Marshall Islands

The Marshall Islands consist of 29 coral atolls and 5 islands arranged in two roughly parallel chains — the Ratak (Sunrise) and Ralik (Sunset) chains — in the central Pacific Ocean. Like Kiribati to the south, the Marshall Islands sit on the interior of the Pacific Plate, far from any active plate boundary, and therefore generate essentially no local seismicity. The islands themselves do not experience damaging earthquake ground shaking from local sources. The principal natural hazard from seismic events is the [[tsunami]] generated by distant plate boundary earthquakes around the Pacific Rim — particularly from the Japan-Kuril-Kamchatka subduction system to the northwest, the Tonga-Kermadec system to the south, and the Alaska-Aleutian arc to the north. All inhabited islands in the Marshall Islands sit less than 2 metres above mean sea level, making the [[tsunami]] hazard particularly acute despite the distances involved. Majuro, the capital, sits on an atoll with a maximum elevation of approximately 3 metres.

The [[tsunami]] history of the Marshall Islands includes several documented inundation events. The 1946 Aleutian [[tsunami]] caused significant flooding on Bikini and other northern Marshall Islands. The 1952 Kamchatka [[tsunami]] (magnitude 9.0 source) reached the Marshall Islands with wave heights sufficient to cause concern. The 1960 Chilean mega-earthquake (magnitude 9.5) generated a [[tsunami]] that traversed the entire Pacific and caused inundation in the Marshall Islands roughly 19 hours after the source event. The 2011 Tohoku [[tsunami]] generated wave heights of approximately 0.5 to 1.5 metres at Marshall Islands tide gauges. The greatest long-term concern for the Marshall Islands is not a single [[tsunami]] event but the combination of rising baseline sea levels with the occasional [[tsunami]] event, which together could make low-frequency flooding more severe and frequent. The Pacific Tsunami Warning Center in Hawaii provides the primary warning service for the Marshall Islands.

The geology of the Marshall Islands atolls follows the same pattern as other Pacific atoll chains: the basement is composed of basaltic volcanic seamount material erupted as the Pacific Plate passed over the Marshall Islands hotspot approximately 80 to 100 million years ago. As the plate continued moving northwest, the volcanic islands subsided and coral reef growth constructed the atoll structures that persist today. The coral limestone thickness reaches up to 1,300 metres on Enewetak Atoll, where Cold War nuclear testing programs drilled to the basaltic basement and revealed the extraordinary depth of carbonate accumulation. The [[seismic-wave]]s that periodically register on Pacific Ocean seismographs and tide gauges associated with the Marshall Islands record distant events rather than local geology. The absence of any compressional, extensional, or transform tectonics in the immediate vicinity means the Marshall Islands' geological future is shaped by sea-level change and continued slow subsidence rather than tectonic activity.