Analysis of Seismic Swarm PS20050328.3 Near Sinabang, Indonesia

Seismic swarm PS20050328.3 occurred in a tectonically active segment of the Sunda subduction zone, 59 km NNE of Sinabang on Simeulue Island, Indonesia. The sequence began at 16:58 on 28 March 2005 and ended at 14:16 on 29 March 2005, spanning 21 hours and 17 minutes during which 14 earthquakes were recorded. All events clustered at shallow to intermediate depths consistent with the regional crustal and upper-mantle structure above the subducting Indo-Australian Plate. The sequence opened with a magnitude 5.1 event at 30 km depth, followed within two hours by a magnitude 5.4 shock at the same depth. Subsequent events maintained magnitudes between 4.2 and 5.9, with depths ranging from 24 km to 33 km until the final pair of shocks at 14:16 on 29 March, which included a magnitude 5.2 event at 101 km depth. Peak activity occurred overnight on 28–29 March, featuring four events above magnitude 5.0 within a 30-minute window, the largest reaching magnitude 5.9 at 05:16 on 29 March. This swarm represents the fifth such episode recorded in the immediate region since 1 January 2000. Earlier swarms occurred in 2002 (two episodes) and 2004 (two episodes). The 2005 swarm is situated within a corridor that has hosted several major earthquakes since 2000, including the magnitude 9.1 Sumatra–Andaman mainshock of 26 December 2004 (67 km from the swarm centroid), the magnitude 7.4 event of 2 November 2002 (32 km distant), the magnitude 7.4 shock of 20 February 2008 (46 km distant), and the magnitude 7.8 earthquake of 6 April 2010 (89 km distant). These events illustrate the persistent strain accumulation and release along the megathrust and its associated splay faults. Geologically, the swarm epicenters lie above the gently dipping portion of the Sunda megathrust, where the Indo-Australian Plate subducts northwestward beneath the Eurasian Plate at rates of approximately 5–6 cm per year. The observed focal depths align with the expected seismogenic zone above the plate interface, while the deeper 101 km event may reflect intraslab deformation. The temporal clustering and moderate magnitudes are characteristic of swarm behavior in this segment, often occurring in the post-seismic phase of great subduction earthquakes. The 2005 swarm therefore provides a useful case for understanding stress redistribution following the 2004 megathrust rupture. Continued monitoring of similar clusters remains essential for refining seismic hazard assessments along the northern Sumatran margin. References USGS Earthquake Catalog (ANSS Comprehensive Catalog) Global CMT Project moment-tensor solutions Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) regional bulletins