The ground shook twice

Twenty years into American rule, Puerto Rico was struck by a major earthquake and a tsunami, a disaster real and well-documented enough that its relative obscurity today feels a little strange.

The San Fermín earthquake of 1918 occurred at 10:14:42 local time on October 11, 1918. It measured 7.1 on the moment magnitude scale and IX, "Violent," on the Mercalli intensity scale, a reading reserved for the most destructive shaking an earthquake can produce, strong enough to cause severe structural damage across a whole region and not just a single town.

Its epicenter lay off the island's northwest coast, near the Mona Passage separating Puerto Rico from the Dominican Republic, somewhere in the Puerto Rico Trench, a seismically active stretch of ocean floor that still generates smaller tremors in the region today.

The quake triggered a tsunami that reached about 6.1 meters, close to 20 feet, in height, a wave large enough to overwhelm low coastal settlements with little or no warning. Damage and loss of life were worst in Mayagüez, on the west coast closest to the epicenter, with lighter damage along the rest of the coast, and aftershocks kept shaking the region for weeks, stretching the fear and disruption long after the initial quake.

An estimated 116 people died in the earthquake itself, and another 40 in the tsunami that followed, bringing the disaster's combined toll to about 156 people in a single day.

It is a disaster that checks every box of significance: a major magnitude, a real tsunami, a documented toll, and a heavily populated coast sitting right in the impact zone. And yet, sandwiched between the catastrophic hurricanes of 1899 and the political convulsions of the 1930s, it rarely makes the island's short list of history. It deserves better than footnote status, if only because the numbers involved were genuinely grave.

Sources: Terremoto de San Fermín de 1918; Historia de Puerto Rico. Freeze 2026-08-22.