The epicenter of the 1918 San Fermín earthquake was located in the Mona Passage off the northwestern coast of the island.

In Aguadilla, the closest town to the earthquake epicenter, the parish church and most of the stone and concrete buildings were either destroyed or partially damaged.

[9][10] In Mayagüez, the largest city to be directly affected, 700 masonry buildings were damaged and 1,000 wooden houses, so many people were homeless.

Major buildings like the church, post office, municipal theater and city hall were severely damaged.

[12][13] The historic center of San Germán was also badly affected, with reported damages to notable structures such as the main town church.

The United States Customs House was destroyed[15] while the Ponce city hall for example was damaged, prompting the mayoral office to temporarily relocated to the Parque de Bombas until 1920.

[17][18] There were damages reported further afield throughout Puerto Rico, for example the original bell towers of the cathedral of Humacao and the main town church of Vega Baja also collapsed.

[24] Several coastal villages were destroyed, and it has been estimated that 40 people drowned (32 in Aguadilla alone) as a direct result of the tsunami.