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What is an Earthquake?

Techtonic MapAn earthquake is the sudden, sometimes violent, ground shaking from the release of energy in the earth's crust. Earthquakes typically occur with little or no warning.

The risks that earthquakes pose to society, including death, injury, and economic loss, can be greatly reduced by better planning, construction, and mitigation practices before earthquakes happen, and providing critical and timely information to improve response after they occur.

Although earthquakes can occur anywhere on earth, the majority of earthquakes worldwide occur at plate boundaries. These earthquakes are known as interplate earthquakes.

Alabama is located within the interior of the North American plate, far from any plate boundary. Earthquakes occuring within a plate are intraplate earthquakes. Little is known as to why intraplate earthquakes occur. The most widely accepted model is that several geologically old fault systems of varying orientation within the subsurface are being reactivated while being subjected to stress, possibly due to the plate tectonics. For hundreds of millions of years, the forces of continental drift have reshaped the earth. Continental drift is based on the concept that the continents bumped into, and slid over and under each other and at some later time broke apart. page

Different geologic materials (soils, sediments, and rocks) react differently to seismic waves Liquefactiongenerated by earthquakes. When shaking intensities are very strong, soft sediments can actually undergo liquefaction, a phenomenon that occurs when wet sediments amplify seismic waves, causing the sediment to shake intensely. Liquefaction can occur in areas underlain by unconsolidated material or in water-saturated soils such as those that are found on river floodplains where recent (Quaternary) soft sediments are saturated by the high water table. The geologic figure (right) shows some of these areas in yellow.