Location:

Period:

28 Mar 2005 16:27:38 - 29 Mar 2005 04:50:07 (12 hours 22 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

5 swarms found nearby.

2005

PS20050328.2(127.1km)

28 Mar

1 day 2 hours

28 earthquakes

PS20050328.1(114.3km)

28 Mar

3 days 8 hours

24 earthquakes

S20050329.1(93.0km)

28 Mar

3 days 4 hours

56 earthquakes

PS20050407.1(91.9km)

7 Apr

1 day 3 hours

7 earthquakes

PS20050730.1(79.0km)

29 Jul

22 hours

5 earthquakes

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Seismic Swarm PS20050329.1 Offshore Singkil, Indonesia

On 28–29 March 2005, a seismic swarm designated PS20050329.1 was recorded 154 km south-southwest of Singkil, Indonesia. The sequence began at 16:27 UTC on 28 March and concluded at 04:50 UTC on 29 March, lasting 12 hours and 22 minutes. Five earthquakes were detected during this interval, with magnitudes ranging from 4.4 to 5.7 and focal depths between 2 km and 38 km.

The events occurred in the following order: a magnitude 5.2 earthquake at 30 km depth at 16:27:38 UTC on 28 March; a magnitude 5.7 event at 38 km depth at 23:13:00 UTC; a magnitude 4.4 shock at 30 km depth at 04:38:57 UTC on 29 March; a magnitude 5.4 earthquake at 2 km depth at 04:50:01 UTC; and a magnitude 5.5 event at 35 km depth at 04:50:07 UTC. These closely spaced occurrences within the overriding plate and near the plate interface are characteristic of swarm activity in subduction settings.

The swarm location lies within the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This tectonic regime has produced repeated megathrust earthquakes, including the 26 December 2004 Mw 9.1–9.3 Sumatra–Andaman event and the 28 March 2005 Mw 8.6 Nias–Simeulue earthquake. The latter event, whose epicenter was situated near Nias Island, generated widespread aftershock sequences across the forearc region, providing a plausible regional context for the observed swarm.

Since 1 January 2000, only two earthquake swarms have been identified in the broader area according to SeismoSight records, with PS20050329.1 representing the first. Such low swarm frequency underscores the dominance of mainshock–aftershock sequences over swarm-type activity along this portion of the margin.

Geological monitoring in the region relies on regional and global seismic networks that routinely detect events above magnitude 4.0. Depths shallower than 40 km, as seen in the swarm, align with both intraslab and plate-interface seismicity typical of the Sumatran forearc.

References

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