Cocos (Keeling) Islands

The Cocos (Keeling) Islands consist of two flat coral atolls rising barely 3 metres above sea level in the eastern Indian Ocean, sitting on the interior of the Indo-Australian Plate approximately 2,750 kilometres northwest of Perth and well to the south of any active plate boundary. The islands are geologically among the most stable places on Earth in terms of local seismicity; the Indo-Australian Plate interior in this region is ancient, cold, and highly rigid, generating virtually no indigenous fault activity. However, the islands are not without seismic context: the mid-ocean ridge system to the south and west — where the Indian and Australian plates began separating — occasionally produces ridge-flank events, and the diffuse boundary between the Indian and Australian sub-plates passes through the Indian Ocean to the north, generating moderate intraplate earthquakes. The primary hazard to the Cocos Islands from seismic sources is tsunami rather than ground shaking.

The 2004 Indian Ocean [[tsunami]] is by far the most significant seismic event in the islands' recorded history. Generated by the [[moment-magnitude]] 9.1 Sumatra-Andaman earthquake, the [[tsunami]] arrived at the Cocos Islands several hours after the source event. Wave heights at Direction Island and Home Island were recorded at approximately 1 to 2 metres above normal tide levels, causing some coastal inundation and minor damage but no casualties — a outcome partly attributable to the atolls' low population and the relatively modest wave heights at this distance from the [[epicenter]]. The [[seismic-wave]]s from the 2004 earthquake were recorded on distant seismographs and showed clear [[p-wave]] and [[s-wave]] arrivals from the Sumatran source. The territory benefits from Australia's participation in the Indian Ocean Tsunami Warning System, which provides advance notification of potential tsunami events from the Sunda megathrust and other active sources.

The geology of the Cocos (Keeling) Islands follows the same coral atoll formation pattern as other tropical Pacific and Indian Ocean atolls: a volcanic seamount formed on the oceanic plate over a hotspot or mid-ocean ridge, which subsided as the plate moved away from the spreading centre, with coral reef growth keeping pace with subsidence to maintain the atoll at or near sea level. The basement basalt is buried tens to hundreds of metres below the coral cap. The surrounding Indian Ocean oceanic crust is Mesozoic in age — among the oldest on Earth — and has long since cooled to a rigid, seismically quiet state. [[seismic-wave]] propagation through old, cold oceanic lithosphere is highly efficient, meaning that the [[p-wave]] arrivals from distant great earthquakes are clearly recorded even on simple instruments. With a permanent population of approximately 600 and no significant infrastructure beyond the settlement, the principal risk remains coastal inundation from distant-source [[tsunami]].