earthquakes

Julie D. per.4



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An **earthquake** (also known as a **quake**, **tremor**, **temblor** or **seismic activity**) is the result of a sudden release of energy in the [|Earth's] [|crust] that creates [|seismic waves]. Earthquakes are recorded with a [|seismometer], also known as a seismograph. The [|moment magnitude] (or the related and mostly obsolete [|Richter] magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly [|imperceptible] and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified [|Mercalli scale]. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake [|epicenter] is located offshore, the seabed sometimes suffers sufficient displacement to cause a [|tsunami]. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. In its most generic sense, the word //earthquake// is used to describe any seismic event — whether a natural [|phenomenon] or an event caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological [|faults], but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its [|focus] or [|hypocenter]. The term [|epicenter] refers to the point at ground level directly above the hypocenter. Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a [|fault plane]. In the case of [|transform] or [|convergent] type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and [|aseismically] only if there are no irregularities or [|asperities] along the boundary that increase the frictional resistance. Most boundaries do have such asperities and this leads to a form of [|stick-slip behaviour]. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the [|stored energy]. This energy is released as a combination of radiated elastic [|strain] [|seismic waves], frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the [|Elastic-rebound theory]. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake [|fracture] growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available [|elastic potential energy] and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.[|[1]] __ [] __