Seismic Swarm Activity Near Sinabang, Indonesia in April 2005

The region surrounding Sinabang, Indonesia, on Simeulue Island lies within the tectonically active Sunda subduction zone. Here the Indo-Australian plate subducts beneath the Eurasian plate at rates of approximately 5–6 cm per year, generating frequent seismicity along the Sunda megathrust. This setting has produced several great earthquakes in recent history, including the magnitude 9.1–9.3 Sumatra–Andaman event of 26 December 2004 and the magnitude 8.6 Nias–Simeulue earthquake of 28 March 2005. Both events caused significant vertical deformation and triggered widespread aftershock sequences across the forearc.

SeismoSight internal records classify swarm PS20050406.1 as a distinct cluster that began at 08:55 on 5 April 2005 and concluded at 01:10 on 6 April 2005, approximately 49 km south-southeast of Sinabang. Within a 16-hour-15-minute window, five earthquakes were registered. The sequence exhibited the following parameters:

• 5 April 2005 08:55:04, magnitude 5.0, depth 33 km
• 5 April 2005 09:37:22, magnitude 5.1, depth 25 km
• 5 April 2005 13:01:58, magnitude 5.3, depth 33 km
• 5 April 2005 22:20:11, magnitude 5.4, depth 33 km
• 6 April 2005 01:10:36, magnitude 4.4, depth 30 km

The events occurred at shallow to intermediate crustal depths consistent with the megathrust interface and its immediate hanging-wall region. Magnitudes remained moderate, yet the tight temporal and spatial grouping distinguishes the swarm from typical mainshock–aftershock decay patterns. Such swarms often reflect fluid migration or aseismic slip transients that load adjacent fault patches without producing a single dominant rupture.

Since 1 January 2000 the same source region has hosted six documented swarms. Earlier episodes occurred in 2002 (one swarm) and 2005 (five swarms, including the present sequence). The April 2005 swarm followed the 28 March 2005 magnitude 8.6 mainshock by roughly one week and was located only 29 km from that event’s epicenter. A subsequent magnitude 7.8 earthquake on 6 April 2010, centered 39 km from the swarm centroid, further illustrates the persistent strain accumulation along this segment of the megathrust.

Collectively, these observations underscore the area’s elevated seismic hazard. Continued monitoring of swarm characteristics can improve short-term forecasting of aftershock productivity and help delineate zones of potential future large ruptures along the Sunda subduction interface.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification database