Seismic Swarm PS20000609.1: Analysis of Earthquake Activity near Southern Sumatra

A seismic swarm designated PS20000609.1 occurred in southern Sumatra, Indonesia, beginning at 04:21 on 9 June 2000 and concluding at 07:41 on 10 June 2000. The events were centered approximately 180 km south-southwest of Pagar Alam. Over 27 hours and 20 minutes, eight earthquakes were recorded, all at a focal depth of 33 km.

The sequence commenced with a magnitude 5.3 event at 04:21:02 on 9 June, followed by a 5.5 event at 05:35:50. Subsequent shocks included another 5.5 at 06:27:26, a 5.1 at 06:47:32, a 4.6 at 07:44:05, a peak magnitude 6.0 at 08:00:24, a 5.0 at 08:24:45, and a final 5.0 at 07:41:29 on 10 June. These events clustered tightly in both time and location, characteristic of swarm behavior where no single mainshock dominates the energy release.

Southern Sumatra lies within the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent seismicity along the megathrust interface and associated crustal faults. The swarm's uniform 33 km depth aligns with typical crustal seismicity in the overriding plate near the trench. The region has experienced significant historical activity, including two documented swarms since 1 January 2000, with PS20000609.1 representing the earliest.

Notable larger events occurred nearby shortly before and after the swarm. A magnitude 7.9 earthquake struck on 4 June 2000, centered 103 km south of Bengkulu and roughly 70 km from the swarm epicenter. Another magnitude 7.4 event followed on 13 February 2001, located 102 km southeast of Bengkulu and about 64 km from the swarm center. These megathrust ruptures highlight the persistent stress accumulation and release along the plate boundary.

Swarm sequences such as this often reflect localized stress perturbations, possibly linked to fluid migration or aseismic slip within the subduction-related fault network. The tight spatiotemporal clustering and moderate magnitudes observed here are consistent with such processes rather than a classic foreshock-mainshock-aftershock pattern. Monitoring of similar activity remains essential for understanding precursory signals in this high-hazard zone.

• USGS Earthquake Catalog
• Global CMT Project
• SeismoSight internal swarm classification records