Kiribati

Kiribati comprises a scattered archipelago of 33 low-lying coral atolls spread across 3.5 million square kilometres of the central and western Pacific Ocean, straddling both the equator and the International Date Line. The islands sit on the Pacific Plate interior, far from any active plate boundary, and therefore experience very low indigenous seismicity; Kiribati does not generate significant earthquakes from local tectonic activity. The primary seismic hazard facing the country is not from shaking but from the [[tsunami]] generated by distant Pacific plate boundary events — particularly those from the Tonga-Kermadec system, the New Hebrides arc, and the Chile subduction zone. The Phoenix Islands in the east are closest to the Tonga-Kermadec [[subduction-zone]] and may experience the shortest warning times for tsunamis from events in that system. The extreme low elevation of all inhabited islands in Kiribati — most land rises less than 3 metres above sea level — means that any significant [[tsunami]] wave represents an existential threat to populations with nowhere to evacuate vertically.

The seismic history specific to Kiribati is limited given the absence of local seismic sources, but the islands have experienced [[tsunami]] impacts from regional events. The 1946 Aleutian earthquake [[tsunami]] traversed the Pacific and reached the Gilbert Islands. The 2011 Tohoku earthquake and [[tsunami]] generated wave heights of approximately 0.5 to 1 metre at some Kiribati monitoring stations, demonstrating the connectivity of the Pacific tsunami system even to islands far from the source. The 1964 Good Friday Alaska earthquake generated a Pacific-wide [[tsunami]] that was recorded at Kiribati tide gauges. The [[tsunami]] threat is compounded by the islands' long-term vulnerability to sea-level rise, which is reducing the natural buffer height and increasing inundation risk from both [[tsunami]] and storm surge events. Monitoring of regional seismicity by distant networks is the primary means of tsunami warning for Kiribati, with PTWC providing bulletins to the Kiribati Meteorological Service.

The geology of Kiribati atolls reflects the subsidence of volcanic seamounts that were originally formed as hotspot volcanoes on the Pacific Plate and have since moved away from the hotspot and subsided below sea level, with coral reef growth keeping pace to maintain the land surface at or near sea level. The basement rock beneath the coral atoll caps is basaltic, but it is buried at depths of tens to hundreds of metres below the surface. The coral limestone and reef debris that forms the atoll ring and motu islands is geologically very young — dominantly Holocene in age — and the total land area of most islands is tiny. The Pacific Plate on which Kiribati sits is ancient and rigid, and the distant [[subduction-zone]] boundaries that generate tsunamis transmit [[seismic-wave]]s efficiently across the cold, dense lithosphere. The absence of local amplification structures and the simple low-relief geology means that the primary hazard model for Kiribati is pure tsunami inundation rather than ground shaking effects.