British Indian Ocean Territory

The British Indian Ocean Territory consists of the Chagos Archipelago, a group of low-lying coral atolls in the central Indian Ocean sitting on the Chagos-Laccadive Ridge — a submarine aseismic ridge extending northward from the Central Indian Ridge toward India. The archipelago sits on the Indo-Australian Plate and is not positioned near any active convergent or transform plate boundary. Diego Garcia, the largest island and site of the joint UK-US military installation, is a coral atoll with a maximum elevation of only a few metres above sea level and experiences no significant local tectonic activity.

The primary seismic threat is not from local geology but from [[tsunami]] generated by distant sources. The catastrophic Indian Ocean [[tsunami]] of 26 December 2004, triggered by the magnitude 9.1 Sumatra-Andaman megathrust earthquake, caused significant wave inundation on Diego Garcia and the other Chagos atolls. The south coast of Diego Garcia experienced waves of approximately 4 to 5 metres, flooding southern portions of the atoll and damaging the military base. The event confirmed that mid-ocean atolls thousands of kilometres from a [[subduction]] source remain vulnerable to major transoceanic [[tsunami]] waves.

The Chagos Archipelago's geological foundation is the same basaltic ridge system that forms the Maldives to the north, created by the passage of the Réunion hotspot beneath the Indo-Australian Plate as it moved northeastward. The coral atolls reflect millions of years of reef growth on subsiding volcanic platforms. The tectonic setting is one of intraplate stability, and the [[seismic-wave]] environment is dominated by distant signals from the active Sunda and Andaman arcs to the northeast. The Indian Ocean Tsunami Warning System now provides monitoring and alerting capacity for the region.