Puerto Rico

Puerto Rico is a United States territory in the northeastern Caribbean that faces some of the highest seismic hazard in the entire Caribbean region, arising from the convergence of the North American Plate and the Caribbean Plate along one of the most geologically complex boundaries in the Atlantic basin. The Puerto Rico Trench, immediately north of the island, is the deepest point in the Atlantic Ocean, reaching 8,376 metres below sea level, and represents the zone where the North American Plate descends beneath the Caribbean Plate at approximately 2 centimetres per year. The seismic setting is further complicated by the Muertos Trough south of Puerto Rico, the Anegada Trough fault zone to the east, the North America-Caribbean transform boundary expressed in the Septentrional and other faults, and a series of reverse and thrust faults within the island itself. The southwest of Puerto Rico, underlain by the Lajas Valley and offshore fault systems including the Guayanilla and Punta Montalva faults, represents the most seismically active onshore zone in recent decades. Seismic hazard is rated very high, with probabilistic ground motion values among the highest in the U.S. National Seismic Hazard Model for any populated territory.

Puerto Rico has a rich and damaging earthquake history. On 11 October 1918, a magnitude 7.1 earthquake struck off the northwest coast near Mayagüez, generating a destructive [[tsunami]] with wave heights reaching 6 metres that killed approximately 116 people and caused massive destruction to coastal infrastructure along the western coast. Beginning in late December 2019, a prolonged sequence of earthquakes struck the southwestern corner of the island, with a magnitude 5.8 event on 28 December 2019 followed by a magnitude 6.4 mainshock on 7 January 2020 — the largest earthquake to strike Puerto Rico since 1918. The 2020 sequence killed one person, injured hundreds, and rendered thousands of buildings uninhabitable in the municipalities of Guánica, Guayanilla, and Yauco; [[aftershock]] activity exceeded 10,000 events over the following year and a half. Earlier significant events include a magnitude 7.0 earthquake in 1943 and numerous M5–6 events throughout the instrumental record. The Puerto Rico Trench is considered capable of hosting a great [[subduction-zone]] earthquake in the magnitude 8–9 range, which would generate a major [[tsunami]] threatening Puerto Rico's north coast.

Puerto Rico's geology reflects complex Cretaceous island-arc volcanism, Eocene to Oligocene carbonate platform development, and Neogene to Quaternary deformation under the current Caribbean-North American convergence. The island's rugged interior consists largely of Cretaceous volcanic and plutonic rocks, while the coastal plains and valleys are underlain by Quaternary alluvial and marine sediments susceptible to [[liquefaction]] and [[seismic-wave]] amplification. The Puerto Rico Seismic Network (PRSN), operated by the University of Puerto Rico at Mayagüez, provides real-time monitoring and participates in the USGS ShakeAlert research programme. The PRSN also operates the Caribbean Tsunami Warning Programme, coordinating [[tsunami]] detection and alert dissemination for the northeastern Caribbean. The 2020 earthquake sequence generated significant attention to the need for improved building codes and housing retrofits, particularly for the many concrete-block structures in the southwestern municipalities.