The San Fermín Earthquake and Tsunami (1918)

On the morning of October 11, 1918—the feast day of San Fermín—a magnitude 7.1 earthquake struck the Mona Passage off western Puerto Rico at 10:14 AM. The quake, registering IX (Violent) on the Mercalli intensity scale, was the most powerful seismic event to hit the island in modern history. Within minutes, a tsunami with waves reaching up to 6 meters (20 feet) lashed the western coastline, destroying entire fishing villages and drowning dozens of people.

Mayagüez, the largest city in the affected zone, suffered catastrophic damage. Over 700 masonry buildings were severely damaged or destroyed, along with 1,000 wooden structures. The city's church, post office, municipal theater, and city hall were all rendered unusable. In Aguadilla, 32 people drowned from the tsunami alone. The tsunami also destroyed the Punta Borinquen lighthouse and devastated coastal settlements at Point Agujereada and Point Jiguero.

The official death toll reached 116 people, with economic losses calculated at $4 million—twice the annual budget for the entire island at the time. Governor Arthur Yager convinced the Secretary of War to send seismologists Harry Fielding Reid and Stephen Taber to investigate, producing a landmark Congressional and scientific publication on the earthquake. A special earthquake commission was formed to inventory damage and disburse relief funds, though the colonial government's response was limited by its constrained fiscal authority under US oversight.

The earthquake exposed the vulnerability of Puerto Rico's infrastructure under colonial administration, where investment in building codes, emergency services, and disaster preparedness remained minimal. The devastation presaged later disasters—including the 2020 earthquake swarm in the same southwestern region—that would repeatedly demonstrate how colonial underinvestment compounds natural hazards into human catastrophes.