Seismic Swarm PS20050401.1 Near Sinabang, Indonesia

On April 1, 2005, a seismic swarm designated PS20050401.1 was recorded 37 km north of Sinabang, Indonesia. The sequence began at 05:55 and concluded at 10:58 local time, spanning five hours and two minutes. During this interval, five earthquakes were detected, with magnitudes ranging from 4.8 to 5.5 and focal depths between 26 and 33 km.

The events unfolded as follows: a magnitude 5.0 quake at 05:55:56 (30 km depth), followed by another magnitude 5.0 at 07:40:26 (33 km depth). Two closely spaced events occurred near 10:37:45 and 10:37:47, registering magnitudes 5.1 and 5.5 at depths of 26 km and 30 km respectively. The swarm ended with a magnitude 4.8 event at 10:58:33 (27 km depth). These shallow to intermediate-depth shocks reflect typical stress release along the plate interface in this tectonically active zone.

The swarm occurred within the Sunda subduction system, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This setting produces frequent seismicity, including both interplate thrust events and intraslab activity. Sinabang lies on Simeulue Island, positioned directly above the locked portion of the megathrust that ruptured catastrophically in the 2004 Sumatra-Andaman earthquake.

Historical records since 2000 show six swarms in the immediate area, with two each in 2002, 2004, and 2005. This pattern indicates episodic clustering of moderate-magnitude events, often preceding or following larger regional ruptures. Strong earthquakes documented since 2000 include the magnitude 9.1 Sumatra-Andaman mainshock of 26 December 2004 (60 km from the swarm center), the magnitude 7.4 event of 2 November 2002 (32 km distant), the magnitude 7.4 shock of 20 February 2008 (46 km away), and the magnitude 7.8 earthquake of 6 April 2010 (95 km distant). These occurrences underscore the persistent seismic hazard along the Sumatran margin.

Swarm activity such as PS20050401.1 provides insight into short-term stress migration and fluid involvement within the subduction interface. Although individual events remained moderate, their temporal and spatial concentration highlights the dynamic nature of the fault system. Continued monitoring is essential given the region’s history of generating great earthquakes.

References

USGS Earthquake Catalog
Global CMT Catalog
NOAA National Centers for Environmental Information