Most of the earthquakes in the world happen at the contacts between great tectonic plates that cover the Earth. These plates have an approximate thickness of 100 km. They can be considered as if they were floating on the deepest zone of the Earth called mantle. They float on a deeper zone of the earth called the mantle, which deform plastically while the plates move more rigidly on top. The bumping and grinding that occurs between rigid tectonic plates as they re-arrange themselves, causes most of the world's earthquakes.
There are different types of contacts between the plates:
1. In some places, such as the west coast of South and Middle America, the plates hit . Ones plate dives, or past each other, in a region called the subduction zone. The greatest earthquakes in the world occur in these zones; they have reached magnitudes up to 9.5.
2. Another type of contact is a "transform" or "strike -slip" fault, such as the San Andres fault in California, in wich the two plates slide past each other. Earthquakes in these zones can also be intense, with magnitudes up to 8.0 historically.
3. In other areas, as in the Himalayas, two plates hit and form great mountain chains. Earthquakes in these zones can also be intense.
4. In zones such as the Mid Atlantic Ridge, new oceanic crush is created as melten rock flows to the surface psolidifies, fusing ontothe sea floor. The plates pull away from the ridge of each other in opposite directions. Earthquakes that occur here are neither deep nor intense
Puerto Rico is located in the limit between the plates of North America and the Caribbean. There is evidence of oblique subduction and lateral displacement between the two plates. The seismic activity is concentrated in eight zones:
1. Puerto Rico Trench
2. Slope faults in the North and South of Puerto Rico
3. Northeast of "Zona del Sombrero"
4. To the west, at the Mona Canyon
5. Mona Passage
6. To the east, in the depressions of Virgin Islands and Anegada
7. Muertos Depression to the South
8. Southeast of Puerto Rico.
The maximum measured magnitude of an earthquake in the vicinity has been of 7.5.
