United States Virgin Islands

The United States Virgin Islands (USVI) occupy one of the most seismically hazardous positions in the entire Caribbean, situated directly within the Anegada Trough — an active east-west-trending extensional and transform fault zone separating the Greater Antilles to the north and west from the Lesser Antilles to the south — and exposed to both the Puerto Rico Trench [[subduction-zone]] to the north and local fault earthquakes from the Anegada Passage fault system. The three main islands — Saint Croix, Saint Thomas, and Saint John — along with the smaller islands, sit on the Virgin Islands Platform, a shallow submarine bank bounded on the north by the steep drop into the Puerto Rico Trench and on the south by the Anegada Trough. The Anegada Trough fault zone is capable of generating M7+ earthquakes from shallow crustal rupture, and the Puerto Rico Trench system poses the threat of a great [[subduction-zone]] earthquake in the M8–9 range that would deliver devastating ground motion and a major [[tsunami]] to the north coast. Seismic hazard in the USVI is rated very high — among the highest of any U.S. territory.

The 18 November 1867 Virgin Islands earthquake is the territory's defining historical seismic event. Centred near Saint Croix in the Anegada Passage at an estimated [[magnitude]] 7.5, the earthquake generated one of the most damaging [[tsunami]]s in Caribbean history. Wave heights of 3 to 5 metres struck Saint Thomas, Saint Croix, and nearby British and Danish colonial settlements, destroying wharves, flooding towns, and killing dozens of people. The [[tsunami]] struck with sufficient force to carry vessels inland and strip coastal infrastructure across multiple islands. [[aftershock]]s continued for weeks. In the instrumental era, the USVI has experienced numerous M4–6 events associated with the Anegada Trough and Puerto Rico Trench, including a magnitude 6.4 earthquake in 1969 near the Anegada Passage. The 2020 Puerto Rico earthquake sequence (M6.4 mainshock) caused felt shaking throughout the USVI. The territory also experienced significant shaking from the 2010 Haiti earthquake and the 2020 Caribbean earthquake (M7.7), demonstrating the breadth of Caribbean seismic sources that affect the islands.

The geology of the USVI reflects a long history of Cretaceous island-arc volcanism, Eocene to Oligocene carbonate platform development, and Neogene to Quaternary structural faulting along the Anegada Trough. Saint Thomas and Saint John are built on Cretaceous volcanic and plutonic basement — geochemically part of the same arc sequence as Puerto Rico — while Saint Croix sits on a carbonate platform basement with a different geological history, having been transported on the Caribbean Plate to its current position. The coastal lowlands of Christiansted and Frederiksted on Saint Croix, and the Charlotte Amalie harbour area on Saint Thomas, are underlain by alluvial and marine sediments susceptible to [[seismic-wave]] amplification and [[liquefaction]] in a major nearby earthquake. Seismic monitoring is conducted by the Puerto Rico Seismic Network (PRSN), which maintains stations in the USVI and provides real-time data to USGS and the Caribbean Tsunami Warning Programme. The territory has invested in [[tsunami]] evacuation route signage and public education following improved hazard mapping of the Anegada Trough and Puerto Rico Trench source zones.