Seismic Swarm PS20050328.1: Analysis of Activity Southwest of Singkil, Indonesia

A seismic swarm designated PS20050328.1 was recorded southwest of Singkil, Indonesia, beginning at 16:21 on 28 March 2005 and concluding at 00:49 on 1 April 2005. Over approximately 80 hours and 28 minutes, the swarm produced 24 earthquakes with magnitudes ranging from 5.0 to 6.0. The events were centered 156 km southwest of Singkil at depths primarily between 22 and 33 km.

The sequence opened with a magnitude 5.8 event at 16:21 on 28 March, followed rapidly by additional shocks including a magnitude 6.0 at 16:38. Activity continued through the evening with multiple events of magnitude 5.0–5.6. On 29 March, five earthquakes occurred, the strongest reaching magnitude 5.2. The following day featured four events, including a magnitude 5.7 at 01:13. Activity tapered on 31 March with five recorded shocks, the largest a magnitude 5.8 at 07:23. The final event, magnitude 5.0, took place at 00:49 on 1 April.

This swarm occurred in close temporal and spatial proximity to a magnitude 8.6 earthquake on 28 March 2005 located 78 km west-southwest of Singkil, approximately 83 km from the swarm centroid. The larger event reflects the tectonic setting of the region, where the Indo-Australian Plate subducts beneath the Eurasian Plate along the Sunda Trench. This convergent margin has generated repeated great earthquakes, including the magnitude 9.1–9.3 Sumatra–Andaman event of December 2004 and the subsequent Nias–Simeulue earthquake of March 2005.

Swarm activity in this segment of the subduction zone is consistent with aftershock sequences and triggered seismicity following major ruptures. Depths of 22–33 km align with the interface and upper plate seismicity typical of the Sumatran forearc. Historical records since 2000 indicate only two prior swarms in the broader area, one in 2002 and the present 2005 sequence, underscoring the episodic nature of clustered moderate-magnitude activity amid infrequent great earthquakes.

The geological framework features the Mentawai Fault and associated thrust systems that accommodate oblique convergence. Strain accumulation along the locked portion of the megathrust, punctuated by periodic release, produces both isolated large events and secondary swarms such as PS20050328.1. Depths and magnitudes observed during the swarm fall within the expected range for interplate and intraslab faulting in this environment.

References

USGS Earthquake Catalog (2005 Nias event parameters)
SeismoSight internal swarm classification PS20050328.1