Antigua and Barbuda

Antigua and Barbuda sit within the northern Lesser Antilles island arc, a chain formed by the westward [[subduction-zone]] of the North American Plate beneath the Caribbean Plate along the Puerto Rico Trench and its southern extension. This ongoing subduction generates frequent low-to-moderate earthquakes throughout the arc, and the twin-island nation experiences regular seismic activity, though it has avoided a major destructive earthquake in the modern era. The dominant seismic source is the [[subduction-zone]] interface approximately 100 to 200 kilometres to the northeast, where convergence occurs at roughly 2 centimetres per year. Secondary seismic sources include shallow crustal faults within the arc platform itself. Antigua, built on older volcanic and coral limestone rocks, is generally less hazardous than the active volcanic islands to the south, but proximity to the subduction system means earthquake exposure remains real. Barbuda's coral limestone platform provides some natural attenuation, but [[tsunami]] vulnerability from Caribbean-wide events remains a concern.

Historical records from the colonial era document several significant earthquakes affecting the islands. An 1843 earthquake centred on the island of Guadeloupe approximately 100 kilometres to the south caused severe damage to buildings in Antigua and killed several people. The 1974 Barbuda earthquake (magnitude 5.5) caused minor structural damage. The 8 December 2004 earthquake (magnitude 6.1) near Antigua produced felt shaking and some minor damage. The great 1755 Lisbon earthquake and [[tsunami]] generated waves that were recorded in the Caribbean, and regional events from the Lesser Antilles subduction zone have historically produced locally significant sea-level changes. The primary long-term threat to Antigua and Barbuda is not a locally generated earthquake but a great subduction event on the Puerto Rico Trench or the southern Lesser Antilles arc, which could generate a destructive [[tsunami]] with limited warning time.

The tectonic architecture of the Lesser Antilles is a classic example of an ocean-ocean [[subduction-zone]] with an associated volcanic island arc. The older, eastern arc — the Limestone or Outer Arc — includes Antigua, Barbuda, and Marie-Galante, and is composed of older submarine volcanic material capped by marine limestone, now uplifted above sea level by arc-flank processes. The younger, more active volcanic arc to the west includes active stratovolcanoes such as Montserrat's Soufrière Hills. Antigua and Barbuda sit on the outer, less volcanically active arc and experience primarily [[seismic-wave]] energy arriving from the main subduction zone rather than local volcanic seismicity. [[magnitude]] estimates for regional events are routinely monitored by the University of the West Indies Seismic Research Centre.