Seismic Swarm PS20080225.1 Near Sungai Penuh, Indonesia

The seismic swarm designated PS20080225.1 occurred 121 km west-southwest of Sungai Penuh in western Sumatra, Indonesia. The sequence began at 08:53 on 24 February 2008 and concluded at 21:02 on 25 February 2008, encompassing 11 earthquakes over 36 hours and 8 minutes. This activity unfolded along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year, generating frequent megathrust and intraslab seismicity.

The swarm exhibited a range of magnitudes and focal depths consistent with regional tectonics. Events included a magnitude 6.5 earthquake at 22 km depth on 24 February, followed by the largest event of magnitude 7.2 at 25 km depth on 25 February. Additional notable shocks reached magnitudes 6.6 and 6.7, both at 25 km depth. Shallower events clustered between 22–27 km, while later activity shifted to depths of 35–38 km. Such depth variations reflect the complex geometry of the subducting slab beneath Sumatra.

Western Sumatra lies within one of Earth’s most seismically active margins. The Sunda Trench has produced great earthquakes throughout recorded history, including the 2004 Indian Ocean event and the 2007 Bengkulu sequence. Since 2000, eight swarms have been documented in the broader area, with prior episodes recorded in 2005 (two swarms) and 2007 (six swarms). These clusters often precede or accompany larger mainshocks along the plate interface.

Two strong earthquakes have occurred near the swarm centroid since 2000. The magnitude 7.2 event of 25 February 2008 struck 164 km west-southwest of Sungai Penuh, only 24 km from the swarm center. Earlier, a magnitude 7.0 earthquake on 13 September 2007 was located 153 km south-southwest of Padang, 32 km from the swarm center. Both events underscore the persistent strain accumulation and release along this segment of the subduction zone.

Seismic swarms in this setting typically arise from fluid migration, afterslip, or stress triggering within the overriding or subducting plates. The 2008 sequence, with its rapid succession of moderate-to-large events, illustrates how short-term clustering can occur amid the longer-term cycle of great earthquakes. Continued monitoring remains essential for assessing evolving hazard in this densely populated region.

References

SeismoSight internal swarm catalog PS20080225.1
USGS Earthquake Catalog (events since 2000)
Global CMT catalog for focal mechanisms and depths