What is an earthquake? An earthquake is a shaking of the ground cause by the breaking and shifting of section of the outer shell of the earth. The energy released by an earthquake may be equal to 10,000 times to amount of an atomic bomb. These natural disasters can cause rivers to change their course, cause landslides, and tsunamis. Although this seems frightning, earthquakes cannot be held responsible for the direct loss of human lives. Instead, they make our buildings crumble over us. They can also create fires through the destruction of power lines. However, not all earthquakes cause noticable damage. In fact, there are about 8,000 earthquakes per day, less than 1,500 of which can even be felt. Of those than can be felt, most are so mild that it feels no different than a car passing you by. There is usually only about one powerful earthquake every two years.

How does an earthquake start? Most earthquakes start at a fault. What is a fault? A fault is an area in the earth's shell where areas of the rock continue to slide past each other. This causes the pieces of rock in the shell to strain and eventually break. When the rock breaks, the pieces move into different positions. This is what causes an earthquake to start. The area where the earthquake begins is called the focus. This point can lie between forty-five miles beneath the surface or up to 450 miles beneath. The point above this one the earth's surface is referred to as the epicenter.

How does an earthquake spread? When the rock beneath the earth's surface breaks, energy is released in the form of seismic waves. These waves move in all directions, growing weaker the farther they travel, thus allowing the quake to spread.

What kind of damage is done by earthquakes? Earthquakes cause damage to buildings, bridges water supply line, sewage lines, and other structures, including natural structures. Earthquakes, because of their impact on water lines, can actually cause widespread disease outbreaks. Earthquakes that occur under the ocean cause tsunamis.

What causes an earthquake? Most earthquakes are caused by the sudden slipping of the geologic faults beneath the earth's surface. This occurs because of the moveement of the techtonic plates. These plates move very slowly along a weak rocky layer. When the plates collide with each other, pressure builds up on the crust. It takes hundreds of years for this pressure to build up enough to cause an earthquake. This is called the elastic rebound theory. This theory came about in 1911 when American seismologist Harry Fielding Reid studied the effects of an earthquake in California in 1906. According to Encarta, he proposed the elastic rebound theory to explain the generation of certain earthquakes that scientists know occur in techtonic areas, usually near plate boundaries. This theory states that during an earthquaks, the rocks under strain suddenly break, creating a fracture along a fault. When these faults slip, the movement causes vibrations that changes the local strain to surrounding rocks. It is this that causes aftershocks.

How are earthquakes measured? Earthquakes are measured on a magnitude scale. Take a look at the chart below for information reguarding the different levels, the number of occurances for each level, and the probable damage.

Earthquake Magnitude Scale