Earthquakes are caused by the abrupt release of stored energy within the earth’s crust. This release of energy is much like that which is released when a stretched rubber band breaks.

The Eastern Caribbean is an example of an island arc system formed at a convergent plate boundary (more specifically, at a subduction zone, where two tectonic plates meet and the denser plate is forced beneath the lighter plate). This is the main cause of the volcanic and seismic activity in the Eastern Caribbean.

1. Most of the earthquakes occurring in the Eastern Caribbean are either tectonic or volcanic in origin.

2. Tectonic earthquakes are generated when plates move as accumulated energy is released.

3. Volcanic earthquakes are generated by the movement of magma within the lithosphere. Since magma is less dense than the surrounding rock, it rises to the surface, breaking the rock as it moves, thereby generating earthquakes.

4. More than 75% of the world's earthquakes occur at convergent plate boundaries. The countries of the Eastern Caribbean are, therefore, highly susceptible to earthquakes

The motion of the ground during earthquakes is recorded by instruments known as seismographs. The ground motion that people notice comes from a release of energy that radiates outward in all directions as seismic waves, which travel through the earth.

There are two basic types of seismic waves- body waves and surface waves. Generally, the first jolt felt during an earthquake is the push-pull body wave, or P wave, as it reaches the surface. A second jolt is another type of body wave, called an S wave.

The fastest wave, and therefore the first to arrive at a given location.Also known as compressional waves, the P wave alternately compresses and expands material in the same direction it is traveling.Can travel through all layers of the Earth.Generally felt by humans as a bang or thump.

The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.Also know as shear waves

Surface waves follow the P and S waves.Also known as Rayleigh and Love waves.These waves travel along the surface of the earth.

The “size” of earthquakes is commonly expressed in two ways- magnitude and intensity.

Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismogram, which plots the ground motion produced by seismic waves.

Magnitude is defined as the logarithm the important thing to remember about magnitude is that the scale is logarithmic, which means that each step in magnitude represents a tenfold increase in amplitude of wave motion. Therefore, an earthquake of magnitude 6.0 has ten times the wave amplitude of an earthquake of magnitude 5.0.

Because magnitude does not describe the extent of the damage, its usefulness is limited to an approximation of whether the earthquake is large, small, or medium-sized.

The Modified Mercalli (MM) Intensity Scale. Developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.

Earthquake Hazards:

The Effect of Ground ShakingThe first main earthquake hazard (danger) is the effect of ground shaking. Buildings can be damaged by the shaking itself or by the ground beneath them settling to a different level than it was before the earthquake (subsidence).

Liquefaction- Liquefaction is the mixing of sand or soil and groundwater (water underground) during the shaking of a moderate or strong earthquake. When the water and soil are mixed, the ground becomes very soft and acts similar to quicksand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet.

Surface Waves - Buildings can also be damaged by strong surface waves making the ground heave and lurch. Any buildings in the path of these surface waves can lean or tip over from all the movement. The ground shaking may also cause landslides, mudslides, and avalanches on steeper hills or mountains, all of which can damage buildings and hurt people.

Ground DisplacementThe second main earthquake hazard is ground displacement (ground movement) along a fault. If a structure (a building, road, etc.) is built across a fault, the ground displacement during an earthquake could seriously damage or rip apart that structure.

FloodingThe third main hazard is flooding. An earthquake can rupture (break) dams or levees along a river. The water from the river or the reservoir would then flood the area, damaging buildings and maybe sweeping away or drowning people.

Tsunamis and seiches can also cause a great deal of damage. A tsunami is what most people call a tidal wave, but it has nothing to do with the tides on the ocean. It is a huge wave caused by an earthquake under the ocean. Tsunamis can be tens of feet high when they hit the shore and can do enormous damage to the coastline. Seiches are like small tsunamis. They occur on lakes that are shaken by the earthquake and are usually only a few feet high, but they can still flood or knock down houses, and tip over trees.

FireThe fourth main earthquake hazard is fire. These fires can be started by broken gas lines and power lines, or tipped over wood or coal stoves. They can be a serious problem, especially if the water lines that feed the fire hydrants are broken, too. For example, after the Great San Francisco Earthquake in 1906, the city burned for three days. Most of the city was destroyed and 250,000 people were left homeless

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