The 1918 San Fermín earthquake, also known as the Puerto Rico earthquake of 1918, struck the island of Puerto Rico at 10:14:42 local time on October 11. The earthquake measured 7.1 on the moment magnitude scale and IX (Violent) on the Mercalli intensity scale. The mainshock epicenter occurred off the northwestern coast of the island, somewhere along the Puerto Rico Trench.

Puerto Rico, officially the Commonwealth of Puerto Rico, is a Caribbean island, Commonwealth, and unincorporated territory of the United States. It is located in the northeast Caribbean Sea, approximately 1,000 miles (1,600 km) southeast of Miami, Florida, between the Dominican Republic and the U.S. Virgin Islands, and includes the eponymous main island and several smaller islands, such as Mona, Culebra, and Vieques. With roughly 3.2 million residents, it is divided into 78 municipalities, of which the most populous is the capital municipality of San Juan. Spanish and English are the official languages of the executive branch of government, though Spanish predominates.