Location:

Period:

26 Dec 2004 02:51:59 - 27 Dec 2004 14:46:46 (1 day 11 hours 54 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

2 swarms found nearby.

2004

PS20041226.4(183.0km)

26 Dec

2 days 11 hours

14 earthquakes

PS20041229.1(63.0km)

28 Dec

13 hours

6 earthquakes

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Seismic Swarm PS20041226.10: Analysis of the December 2004 Andaman Sequence

A significant seismic swarm, designated PS20041226.10, was recorded in the Andaman Sea region beginning at 02:51 UTC on 26 December 2004 and concluding at 14:46 UTC on 27 December 2004. The events occurred approximately 203 km north of Bamboo Flat in India’s Andaman and Nicobar Islands. Over 35 hours and 54 minutes, the swarm comprised 19 earthquakes with magnitudes ranging from 5.0 to 6.3.

The sequence featured multiple events above magnitude 6.0, including a peak magnitude of 6.3 at 10:19 UTC on 26 December. Focal depths remained predominantly shallow, between 2 km and 45 km, consistent with activity along the shallow portion of the plate boundary. The swarm occurred roughly two hours after the great Sumatra–Andaman earthquake of 26 December 2004 (Mw 9.1–9.3), whose rupture extended northward along the Sunda megathrust into the Andaman segment.

Geologically, the Andaman region lies at the convergent boundary where the Indian plate subducts beneath the Burma microplate at rates of approximately 5–6 cm per year. This oblique subduction has produced the Andaman–Nicobar accretionary prism and the associated volcanic arc. The 2004 mainshock released centuries of accumulated strain along a 1,200 km rupture zone, triggering widespread aftershock activity throughout the Andaman Sea. The PS20041226.10 swarm represents a concentrated cluster within this aftershock zone, likely reflecting stress redistribution along subsidiary faults and the megathrust interface.

No prior swarms meeting the internal SeismoSight classification criteria have been identified in the instrumental record since 1 January 2000. Consequently, this sequence constitutes the sole documented swarm in the region during the subsequent monitoring period.

The combination of high event rates, repeated magnitude 6+ shocks, and persistently shallow depths underscores the dynamic stress environment that followed the 2004 megathrust rupture. Such swarms illustrate how large subduction-zone earthquakes can sustain elevated seismicity for days to weeks as the crust adjusts to the new stress field.

References
USGS Earthquake Catalog (2004–2024)
Global CMT Catalog
SeismoSight internal swarm classification records