The United States Geological Survey recorded a Magnitude 4.8 Earthquake at a depth of 42 km roughly 50 km north of Puerto RIco, slightly before 4.15 pm local time (slightly before 8.15 pm GMT) on Friday 18 August 2017. This was a moderate quake, and at some depth as well as some way offshore, and there are no reports of any casualties or serious damage, though the quake was felt across Peurto Rico.
The approximate location of the 18 August 2017 Puerto Rico Earthquake. USGS.
Puerto Rico is located at the northeastern fringe of the Caribbean Tectonic Plate. The Atlantic Plate (strictly speaking, an extension of the South American Plate which runs to the northeast of the Caribbean) is being subducted beneath this. The subduction of the Atlantic Plate beneath the Caribbean Plate is not a smooth process, with the two plates constantly sticking together then breaking apart as the tectonic pressure builds up, causing Earthquakes in the process, though since the boundary between the two plates is some way to the north and east of the islands, Earthquakes in this area tend to be both deep and offshore, which lessens their destructive potential.
The subduction of the Atlantic Plate beneath the Caribbean Plate fuels the volcanoes of the Lesser Antilles Volcanic Arc. George Pararas-Carayannis.
Witness accounts of quakes can help geologists to understand these events and the rock structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) you can report it to the USGS here.
Follow Sciency Thoughts on Facebook.