The island of Puerto Rico is located along the northern boundary of the Caribbean plate, the expression of which is manifested as the Puerto Rico Trench, located immediately north of the island. At the Puerto Rico Trench (the deepest location in the Atlantic Ocean), the North American Plate is being obliquely subducted beneath the Caribbean Plate to the south. This oblique subduction is accommodated by a series of active fault zones, which lie very close to Puerto Rico's northern coast. The presence of these large, active fault zones located just off shore of the island, creates a substantial tsunami threat for the Puerto Rican coast.

On October 11, 1918, the island of Puerto Rico was struck by a magnitude 7.5 earthquake, centered approximately 15 kilometers off island's northwestern coast, in the Mona Passage. In addition to causing widespread destruction across Puerto Rico, the quake generated a medium sized tsunami that produced runup as high as six meters along the western coast of the island (see figure below).

The tsunami caused an estimated 4 million dollars in property and other damages to the coastal communities of Puerto Rico. Of the 116 people killed by the earthquake, 40 of those were victims of the tsunami.

At all locations, eyewitnesses to the tsunami indicate that event was marked first by a large withdrawal of water from the shore (the tsunami trough), followed by a large wave (the tsunami crest). The eyewitnesses indicate that this pattern was then repeated one or two more times, but at a smaller scale.

A description of the tsunami's effects at several locations along Puerto Rico's coast is included in the following section. Tsunami runup values at these locations are shown on the figure below.

The highest runup values of 6 meters, 4.5 meters, and 5.2 meters occurred at Point Agujereada, Point Borinquen, and Point Jiguero, respectively. This makes sense, as these locations are closest to the earthquakes epicenter, and probable tsunami source. Point Borinquen is a topographically low-lying area, and as a result the tsunami inundation reached as far as 100 meters inland. At Point Aguereada, the tsunami destroyed many houses, and killed eight people.

The area surrounding Aguadilla, also located in the northwestern corner of the island, was hit hardest by the tsunami. Though the runup values here of 2.4 to 3.4 meters, were not as great as those elsewhere, the 4 meter waves wiped out a village of huts located along the beach, killing 32 people. The tsunami was also powerful enough to carry several 1000 kilogram limestone blocks up to 75 meters inland from their original location.

At the city of Mayaguez a tsunami runup of 1.5 meters flooded the lower floors of waterfront buildings, and destroyed several native huts located near the shore (see the photographs below). On Mona Island the tsunami generated a four meter runup, destroying a pier. The town of Isabella, located on the northern shore of Puerto Rico just beyond Point Borinquen, reported a 2 meter runup.

Additional runup data from locations along Puerto Rico's northern and southern shores, indicate that the tsunami's energy decreased rapidly with distance form the source area. Boqueron, located near the southwestern corner of Puerto Rico, received only one meter waves. Guanica, located on the southern shore, and Isla Caja de Muertos, located just south of the southern coast, received only 0.5 and 1.5 meters of runup, respectively. At Arecibo, located on the northern shore, the runup was only a maximum of 60 centimeters. The tsunami was not even noticed in San Juan Harbor.

Please click on the photos below for an illustration of damage from the 1918 tsunami.

Though the 1918 tsunami was fairly large at some locations, evidence exists supporting the theory that a much larger tsunami may have struck Puerto Rico in geologically recent time. The results of studies involving bathymetric mapping and seismic reflection profiling of the sea bottom off Puerto Rico's northern shore, have uncovered evidence of an enormous submarine landslide. The the suspect slide extends across roughly 57 square kilometer area, and evidence indicates that a slide of this side would have involved approximately 1000 cubic kilometers of material (Grindlay 1998).

Assuming that the slide occurred as a single event, an underwater slide of this magnitude would have generated a tsunami of frightening proportions. The effects of such a tsunami on present day Puerto Rico would be disastrous, and considering that the region is still tectonically active, an event like this could likely occur again at some point in the future.