The San Fermín earthquake, registering 7.1 Mw, rocked Puerto Rico on October 11, 1918. With its epicenter near the island's southwestern coast, the quake unleashed a catastrophic tsunami that overwhelmed coastal regions, specifically impacting cities like San Germán and Mayagüez. Residents felt the tremors as they disrupted daily life, resulting in panic as buildings swayed and crumbled. Following the initial quake, massive waves up to 20 feet high surged inland, leading to widespread devastation and loss of life. Emergency responses were hampered by the extensive destruction of communication lines and transportation infrastructure.

The aftermath revealed a disturbing picture; the historic death toll reached up to 116 people while thousands were left homeless. The earthquake prompted an urgent assessment of Puerto Rico's earthquake preparedness and building codes. The government acknowledged the need for improved infrastructure and promoted public awareness regarding seismic safety.

In terms of geological significance, the earthquake marked a pivotal point in understanding the tectonic activity of the region, reigniting interest in monitoring seismic events. The event underscored Puerto Rico's vulnerability to such natural disasters, particularly within the context of the Caribbean plate tectonics.