Seismic Swarm Activity in the Nicobar Islands Region: Analysis of the June 2010 Event

The Nicobar Islands, part of the Andaman-Nicobar archipelago in the eastern Indian Ocean, lie at a tectonically complex boundary where the Indian Plate subducts beneath the Burma Plate. This subduction zone drives frequent seismic activity, including earthquakes and occasional swarms—clusters of events occurring in close temporal and spatial proximity without a single dominant mainshock. The region’s geology features volcanic arcs, back-arc basins, and thrust faults that accommodate oblique convergence at rates of approximately 4–5 cm per year. Historical records indicate elevated seismicity following the great 2004 Sumatra-Andaman earthquake (Mw 9.1–9.3), which ruptured a vast segment of the megathrust and triggered widespread aftershocks across the Nicobar segment.

On 12–13 June 2010, a seismic swarm designated PS20100613.1 was recorded in the Nicobar Islands, India region. The sequence began at 19:26 on 12 June and concluded at 07:05 on 13 June, encompassing six earthquakes over 11 hours and 38 minutes. The largest event reached magnitude 7.5 at a depth of 35 km, followed by five additional shocks with magnitudes ranging from 4.7 to 5.2 and depths between 27 and 37 km. These events clustered near the epicenter of the initial M 7.5 shock, illustrating typical swarm behavior where energy release occurs through multiple comparably sized events rather than a classic foreshock-mainshock-aftershock pattern.

Since 1 January 2000, five swarms have been identified in the region, with earlier episodes occurring in 2004 (three swarms) and 2005 (two swarms). The 2010 swarm aligns with this pattern of episodic clustering, often linked to stress transfer along the subduction interface or fluid migration within the crust. Notably, the M 7.5 event of 12 June 2010 occurred 19 km from the swarm center, while an earlier M 7.2 shock on 24 July 2005 was located 42 km away, underscoring the persistent seismic hazard in this segment of the plate boundary.

Such swarms contribute to ongoing monitoring efforts aimed at understanding strain accumulation and release along the Andaman-Nicobar trench. The 2010 sequence, though moderate in total energy compared with great subduction events, highlights the value of dense seismic networks in capturing fine-scale activity that may precede or accompany larger ruptures. Continued observation remains essential given the region’s history of generating both moderate swarms and infrequent great earthquakes.

References

United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
Global Centroid Moment Tensor Project (globalcmt.org)
Indian Meteorological Department Seismological Reports