Geology And Geophysics

Earthquakes Tectonic Volcanic Seismic Waves



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Did you know that there are almost one thousand earthquakes occurring in only one day? The movements, which are often the result of the natural shaking of the Earth's surface, can go undetected. However, eruptions can also be so violent that they bring death and destruction to any area of the world. They have been occurring since prehistoric times and continue to wreak havoc when they occur.

A major earthquake can bring about many different changes in the surface of the Earth. One that is only modest in intensity can create fissures, cracks and slumps in the ground and set off avalanches and landslides. Some earthquakes have changed the course of rivers and fires, which often result, make the damage from the shocks much worse. Tsumanis can also result when the source of the earthquake occurs under the water.

Quite often there are warning signs of earthquakes, such as foreshocks. However, a major earthquake does have many aftershocks, which decrease in intensity, but can last for days or weeks.

The Causes of Earthquakes

There are two main causes of earthquakes - tectonic and volcanic. The majority of earthquakes are the result of a sudden movement of the earth's tectonic plates along an existing fault or fracture. This occurs when the rocks are no longer able to withstand the forces and tension that has built up within them and around them. This causes them to suddenly snap and move position and rebound in order to relieve this pressure. When the rocks snap back in a horizontal position, this causes a strike-slip fault. When they snap back in a vertical position, it causes a dip-slip fault resulting in an uplift on one side and a sink on the other.

Earthquakes that are volcanic in nature are the result of sudden movements of lava or hot gases deep within the earth's core. The heat causes the rock to fracture, resulting in an earthquake. These kinds of earthquakes are generally local in nature and only affect the surrounding area.

Shock Waves

Many kinds of shock waves occur on the surface when an earthquake occurs. These shocks are called seismic waves and there are three main types. Primary seismic waves travel very quickly and pass through the rock causing a vibration. Secondary seismic waves are slower and because they pass through the rock at a slower pace, the rock vibrates at right angles. They do not however, travel through liquids. Long seismic waves are the slowest of all waves. They travel along the surface of the earth producing slow undulating shock waves. They are much larger than the others and are the most important shock waves associated with an earthquake.

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