Seismic Swarm PS20100407.1: Analysis of Activity Near Sinabang, Indonesia

The seismic swarm designated PS20100407.1 occurred offshore approximately 74 km south-southeast of Sinabang on Simeulue Island, Indonesia. The sequence began at 22:15 UTC on 6 April 2010 and concluded at 04:22 UTC on 7 April 2010, spanning six hours and seven minutes. During this interval, five earthquakes were recorded, with the largest event reaching magnitude 7.8 at a depth of 31 km.

This region lies along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. The tectonic setting produces frequent megathrust earthquakes and associated seismic sequences. Simeulue Island and adjacent waters have experienced repeated episodes of clustered seismicity, reflecting stress transfer along the plate interface and within the overriding plate.

The swarm comprised the following events:

• 6 April 2010, 22:15:01 UTC, magnitude 7.8, depth 31 km
• 6 April 2010, 22:54:06 UTC, magnitude 5.3, depth 35 km
• 6 April 2010, 23:35:38 UTC, magnitude 5.1, depth 28 km
• 7 April 2010, 04:03:48 UTC, magnitude 4.0, depth 35 km
• 7 April 2010, 04:22:15 UTC, magnitude 5.1, depth 35 km

The mainshock of magnitude 7.8 was located 24 km from the swarm centroid and 75 km east of Sinabang. It occurred in the immediate aftermath of the 28 March 2005 magnitude 8.6 Nias–Simeulue earthquake, whose epicenter lay 38 km from the swarm center near Singkil. These large events illustrate the capacity of the subduction interface to release strain in both isolated great earthquakes and subsequent clustered sequences.

Historical records since 1 January 2000 document eleven swarms in the same area. Earlier episodes occurred in 2002 (two swarms), 2005 (eight swarms), and 2008 (one swarm). The elevated swarm frequency in 2005 coincides with the aftershock period of the magnitude 8.6 mainshock, suggesting that post-seismic stress adjustments can trigger multiple short-lived clusters.

Seismic swarms in this setting typically reflect fluid migration, aseismic slip, or localized stress perturbations rather than a single foreshock–mainshock–aftershock progression. The 2010 sequence, dominated by an initial magnitude 7.8 event followed by smaller shocks at similar depths, is consistent with rapid stress release along a limited rupture patch within the broader subduction interface.

Continued monitoring of such swarms contributes to understanding spatiotemporal patterns of strain accumulation and release along the Sunda margin. The data underscore the persistent seismic hazard in the vicinity of Simeulue and Nias islands, where both great earthquakes and secondary clustered activity remain characteristic features of the tectonic regime.

References

• United States Geological Survey Earthquake Catalog (USGS)
• SeismoSight internal swarm classification records