Seismic Swarm PS20050410.1: Insights into Regional Seismicity Southwest of Padang

The seismic swarm designated PS20050410.1 occurred 113 km southwest of Padang, Indonesia, within the tectonically active Sunda subduction zone. This zone marks the convergent boundary where the Indo-Australian plate subducts beneath the Eurasian plate at rates of approximately 5–6 cm per year, generating frequent earthquakes along the Sunda Trench and associated strike-slip faults such as the Great Sumatran Fault. The swarm initiated at 10:29 on 10 April 2005 and concluded at 18:19 on 11 April 2005, spanning 31 hours and 50 minutes during which 24 earthquakes were recorded.

Events within the swarm exhibited magnitudes ranging from 5.0 to 6.7, with the largest shock measuring 6.7 at a depth of 19 km occurring at the onset. Subsequent activity included multiple events above magnitude 5.5 clustered in the first several hours, such as paired shocks of 6.3 and 6.5 at 11:14, followed by a series of 5.0–5.9 magnitude tremors through the afternoon and evening of 10 April. Depths predominantly centered around 20–33 km, consistent with intermediate-depth seismicity in the subducting slab, though a few shallower events reached depths as low as 2 km. Activity tapered on 11 April with a final 5.0 magnitude event at 18:19. This temporal pattern reflects typical swarm behavior, characterized by a rapid onset, lack of a single dominant mainshock-aftershock sequence, and distributed energy release over a compact area.

The Padang region has experienced significant seismic history tied to the same subduction dynamics. Since 2000, only this single swarm has been documented in the immediate vicinity. Notable larger events nearby include the magnitude 7.6 earthquake on 30 September 2009 located 30 km west-southwest of Pariaman (78 km from the swarm center) and the magnitude 7.0 event on 13 September 2007 situated 153 km south-southwest of Padang (81 km from the swarm center). These underscore the persistent potential for both moderate swarms and great subduction-zone earthquakes in West Sumatra.

Seismic swarms in this setting often arise from fluid migration or stress perturbations along the plate interface, providing valuable data for understanding precursory activity ahead of larger ruptures. Monitoring such episodes contributes to refined hazard assessments for coastal populations in Sumatra, where rapid urbanization intersects with high seismic risk.

References
SeismoSight internal swarm classification records (PS20050410.1 dataset).
USGS Earthquake Catalog (historical events since 2000).
Global CMT Project (moment tensor solutions for regional events).