1918 San Fermín earthquake and tsunami

The morning the sea stepped back

It began with an ordinary shore morning: fishermen hauling nets, carts rumbling toward the docks, children watching the tide. Then, according to the people who lived it, the water betrayed them. In places the sea seemed to withdraw — leaving boats resting on newly exposed sand and reefs — like someone pulling a tablecloth too quickly. The pause lasted only moments. The return came as a wall of water; in other accounts it arrived as a series of surges. Boats were torn from moorings, wharves were smashed, and waterfront streets filled in a matter of minutes.

That withdrawal and return is the image most people remember, because it makes clear what kind of danger local residents faced: not only ground that shook beneath their feet, but the ocean itself that turned from friend to foe. What most could not know then was why the wave had acted the way it did. In the decades since, scientists would assemble a picture that mixed seismic rupture with a catastrophic underwater slope failure, but in 1918 there were only stunned witnesses and ruined piers.

A coast that lived with the water — and little warning

Puerto Rico in 1918 was still adapting to a new colonial reality. The island had been a U.S. territory for two decades. Its western towns — Aguadilla, Añasco, Rincón and Mayagüez — were tied to the sea: fishing boats, sugar and coffee shipments, warehouses and wharves crowded the shoreline. Buildings clustered near the water, built of masonry or timber, often without reinforcement against shaking. There were no tsunami sirens, no island-wide seismic network, and no organized early-warning system. Seismology existed mostly in far-off university labs and government offices; what mattered on the coast was human memory and luck.

Geology, however, had been quietly setting the stage. Puerto Rico sits where the North American Plate and the Caribbean Plate grind past one another in a tangle of faults. The Mona Passage — the deep channel that separates the island from Hispaniola — is a rugged, active stretch of seafloor. Faults slice through its depths; steep submarine slopes and canyons make it susceptible not only to seismic rupture but to landslides that can throw enormous volumes of sediment into motion. That combination — a strong quake plus unstable offshore slope — would prove deadly.

The shaking that changed a morning

In the hours and days leading to October 11 some residents later reported light tremors, little signs of a larger thing coming. Instrumental records were sparse in the Caribbean at that time; what mattered to contemporary accounts was what people felt and what fell. Then, in the local morning, a large earthquake struck the Mona Passage region. Modern re-analyses of the event place its size in the range of moment magnitude 7.1–7.5 (many authorities use a working figure near Mw 7.3). For those on shore it felt like a calamity: chimneys toppled, unreinforced masonry cracked or collapsed, buildings shook violently and the dance of cargo and crates on the quays turned into disorder.

The pattern of damage was concentrated along the west coast. Ports and docks — the physical arteries of towns built to trade — took a heavy hit. Warehouses that stored sugar or coffee, fragile piers, and small fishing vessels were destroyed or carried inland by the water. Some people died when buildings fell; others were swept away in the sudden floods that hit low-lying streets and shorelines. Marinas and wharves that had been hubs of commerce lay in splinters.

The sea became an unkind neighbor

Eyewitnesses recorded similar sequences across towns: an initial withdrawal of water, a brief exposed shoreline, then violent inundation. The tsunami did not behave uniformly. Runup — the height the wave reached onshore — varied greatly from place to place. Historical accounts and later studies report local runups usually of a few meters, with some areas experiencing measured or reported heights approaching six meters. That uneven pattern hinted at something more complex than simple fault displacement.

Fishing boats were among the most visible losses. Many small vessels were either smashed on reefs, hurled up onto quays, or carried inland and left stranded. Men who had gone to their boats to check nets were among the casualties; others drowned onshore as waves pushed into marketplaces and pier warehouses. Contemporary tallies, later compiled by researchers, report approximately 116 fatalities — a number that has become the commonly cited total, though historians acknowledge uncertainty because record keeping was incomplete and some deaths at sea or in remote locales went uncounted.

Cleaning up the docks and counting the dead

In the hours after the shaking and the surges, communities pivoted toward rescue. Municipal officials, neighbors, and whoever could muster muscle and carts went to work. Bodies were recovered where possible. Debris was cleared enough to allow salvaging of goods. Where docks remained passable, shipments resumed on a reduced scale. Local authorities and U.S. territorial administrators moved to assess damage and coordinate relief, though resources were limited and communications sometimes faltered amid the rubble.

