Seismic Swarm PS20071021.1: Analysis of Earthquake Activity West of Bengkulu, Indonesia

On 21 October 2007, a seismic swarm designated PS20071021.1 was recorded approximately 171 km west of Bengkulu, Indonesia. The sequence began at 12:34 local time and concluded at 16:25, encompassing six earthquakes over a span of 3 hours and 51 minutes. This event occurred within the tectonically active Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate along the Sunda Trench.

The earthquakes exhibited magnitudes ranging from 4.4 to 5.6 and focal depths between 23 and 35 km. The sequence unfolded as follows: a magnitude 5.2 event at 12:34:01 (35 km depth), followed by another magnitude 5.2 at 12:40:07 (23 km depth). Activity intensified with a magnitude 5.5 quake at 14:24:20 (35 km depth), a magnitude 4.4 at 14:27:23 (26 km depth), a magnitude 5.0 at 14:31:31 (23 km depth), and culminated in a magnitude 5.6 event at 16:25:35 (27 km depth). Such swarms represent clusters of seismicity without a dominant mainshock, often linked to fluid migration or stress adjustments in subduction environments.

This swarm aligns with the region's established seismic patterns. Since 1 January 2000, seven swarms have been documented in the area, with notable concentrations in 2000 (one swarm), 2001 (one swarm), and 2007 (five swarms). The 2007 activity underscores heightened seismicity during that period, consistent with the broader tectonic loading along the Sumatran margin.

Proximity to larger events further contextualizes the swarm. A magnitude 7.9 earthquake struck 87 km southwest of Sungai Penuh on 12 September 2007, located 97 km from the swarm epicenter. Similarly, a magnitude 7.8 event occurred 215 km southwest of Sungai Penuh on 25 October 2010, 74 km from the swarm center. These instances illustrate the potential for moderate swarms to occur amid or following great subduction earthquakes.

Bengkulu lies on the western coast of Sumatra, where ongoing plate convergence at rates of approximately 5–6 cm per year generates frequent seismic and volcanic activity. Historical records indicate recurrent large-magnitude events in this segment of the Sunda Arc, driven by locked and creeping portions of the megathrust interface. Depths in the 20–40 km range, as observed in the swarm, typically correspond to the seismogenic zone of the subducting slab.

SeismoSight internal classification identifies swarm PS20071021.1 as a distinct cluster based on spatiotemporal parameters. Continued monitoring of such sequences contributes to understanding stress transfer and precursory signals in this high-hazard region.

References

• SeismoSight internal swarm database (classification PS20071021.1 and historical statistics since 2000.01.01)
• USGS Earthquake Catalog (events of 12 Sep 2007 and 25 Oct 2010)