United States Minor Outlying Islands

The United States Minor Outlying Islands comprise eight unincorporated U.S. territories scattered across both the Pacific Ocean and the Caribbean Sea, with fundamentally different tectonic settings and seismic hazard profiles. In the Pacific, Johnston Atoll is an isolated low-lying coral atoll on the Hawaiian-Emperor seamount chain, far from any active plate boundary; Wake Island and Midway Atoll are similarly remote Pacific atolls on the Pacific Plate with minimal indigenous seismicity. Palmyra Atoll, Kingman Reef, Jarvis Island, Howland Island, and Baker Island lie in the equatorial Pacific, also on the Pacific Plate intraplate setting. By contrast, Navassa Island in the Caribbean is located approximately 50 kilometres west of Haiti and is geologically situated near the Cayman Trough transform fault system and the Caribbean-North American plate boundary, giving it the highest seismic hazard among the group. Seismic hazard across the territories ranges from minimal (Pacific atolls) to moderate to high (Navassa Island).

The earthquake history of the U.S. Minor Outlying Islands reflects their diverse settings. Navassa Island, lying within 50 to 100 kilometres of the Cayman Trough transform fault and the fault systems of western Haiti and eastern Cuba, is exposed to the same Caribbean plate boundary seismicity that affects the wider region. The January 2020 magnitude 7.7 earthquake on the Oriente Fault — the largest Caribbean earthquake in decades — would have generated strong shaking near Navassa, which is uninhabited but for periodic scientific visits. Major Hispaniola earthquakes, including the 2010 and 2021 events, would have been felt strongly there. The Pacific atolls lie at varying distances from Pacific subduction zones and mid-ocean ridge systems; Wake Island, historically important as a military outpost, has experienced felt events from remote Pacific sources, and the equatorial atolls are entirely removed from active seismicity. [[tsunami]] hazard is high for all low-lying Pacific atolls, which are exposed to Pacific-wide tsunami propagation from subduction events around the Ring of Fire.

The geological character of the U.S. Minor Outlying Islands reflects their origins as coral atolls built on submerged seamounts (Pacific) or as exposed Cretaceous limestone and igneous rocks (Navassa Island). The Pacific atolls are typically less than 2 metres above sea level and would be completely inundated by a significant [[tsunami]]; this makes them among the most physically vulnerable territories in the U.S. system to ocean hazards, though their negligible permanent populations limit human exposure. Navassa Island's carbonate limestone geology reflects its position on the Cuban-Haitian carbonate platform. Scientific monitoring infrastructure across most of these islands is minimal or absent; data from regional seismic and ocean networks provides the principal basis for understanding seismic and [[tsunami]] activity near these remote territories.