Earthquake

An earthquake is a trembling or shaking movement of the earth's surface. Earthquakes occur every day, however the vast majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life. Large earthquakes usually have a substantial initial tremor, followed by numerous aftershocks (smaller tremors). The underground point of origin of an earthquake is called its "focus" and the location on the surface directly above the focus is the epicentre.

The magnitude and intensity of an earthquake used to be measured using the Richter scale. Now there are multiple scales used to measure an earthquake's magnitude and intensity. The scale reported to the public is so close to the Richter scale that the seismologists don't bother to explain the difference.

Earthquakes typically result from the movement of continental and oceanic plates, as described by plate tectonics. Pressures are built up along the boundaries of these plates in regions called faults until this pressure is released by a slippage of the plates some distance. The resulting release of energy creates a wave in the surrounding terrain, causing it to shake in an earthquake.

Some earthquakes are caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes are caused by the build-up of large masses of water behind dams, such as the Kariba Dam.

Several types of waves, called seismic waves, are generated during an earthquake: the two strongest are the primary (P) and secondary (S) waves. (P) waves are compressional waves (or longitudinal waves), and (S) waves are tranverse waves (caused by up-and-down motions). The two types of wave differ in travel time, depending on the material they travel through. In general, (P) waves travel twice as fast through a material than (S) waves. (P) waves also travel through any material, but (S) waves travel only through solids. The combination of these factors has allowed scientists to locate earthquake epicenters precisely and to determine in general the structure of the earth's interior. The study of this process is known as seismology.

An interesting technique for looking at and understanding earthquake data is to use the 3D presentation system called the GeoWall. Go to http:/wwww.geowall.org for more information.