Earthquakes

Wikipedia defines an earthquake as a trembling or shaking movement of the Earth's surface.  Earthquakes usually are caused by movement of faults. The earth is in slow and constant motion and earthquakes happen when the stress is larger than what the Earth materials can handle. This happens most often where the tectonic plates are divided into Earth's lithosphere. Events that occur at plate boundaries are called interplate earthquakes. Earthquakes that occur in the interior of the lithospheric plates don't occur as often and are called intraplate earthquakes.

Some earthquakes are so slight, and some occur in such remote areas, that they are barely noticed by people. Others are so violent that they cause massive destruction. The intensity of an earthquake becomes weaker outward from the epicenter, but different types of ground absorb the earthquake waves differently. Buildings on filled ground are damaged more than buildings built on solid rock, even when they are the same distance from the epicenter.

The strength of the earthquake is called the magnitude, which is a single number. The magnitude is the total energy released by an earthquake at its focus. Earthquakes of large magnitude are stronger and more destructive than those of smaller ones. Large earthquakes are followed by many aftershocks, which may persist for days or weeks, but the first shock is usually the most damaging.

To understand magnitude and epicenters, visit the Virtual Earthquake site.

Besides producing floods and destroying buildings, earthquakes that take place under the ocean can sometimes cause tsunamis, or tidal waves. These waves are high and long walls of water which can travel at a very rapid rate. They are notorious for destroying entire populations and cities near coastlines.

Earthquakes are measured by either the Mercalli or Richter Scales so we can compare the strengths.  They occur where near any of the tectonic plates are found in many different places throughout the world, including in the United States.  The Great San Francisco Earthquake and the 1959 earthquake in Hebgen, Montana are two of the worst felt in our country.

Picture courtesy of BBC News

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