Seismic Swarm Analysis: North Indian Ocean Event of April 2012

The seismic swarm designated PS20120411.1 occurred in the North Indian Ocean, centered in the Wharton Basin region. This sequence began at 08:55 on 11 April 2012 and concluded at 02:54 on 12 April 2012, spanning 17 hours and 58 minutes. During this period, 23 earthquakes were recorded, ranging in magnitude from 5.0 to 8.2, with focal depths predominantly between 2 km and 25 km.

The events clustered tightly in both space and time, characteristic of swarm activity rather than a classic foreshock-mainshock-aftershock pattern. The largest event, magnitude 8.2 at 10:43:10 on 11 April, occurred at 25 km depth and aligns with the documented 2012 Wharton Basin aftershock. Earlier activity included multiple magnitude 5.8 events within the first minutes, followed by a magnitude 6.0 at 09:27:56. Later shocks tapered in energy but maintained shallow depths, with several at or below 10 km.

Geologically, the Wharton Basin forms part of the diffuse boundary between the Indian and Australian plates within the Indo-Australian composite plate. This area exhibits elevated intraplate seismicity due to regional compression transmitted from the Himalayan collision zone and subduction along the Sunda Trench. The oceanic crust here dates primarily to the Cretaceous, featuring fracture zones that act as pre-existing weaknesses facilitating strike-slip faulting.

Historical context includes the great 2004 Sumatra-Andaman earthquake, which altered regional stress fields, and the 2012 mainshock doublet (magnitudes 8.6 and 8.2) that ruptured multiple orthogonal faults in an unprecedented manner for oceanic lithosphere. The 2012 sequence released energy equivalent to a single magnitude 8.6–8.8 event and triggered aftershocks across a broad area, including the swarm documented here approximately 73 km from the primary epicenter.

Such swarms highlight the complex stress interactions in the Wharton Basin, where strike-slip mechanisms dominate over typical subduction-related thrust events. Monitoring by global networks continues to refine models of plate deformation in this tectonically active corridor.

References

USGS Earthquake Catalog
Global Centroid Moment Tensor Project
Nature Geoscience publications on 2012 Indian Ocean events