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ALAN M. PERIOD 4 3d. Students know why and how earthquakes occur and the scales used to measure their intensity and magnitude.

media type="custom" key="5469295" width="140" height="140" 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]. 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.