Cook Islands

The Cook Islands comprise 15 islands scattered across approximately 2.2 million square kilometres of the south-central Pacific Ocean, all sitting on the interior of the Pacific Plate far from any active plate boundary. The islands were formed as the Pacific Plate passed over the Cook-Austral hotspot chain, which produced a series of volcanic edifices — some now above sea level as fertile volcanic islands such as Rarotonga, and others submerged as guyots and seamounts. Rarotonga, the most populous island, is the youngest and least eroded of the volcanic islands, with its shield volcano reaching 652 metres at Te Manga. Because the Cook Islands sit on stable Pacific Plate interior, indigenous seismicity is essentially absent; there are no active tectonic plate boundaries nearby and no known active fault structures beneath the island chain. The primary seismic-related hazard facing the Cook Islands is the [[tsunami]] generated by great earthquakes in the surrounding Pacific Ring of Fire.

The most significant [[tsunami]] threat to the Cook Islands comes from three principal source zones: the Tonga-Kermadec [[subduction]] system to the west, where the Pacific Plate is consumed at extreme rates; the Chile subduction zone on the far eastern margin of the Pacific; and the New Hebrides arc to the northwest. The 1960 Chilean mega-earthquake ([[moment-magnitude]] 9.5) generated a Pacific-wide [[tsunami]] that caused wave heights of approximately 1 metre at Rarotonga tide gauges. The 2009 Samoa [[tsunami]], generated by the [[moment-magnitude]] 8.1 outer-rise earthquake, reached the Cook Islands with wave heights of approximately 0.5 metres. The 2011 Tohoku [[tsunami]] produced wave heights of around 0.3 to 0.8 metres at Cook Islands monitoring stations. The 15 January 2022 Hunga Tonga-Hunga Ha'apai volcanic [[tsunami]] produced wave heights of approximately 1 metre at Rarotonga, reaching the island roughly 2 hours after the eruption.

The geological foundation of the Cook Islands reflects 20 million years of Pacific Plate motion over the Cook-Austral hotspot. Rarotonga, approximately 2 million years old, retains its rugged volcanic topography and active hydrothermal systems, while the more northerly Manihiki Plateau seamounts are far older. The southern group of volcanic islands — Rarotonga, Atiu, Mauke, Mitiaro, and Mangaia — shows a clear age progression from youngest to oldest consistent with northwestward Pacific Plate motion at approximately 70 millimetres per year over the hotspot. Mangaia, the geologically oldest, is also the most eroded and has been transformed from a shield volcano to a low coral-capped remnant. The absence of any active [[subduction]] boundary or [[fault]] system near the Cook Islands means [[seismic-wave]] arrivals recorded at the islands are entirely from distant sources. The Cook Islands Meteorological Service receives [[tsunami]] warnings from the Pacific Tsunami Warning Center and maintains community-level evacuation planning for the volcanic island group.