries. The 2000 swarm occurred in a zone that experienced a magnitude 7.4 earthquake on 13 February 2001, centered 102 km southeast of Bengkulu and only 22 km from the swarm centroid, underscoring persistent seismic productivity.

Analysis of the swarm reveals a classic foreshock–mainshock–aftershock pattern superimposed on swarm-like characteristics, with multiple events exceeding magnitude 5.0 within hours of the mainshock. Depths concentrated near 33 km suggest activation along the plate interface rather than deeper intraslab processes. Such sequences contribute to regional hazard assessment by delineating areas of elevated stress transfer, potentially influencing rupture potential on adjacent fault segments.

The broader Sumatran margin remains capable of generating great earthquakes, as demonstrated by subsequent events elsewhere along the trench. Continued monitoring of microseismicity and geodetic strain in the Bengkulu sector supports refined models of subduction segmentation and recurrence intervals.

References

USGS Earthquake Catalog (events 2000–2001)
Global CMT Project focal mechanism solutions
SeismoSight internal swarm classification PS20000604.1