Seismic Swarm Activity Near Sungai Penuh, Indonesia: The September 2007 Event

The region surrounding Sungai Penuh in western Sumatra lies within the tectonically active Sunda subduction zone, where the Indo-Australian Plate converges with the Sunda Plate at rates of approximately 5–7 cm per year. This oblique subduction produces frequent seismic activity along the megathrust interface and associated strike-slip faults such as the Great Sumatran Fault. The area experiences both deep intraslab events and shallower crustal earthquakes, contributing to a high seismic hazard profile documented over decades of instrumental recording. Swarm PS20070924.1 was recorded 145 km SSW of Sungai Penuh. The sequence began at 16:17 on 23 September 2007 and concluded at 08:15 on 24 September 2007, encompassing eight earthquakes over 15 hours and 58 minutes. Magnitudes ranged from 4.5 to 5.7, with focal depths varying between 18 km and 100 km. The events clustered temporally, with the strongest shock (magnitude 5.7 at 18 km depth) occurring at the sequence’s end. Earlier events included a magnitude 5.4 shock at 35 km depth and several magnitude 5.0 earthquakes at depths of 26–40 km, illustrating a rapid succession typical of swarm behavior rather than a classic foreshock-mainshock-aftershock pattern. Historical records since 2000 indicate six swarms in the vicinity, with five occurring in 2007 alone and one in 2001. This concentration underscores episodic swarm activity superimposed on the region’s background seismicity. Strong earthquakes in the same period include a magnitude 7.9 event on 12 September 2007 located 87 km southwest of Sungai Penuh (approximately 60 km from the swarm center) and a magnitude 7.8 event on 25 October 2010 situated 215 km southwest of the city (82 km from the swarm center). These megathrust ruptures highlight the capacity for large events to influence subsequent smaller sequences through stress transfer. Geological context places the swarm within the broader framework of Sumatran tectonics, where the subduction zone has generated great earthquakes historically, including the 2004 Indian Ocean event and the 2007 Bengkulu sequence. Depths spanning 18–100 km suggest involvement of both the overriding plate and the subducting slab, consistent with the complex geometry of the margin. Updated monitoring by global networks continues to refine understanding of such clusters, aiding in distinguishing swarms from more hazardous mainshock-aftershock sequences. References
USGS Earthquake Catalog (events since 2000)
Global CMT Catalog (focal mechanisms and depths)
Scientific literature on Sumatran subduction zone dynamics (e.g., reviews in Tectonophysics and Journal of Geophysical Research)