Economic losses were severe for coastal towns. Contemporaneous estimates placed direct property damage at roughly $4 million in 1918 dollars — a shock to local economies dependent on maritime commerce and the agri-industries tied to export. Converted into modern dollars using broad consumer price indices, that translates into tens of millions by mid‑21st-century measures — a rough sense of scale rather than a precise figure. The immediate human needs — shelter, food, and basic services — consumed whatever local and territorial resources were available.

Questions that could not be answered that week

Scientists and officials in 1918 faced hard questions with few tools. Instrumental coverage in the Caribbean was sparse: there were limited seismographs, no comprehensive island network, and almost no way to map the seafloor in detail. The eyewitness record was rich in feeling and practical detail, but it could not, on its own, explain why some stretches of coast were hammered harder than others.

Early investigations catalogued damage and collected testimonies; later, geologists and seismologists would return to this event with new techniques. They would study distant seismograms, compile macroseismic reports, and, crucially, model how both fault displacement and underwater landslides could create waves. These decades-later reconstructions pointed to a mixed origin: the earthquake’s rupture moved the seafloor, and the quake also likely triggered one or more submarine slope failures in the Mona Passage — massive landslides that amplified and complicated the tsunami pattern along the coast.

The slow work of rebuilding and remembering

Reconstruction began quickly, as it always does after a disaster, in fits and starts. Wharf timbers were salvaged where possible; new docks were built or patched, and warehouses were repaired. For many residents, however, the loss of income from damaged boats and ruined cargo lasted far longer than the visible rubble. Families who depended on fishing or on jobs at the port felt the economic sting into the following seasons.

Policy and institutional change after 1918 was incremental. The earthquake and tsunami became part of Puerto Rico’s hazard record — a warning that offshore quakes could produce destructive local tsunamis, especially where submarine slopes were steep and unstable. But comprehensive seismic networks, robust building codes, and formal tsunami-warning systems would not emerge overnight; they developed gradually over the mid- to late-20th century as instrumentation, scientific understanding, and political will expanded.

What the sea left behind for science

The 1918 San Fermín event is now a case study in how earthquakes can make the ocean do unexpected things. Modern re-analyses place the earthquake’s size in the Mw 7.1–7.5 range and point to a combined tsunami source: both tectonic displacement and submarine landslides likely contributed to the observed wave heights and the patchy pattern of coastal impact. The Mona Passage remains closely watched because of that combination of strong faults and steep underwater slopes.

The casualty count — commonly cited as about 116 dead — is grim but incomplete. Historians and seismologists note the probability of undercounting, particularly among those lost at sea or living in isolated settlements. The economic damage figure of roughly $4 million in 1918 provides a quantitative anchor, but it does not fully capture the long-term disruption to livelihoods and local commerce.

The island’s memory and a continuing lesson

Disasters leave both scars and lessons. The San Fermín earthquake and the tsunami it produced entered Puerto Rico’s collective memory as a reminder of how quickly the familiar can be altered: docks turned to wreckage, markets flooded, boats lifted from their moorings. For scientists and emergency planners, the event remains a clear example of the hazard presented when a strong offshore quake coincides with unstable submarine slopes.

Today, seismic networks and tsunami hazard models are far more advanced. Yet the essential vulnerability has not vanished: coastal towns still cluster near the water they rely on, and submarine slopes remain as dangerous as they were a century ago. The work begun in the aftermath of 1918 — cataloging damage, re-examining seismograms, and modeling tsunamis — continues. Geologists still probe the Mona Passage, searching for sedimentary fingerprints of past tsunamis and mapping slopes that could fail again.

The photographs that survive from the days after October 11, 1918 show overturned boats, toppled chimneys and people gathered on ruined quays, sheltering their losses beneath the same sky that once bore the wave. Those images are not only a record of destruction; they are a warning. The ocean reclaimed and reshaped part of Puerto Rico’s coast that morning, and the lessons of 1918 — about risk, preparedness and the hidden instability of the seafloor — remain part of the island’s story.