What is an earthquake?
Is the ground shaking caused by a sudden movement of rocks in the Earth’s crust. Such movements occur along faults, which are thin zones of crushed rock separating blocks of crust. When one block suddenly slips and moves relative to the other along a fault, the energy released creates vibrations called seismic waves that radiate up through the crust to the earth’s surface, causing the ground to shake.
How do they occur?
Most earthquakes occur mainly due to the movement of tectonic plates, but some may occur due to volcanic eruption. As it is known, the crust or lithosphere is broken into plates and sub-plates which are called tectonic plates. These either moves apart, collide toward each other or displaces in lateral motion. In our region, there are two major and several smaller plates. To the North of Puerto Rico is the North American plate, to the South is the South American plate, Cocos Plate to the West and the Caribbean Plate in the center.
There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. These seismic waves are described below.
P waves or longitudinal (Primary wave)
This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the wave is moving in, which is the direction that the energy is traveling in.
S waves (Secondary waves)
Is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock particles up and down, or side-to-side--perpendicular to the direction that the wave is traveling in (the direction of wave propagation).
The first kind of surface wave is called a Love wave. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.
The other kind of surface wave is the Rayleigh wave. This wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.
Boundaries of Tectonic Plates
The tectonic plates have different directions of travel or contact edges:
A tectonic boundary where two plates are moving toward each other. If the two plates are of equal density, they usually push up against each other, forming a mountain chain. If they are of unequal density, one plate usually sinks beneath the other in a subduction zone. Major earthquakes can occur along convergent boundaries.
A tectonic boundary where plates move apart and magma rises from the asthenosphere and erupts along the axis of the ridges, extending the Earth crust. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. Shallow and mild quakes can occur as pressure is equalized in the spreading sea floor, but the act of pulling the crust apart serves to decrease pressure, not increase it.
Transformation plate (lateral displacement):
When two tectonic plates slide laterally next to each other, along a fault. In this case no separations or collisions occur. Both plates move laterally, as in the San Andreas Fault in California. Earthquakes in these zones can be strong, historically up to 8 on the scale of Richter.
The friction between plates can also cause aftershocks, earthquake swarms and sequences.
They are subsequent to a main event or events; in other words are all seismic events that occur in the same region after a main event. The aftershocks may last for days, months or even years, they may be less than or equal magnitude than the main event. For example, in November 2, 2008, an event of magnitude 5.5 (Mw) in the Puerto Rico Trench, generated 369 aftershocks that were recorded.
Swarms are many seismic events that occur in the same area with the same characteristics (similar magnitudes) in a given period of time. In our area, for example twelve swarms were generated in the region of Puerto Rico and the Virgin Islands in 2011. One of these swarms occurred in the Sombrero Seismic Zone and the platform of Virgin Islands, from 14 to 15 in July 2011, where 88 earthquakes were located in 33 hours. For more information please refer to the official report of the Puerto Rico Seismic Network at the following link: http://redsismica.uprm.edu/Spanish/sismos/repanual.php