Seismic Swarm PS20050328.2: Analysis of the 2005 Nias Aftershock Sequence

The seismic swarm designated PS20050328.2 was recorded southwest of Singkil, Indonesia, beginning at 16:09 on 28 March 2005 and concluding at 18:29 on 29 March 2005. Over 26 hours and 20 minutes, the sequence comprised 28 earthquakes, including a mainshock of magnitude 8.6 at 30 km depth. Subsequent events ranged from magnitude 4.7 to 6.2, with focal depths predominantly between 10 km and 60 km.

This activity occurred along the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 5–7 cm per year. The region features the Mentawai and Sumatra fault systems, which accommodate oblique convergence through megathrust and strike-slip mechanisms. The swarm’s location, 105 km southwest of Singkil on the western coast of Sumatra, aligns with the rupture area of the 28 March 2005 Nias–Simeulue earthquake.

Geological records indicate that the Sunda margin has produced repeated great earthquakes. The 2004 Indian Ocean event (magnitude 9.1–9.3) ruptured adjacent segments to the north, transferring stress southward and triggering the 2005 mainshock. Historical seismicity includes the 1861 Sumatra earthquake (estimated magnitude 8.5) and earlier events in the 18th and 19th centuries, demonstrating a recurrence pattern of large-magnitude ruptures every 150–200 years in this segment.

The temporal distribution of the swarm shows an immediate aftershock response following the magnitude 8.6 mainshock, with events clustered in the first 12 hours and tapering thereafter. Depths remained shallow to intermediate, consistent with afterslip on the megathrust interface and activation of subsidiary faults. Magnitudes declined progressively, illustrating typical Omori-law decay in aftershock productivity.

Such sequences provide insight into post-seismic relaxation processes. The inclusion of multiple events above magnitude 5.0 within 26 hours reflects elevated stress transfer along the plate boundary. Monitoring data from this swarm contributed to refined models of rupture segmentation between the 2004 and 2005 events, highlighting barriers that limited further northward propagation.

Regional hazard assessments incorporate these observations to update probabilistic forecasts. The tectonic setting remains active, with ongoing microseismicity and GPS-measured strain accumulation indicating continued seismic risk along the Sumatran margin.

References

USGS Earthquake Catalog, 2005 Nias event records.
Global CMT Project, focal mechanism solutions.
Sieh et al., 2008, Tectonic setting of the Sumatran subduction zone.