Seismic Swarm S20050410.1: Earthquake Activity Southwest of Padang, Indonesia

Seismic swarm S20050410.1 occurred 104 km southwest of Padang, Indonesia, from 03:50 on 10 April 2005 to 18:50 on 15 April 2005. Over 135 hours, 98 earthquakes were recorded, providing insight into transient seismic behavior along the Sunda subduction zone.

The swarm region lies offshore western Sumatra within the Mentawai segment of the Indo-Australian Plate subduction beneath the Sunda Plate. Convergence rates average 5–6 cm per year, producing frequent moderate-to-large earthquakes at depths of 20–45 km. Most events in the swarm clustered between 20 and 40 km depth, consistent with the interface and upper-plate seismicity typical of this tectonic setting.

Padang and the surrounding West Sumatra coast have a long history of destructive earthquakes. The great 1797 and 1833 events generated major tsunamis, while the 2004 Mw 9.1–9.3 Sumatra–Andaman earthquake, located farther north, produced widespread regional stress perturbations. The April 2005 swarm began four months after that megathrust rupture and may reflect delayed afterslip or stress transfer along the Mentawai patch.

Within the swarm, magnitudes ranged from 3.8 to a peak of 6.4. The largest event occurred on 10 April at 17:24 UTC at 30 km depth. Activity was intense on the first day, with more than 30 events above magnitude 4.0, then declined steadily through 15 April. Depths remained stable, indicating rupture on a consistent structural level rather than migration to shallower crustal levels.

Swarm characteristics differ from classic mainshock–aftershock sequences by lacking a single dominant event followed by Omori-law decay. Instead, multiple events of comparable size (magnitudes 4.8–5.4) occurred throughout the five-day period, suggesting fluid migration or aseismic slip pulses modulating the fault. Such swarms are documented in other subduction zones and often precede or accompany slow-slip episodes.

No damage or casualties were reported from this swarm, although shaking was felt in coastal West Sumatra. The activity underscores the persistent seismic hazard along the Mentawai segment, which last ruptured in 1833 and is currently considered capable of producing another great earthquake.

References

SeismoSight internal classification and catalog for swarm S20050410.1 parameters.
USGS Earthquake Hazards Program regional tectonic summaries for Sumatra subduction zone.
Global CMT and ISC catalogs for event magnitudes and depths in the Mentawai region.