October 11, 1918 Earthquake: Mona Passage, Puerto Rico, US Territories

A magnitude 7.1 Mw (Ref #10476) earthquake, at 10:14 Local Time in the Mona Passage, west of Puerto Rico, was caused by displacement along four segments of a normal fault, oriented N-S in the Mona Canyon (Ref #267). Ref #1914 and 9910 list 7.3 Mw (MMI VIII). The map in Ref #1078 lists a MMI intensity IX at Punta Borinquen, closest dry land to the epicenter. Immediately following the shock a tsunami broke upon the shore, drowning many persons and destroying many native dwellings.Earthquake Damage (from Reference #1128):

Most extensive property damage reports were received from Aguada and Anasco, both located on alluvium in western Puerto Rico. In Aguada, masonry buildings were largely demolished. Some walls, badly cracked by the main shock were thrown down by the aftershock of October 24. The church, built prior to 1867, was entirely destroyed. Some concrete walls and foundations without reinforcement were wrecked. A one-story reinforced concrete school was practically undamaged, and frame buildings were not damaged appreciably.

At Anasco, all brick buildings were destroyed or condemned. Several concrete structures with very little reinforcmenet were generally wrecked during aftershocks, but those which were well reinforced sustained little damage. Wood frame buildings were not damaged except where timbers had rotted.

At Aguadilla, most of the buildings on alluvium were badly damaged or destroyed. As a rule, masonry structures were badly cracked.

At Mayaguez, south of Anasco, the brick Catholic Church had to be razed, but the brick Presbyterian Church was only slightly damaged. Several persons were killed in the collapse of a two-story, poorly built, concrete cigar factory.

Many bridges were damaged; railroad tracks were bent and displaced; pipelines and flumes were wrecked; and tall brick chimneys were thrown down. Two cable links were broken in Mona Pasage.

Tsunami Damage (from Reference #1182):

Before the tsunami arrived, the ocean withdrew and exposed reefs and stretches of seafloor never visible during low tide. A tsunami with runup heights reaching 6 m followed the earthquake, causing extensive damage along the western and northern coasts of Puerto Rico, especially to those villages established in a flood plain.

Deaths and Total Damage:

The official death toll attributed to both earthquake and tsunami was 116 persons according to Reference #1078. Reference #1492 said, "Since more than 100 perished at Mayaguez, plus 4 at Ponce, plus 40 tsunami deaths at northwestern Puerto Rico, the tally would have been more than 116. The greatest number of fatalities was said to be due to the tsunami." Reference #1591 reported 91 killed by the tsunami in northwestern Puerto Rico.

References #1128 and #1492 list $4 million damage in Puerto Rico; Reference #1250 lists $29 million damage.

In Mayaguez most stone buildings either fell or were very badly damaged. Many streets were made impassable with debris. The Science building of the Agricultural College was destroyed by fire. More than one hundred deaths occurred in Mayaguez. In Ponce many buildings were seriously damaged, including

the theatre and the Cathedral. Four were killed. Most of the towns of the island reported some damage to buildings. A tidal wave following the earthquake caused thirty deaths in Aguadilla. Damage to property in Puerto Rico exceeded $4,000,000 (ref #364)

View a calculated

tsunami-travel-time map.