Seismic Swarm PS20070912.1: Analysis of Events Near Sungai Penuh, Indonesia

The seismic swarm designated PS20070912.1 occurred in a tectonically active region of western Indonesia, centered 96 km south of Sungai Penuh. This sequence began at 11:40 on 12 September 2007 and concluded at 04:03 on 14 September 2007, spanning 40 hours and 23 minutes during which 17 earthquakes were recorded. The events clustered at depths primarily between 10 km and 43 km, reflecting activity within the crust above the subduction interface.

The swarm initiated with a magnitude 5.5 event at 35 km depth, followed rapidly by additional shocks of magnitudes 5.2, 5.6, 5.0, 5.3, and 5.2 within the first two hours. Activity intensified through the afternoon, culminating in a magnitude 5.9 event at 14:40 and a magnitude 5.8 at 16:37, both at 35 km depth. A notable magnitude 7.9 earthquake struck at 23:49 on 12 September at 35 km depth, marking the largest event in the sequence. Subsequent aftershocks on 13 and 14 September included magnitudes ranging from 5.0 to 5.5 at depths of 29 km to 41 km.

This swarm unfolded in the context of the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. The resulting megathrust fault system generates frequent seismic activity across Sumatra and adjacent offshore areas. Sungai Penuh lies inland from the Mentawai Islands, positioning the swarm near the downdip extent of the locked portion of the plate boundary. Historical patterns in the region demonstrate recurrent great earthquakes, with the 2007 sequence contributing to stress redistribution along adjacent fault segments.

Since 1 January 2000, only three swarms have been identified in this locale, with prior episodes limited to two events in 2005 and one additional swarm in 2007. Strong earthquakes recorded since 2000 further illustrate the area's elevated hazard. These include a magnitude 7.2 event 164 km west-southwest of Sungai Penuh on 25 February 2008, located 24 km from the swarm center; a magnitude 7.0 event 153 km south-southwest of Padang on 13 September 2007, 52 km from the center; and the magnitude 7.9 mainshock of the swarm itself, 93 km from the swarm center.

The temporal clustering and depth distribution of the 17 events indicate a foreshock-mainshock-aftershock pattern typical of subduction-related sequences. Depths remained consistent around 30–35 km for most larger events, consistent with rupture on or near the plate interface. Such swarms provide insight into preparatory processes preceding larger ruptures, highlighting the dynamic stress interactions within the Sumatran tectonic framework.

References

USGS Earthquake Catalog (prompt data utilized as authoritative for swarm parameters)
SeismoSight internal classification records