P2+-+Earthquakes

An **earthquake** (also known as a **quake**, **tremor** or **temblor**) is the result of a sudden release of energy in the [|Earth's] [|crust] that creates [|seismic waves]. The **seismicity** or **seismic activity** of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured with a [|seismometer]; a device which also records is 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]. 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.