1918 San Fermín earthquake

Damage caused to the "La Habanera de Infanzón y Rodríguez" building in Mayagüez

Numerous structures in the west coast suffered irreparable damage. Factories and production facilities were virtually destroyed, while bridges and roads were severely damaged. The earthquake caused several mudslides in areas where the intensity exceeded Level VII, but none caused numerous deaths. Also, the river currents were affected, which, in many cases affected the foundations of many bridges, resulting in their collapse. Telegraph cables under the ocean were damaged, cutting off the island from outside communication for a time.[7]

Simulation of the results of the tsunami on the coast of Aguadilla, Puerto Rico

The reported casualties of the earthquake have been estimated somewhere between 76 and 116 deaths. Approximately 40 of these deaths were caused by the tsunami which swept shore communities. Damage to property was estimated to be between $4 and 29 million.

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.[8] The nearby Spanish-built Punta Higuero lighthouse of Rincón also was severely 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. With fear because of the aftershocks, many people camped out in the hills for weeks.[7] Some of the buildings of the recently founded University of Puerto Rico campus were also damaged or destroyed. The Edificio José de Diego suffered structural damages and the Degetau Hall was destroyed with its main entrance portico being the only standing structure left intact.[11] These ruins were later preserved, and its portico would later on become a local landmark and the official emblem of the institution today.[12][13]

The historic center of San Germán was also badly affected, with reported damages to notable structures such as the main town church.[14] Cities throughout the southern coast were also affected. 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.[16] Other damaged buildings in Ponce were the Tricoche Hospital and the Armstrong-Poventud House.[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.[19][20] The Corregimiento Plaza Theater, the cathedral and the city hall in Arecibo also sustained damages.[21][22]

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 estimated at 6.0 m (19.7 ft) in Point Agujereada, 4.5 m (15 ft) at Punta Borinquen (where it destroyed the lighthouse),[23] and 5.2 m (17 ft) at Point Jiguero.[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.[25] The earthquake and consequent tsunami destroyed most historic vernacular residences in downtown Aguadilla with only a few, such as the Amparo Roldán residence,[26] surviving. Many of the historic tombs and mausolea of the historic cemetery were also badly damaged by the waves.[27]

Several aftershocks were reported immediately after the main earthquake. On October 24 and November 12, two strong aftershocks were reported on the island. However, no damage was reported as a result.