| Date | October 11, 1918 |
|---|---|
| Magnitude | 7.5 Mw |
| Countries or regions | Puerto Rico |
| Casualties | 116 |
The San Fermín earthquake[1][2][3], also known as the Puerto Rico earthquake of 1918, was a major earthquake that struck the island of Puerto Rico at 10:14am on October 11, 1918. The magnitude for the earthquake has been reported at around 7.5 (or Level IX in the Rossi-Forel scale used at that time); however, that might not be an exact number. The main-shock epicenter occurred offshore about 5 km (3.1 mi) from the northwestern coast of the island, somewhere along the Puerto Rico Trench.
The earthquake triggered a tsunami with waves measured at approximately 5.5 meters (20 feet) that lashed the west coast of the island and is remembered as one of the worst natural disasters that have struck the island. The losses resulting from the disaster were approximately 116 casualties and $4 million in property.
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The epicenter of the 1918 Puerto Rico Earthquake was located in the Mona Passage about 16 km (9.9 mi) from the northwestern coast of the island, somewhere along an old left-lateral strike-slip fault close to the Mona Passage. The strongest ground shaking has been estimated at around a magnitude 7.3 or 7.5, or Level IX (Rossi-Forel scale). The resulting tsunami affected primarily the west coast city of the Island (primarily Mayaguez) and other adjacent towns as well.
As a result of the earthquake, numerous structures in the west coast suffered irreparable damages. Factories and production centrals were virtually destroyed, while bridges and roads were severely damaged.
The earthquake caused several mudslides in areas where the magnitude exceeded Level VII, but none of it was deemed as tragic. Also, the river currents were affected, which, in many cases affected the foundations of many bridges which ended up collapsing.
The reported casualties of the earthquake have been estimated somewhere between 91 to 116 deaths. Approximately 40 of these deaths were caused by the tsunami. Also, damages to property were estimated at around $4 million, which was a huge amount at that time.
As a result of the earthquake, a tsunami lashed the west coast of the island, probably 4–7 minutes after the main shock. The highest waves were measured at 5.5 to 6 meters (20 feet) and ended up destroying several coast-side villages. It has been estimated that 40 people were drowned as a direct result of the tsunami.
The tsunami reached Galveston, Texas as a disturbance on tide gauges.
Several aftershocks were reported immediately after the main earthquake. On October 24 and November 12, two strong aftershocks were reported in the island. However, no damages were reported as a result.