earthquake: Causes of Earthquakes

Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface (see lithosphere ). The immediate cause of most shallow earthquakes is the sudden release of stress along a fault , or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. An earthquake on one fault can trigger the sudden release of stress on a nearby fault and even cause a complex earthquake that propagates through several neighboring faults under the appropriate geological conditions. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Earthquakes have also been associated with hydraulic fracturing, or fracking, a drilling technique that injects slurry under pressure into a well, and with the disposal of fracking wastewater in old wells, most notably in N and central Oklahoma, where injection of wastewater in disposal wells has led to a significant increase in the number and strength of earthquakes since 2009. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.

See also plate tectonics .

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