Seismic Swarm PS20070912.2: Analysis of Events off Bengkulu, Indonesia

Seismic swarm PS20070912.2 occurred in the waters 147 km west-southwest of Bengkulu, Indonesia, within the tectonically active Sunda subduction zone. This region marks the convergence of the Indo-Australian and Eurasian plates, where subduction generates frequent megathrust earthquakes and associated aftershock sequences. The swarm initiated at 11:10 on 12 September 2007 and concluded at 03:07 on 14 September 2007, spanning 39 hours and 56 minutes with a total of 21 recorded events.

The sequence began with a mainshock of magnitude 8.4 at a depth of 34 km. Subsequent events ranged from magnitude 3.9 to 6.2, with depths varying between 2 km and 53 km. Notable activity included a magnitude 6.2 event at 10 km depth and a magnitude 6.0 at 53 km on 13 September, alongside multiple magnitude 5.0–5.5 shocks clustered around 30–35 km depth. These patterns reflect typical aftershock behavior following a large megathrust rupture, where stress redistribution triggers smaller events along the plate interface and adjacent crustal faults.

Geologically, the Bengkulu segment lies along the Mentawai Fault and the deeper subduction interface, part of the broader Sumatran fault system. Historical seismicity in this area includes major events driven by the same plate-boundary dynamics, with recurrence intervals for great earthquakes often spanning decades to centuries. Since 2000, only two prior swarms have been documented in the region—one in 2000 and one in 2001—indicating that such clustered sequences remain relatively infrequent compared to isolated mainshock-aftershock series.

A magnitude 7.9 earthquake on 12 September 2007, located 87 km southwest of Sungai Penuh and 53 km from the swarm centroid, occurred in close temporal and spatial proximity. This event underscores the interconnected nature of rupture segments in the subduction zone, where slip on one patch can influence adjacent areas.

The 2007 swarm provides insight into post-rupture stress evolution in a locked subduction segment. Depths predominantly between 25–40 km align with the expected megathrust seismogenic zone, while shallower outliers suggest activation of splay faults or upper-plate structures. Such data contribute to refined models of seismic hazard along the western Sumatran margin.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (regional tectonic framework).
Global CMT Project (moment tensor solutions for Sumatran events).