Seismic Swarm PS20090816.1: Analysis of Activity off Padang, Indonesia

Seismic swarm PS20090816.1 occurred approximately 144 km west-southwest of Padang, Indonesia, within the tectonically active Sunda subduction zone. This region lies along the Mentawai segment of the megathrust, where the Indo-Australian Plate converges with the Eurasian Plate at rates of 5–6 cm per year. Subduction-driven stress accumulation produces frequent seismic events, including both isolated large earthquakes and clustered swarms.

The swarm initiated at 07:38 on 16 August 2009 and concluded at 13:55 on 17 August 2009, spanning 30 hours and 17 minutes. A total of 12 earthquakes were recorded, with magnitudes ranging from 4.5 to 6.7 and focal depths between 10 km and 51 km. The sequence began with a magnitude 6.7 event at 20 km depth, followed by multiple aftershocks clustered at shallow depths around 10 km and several deeper events near 50 km. Key events included two magnitude 5.4 shocks occurring within seconds of each other, a magnitude 5.8 event, and additional magnitude 5.0 earthquakes distributed throughout the period.

This pattern reflects typical swarm behavior in subduction forearc settings, where fluid migration and stress transfer along the plate interface can trigger multiple moderate events without a single dominant mainshock-aftershock sequence. Depths indicate activity both near the plate interface and within the overriding plate.

Historical records since 2000 show five prior swarms in the vicinity: two in 2005, two in 2007, and one in 2008. These clusters highlight recurring episodic seismicity in the area. Notable larger earthquakes nearby include the magnitude 7.6 event on 30 September 2009 located 30 km west-southwest of Pariaman (91 km from the swarm center) and the magnitude 7.0 event on 13 September 2007 located 153 km south-southwest of Padang (78 km from the swarm center). Both underscore the high seismic hazard of the region due to locked patches on the megathrust capable of generating great earthquakes.

Continued monitoring of such swarms provides valuable data on stress evolution along the subduction interface and aids in refining regional hazard assessments.

References:
SeismoSight internal classification data for swarm PS20090816.1 and historical swarm statistics.
Prompt-provided event parameters and regional context.