Seismic Swarm PS20000608.1 Near Bengkulu, Indonesia: Geological Context and Event Analysis

The seismic swarm designated PS20000608.1 occurred approximately 79 km south-southwest of Bengkulu on the island of Sumatra, Indonesia. It commenced at 23:45 on 7 June 2000 and concluded at 12:59 on 8 June 2000, spanning 13 hours and 14 minutes. During this interval, six earthquakes were recorded, all at a focal depth of 33 km. The sequence began with a magnitude 6.7 mainshock, followed by five aftershocks ranging from magnitude 4.7 to 5.1.

Bengkulu lies along the western margin of Sumatra within the Sunda subduction zone, where the Indo-Australian Plate converges with and subducts beneath the Eurasian Plate at a rate of approximately 5–6 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes, with the megathrust interface capable of generating events exceeding magnitude 8. The June 2000 swarm took place in the overriding plate or upper portion of the subducting slab, consistent with the observed 33 km depths.

The swarm followed closely after a magnitude 7.9 earthquake on 4 June 2000, located 33 km from the swarm center. Additional strong events in the vicinity since 2000 include a magnitude 7.4 earthquake on 13 February 2001 (62 km from the swarm center) and a magnitude 8.4 event on 12 September 2007 (76 km from the swarm center). These occurrences underscore the persistent seismic hazard in the Bengkulu segment of the Sumatran margin.

Event parameters for the swarm are as follows:

• 7 June 2000, 23:45:26 UTC – magnitude 6.7
• 8 June 2000, 00:08:55 UTC – magnitude 5.0
• 8 June 2000, 00:11:09 UTC – magnitude 5.1
• 8 June 2000, 04:34:50 UTC – magnitude 5.0
• 8 June 2000, 04:56:19 UTC – magnitude 4.7
• 8 June 2000, 12:59:57 UTC – magnitude 5.1

All events clustered tightly in both time and space, characteristic of swarm behavior rather than a classical mainshock-aftershock sequence. Historical records indicate this was the sole swarm recorded in the Bengkulu region since 1 January 2000.

The geological framework of western Sumatra features the Great Sumatran Fault, a right-lateral strike-slip system that accommodates oblique convergence. Seismicity at depths around 33 km commonly reflects intraslab deformation or slip along the plate interface. The 2000 swarm likely represents triggered slip following the 4 June mainshock, illustrating stress transfer within the subduction complex.

References
SeismoSight internal swarm classification database
USGS Earthquake Hazards Program catalog (historical events)
Global CMT Project moment tensor solutions
Tectonic framework derived from published studies on the Sunda Arc subduction zone