Seismic Swarm PS20070915.1: Analysis of Activity near Sungai Penuh, Indonesia

A seismic swarm designated PS20070915.1 occurred 131 km SSW of Sungai Penuh, Indonesia, beginning at 01:41 on 15 September 2007 and concluding at 11:37 on 16 September 2007. Over 33 hours and 56 minutes, the swarm produced nine earthquakes, providing a focused record of clustered seismic energy release in a tectonically active zone.

The sequence began with a magnitude 5.0 event at 35 km depth. Subsequent shocks included a magnitude 5.5 at 14:45 on 15 September, also at 35 km depth, followed by two events within two minutes at 15:04 (magnitudes 5.2 and 5.0). A magnitude 5.0 shock occurred later that evening at 22:53 and 30 km depth. On 16 September, additional events registered magnitudes of 5.2 at 01:15 (38 km depth), 5.2 at 09:10 (35 km depth), and 5.3 at 11:37 (35 km depth). Depths remained shallow throughout, concentrated between 30 and 38 km.

This swarm fits within a broader pattern of seismic activity in the region. Since 1 January 2000, four swarms have been recorded near Sungai Penuh, with one occurring in 2001 and three in 2007. The 2007 sequence followed closely after a magnitude 7.9 earthquake on 12 September 2007, located 87 km southwest of Sungai Penuh and 38 km from the swarm center. Such proximity suggests possible stress redistribution along regional fault structures.

Geologically, the area lies along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This convergence drives megathrust earthquakes and associated aftershock sequences or swarms. Shallow to intermediate-depth events predominate due to the geometry of the subducting slab. Historical records indicate recurrent moderate-to-large earthquakes, consistent with the ongoing plate-boundary dynamics that have shaped Sumatra’s seismic landscape for millions of years.

Swarm events like PS20070915.1 typically reflect fluid migration or localized stress adjustments rather than a single mainshock-aftershock cascade. The nine events clustered tightly in time and space, with magnitudes between 5.0 and 5.5 dominating the energy release. This pattern aligns with known swarm behavior in subduction-related settings, where episodic slip on subsidiary faults can produce dense sequences without a dominant rupture.

Continued monitoring of such swarms contributes to understanding precursory signals and regional hazard assessment. The 2007 activity underscores the persistent seismic potential of the Sunda margin and the value of detailed event catalogs for refining models of crustal deformation.

References

SeismoSight internal swarm classification records.
USGS earthquake catalog for regional context (2000–present).