Earthquakes occur every day on Earth, but the majority of them are minor and cause no damage(those less than 5 on ritcher scale). Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage.

Damage from the 1906 San Francisco earthquake

Section of collapsed freeway after the 1989 Loma Prieta earthquake.
Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. It is usually possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" and usually proves to be the point at which the fault slip was initiated. The position of the focus is known as the "hypocenter" and the location on the surface directly above it is the "epicenter". The fault may slip well beyond its epicenter, though. Just as a large loudspeaker can produce a greater volume of sound than are capable of higher magnitude earthquakes than smaller faults are.

Earthquakes, especially those that occur beneath oceans or seas, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.

There are four types of seismic waves all that are generated simultaneously. They arrive in the following order: first the body waves P-waves (primary or pressure waves) then S-waves (secondary or shear waves), next the surface waves (Love waves) then Rayleigh waves.

A class of earthquakes known as silent earthquakes are thought to be caused by very slow slippage. They are of extremely low intensity but can last for days or weeks releasing as much energy as large earthquakes.

In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes, which has come to be called the Richter scale. Since Richter, seismologists have developed a number of magnitude scales. Most of the scales in use in the Western world (such as the moment magnitude scale) are mutually consistent to a sufficient extent that the term "Richter scale" is routinely used in reporting these numbers to the public. Other scales (and other ways of describing the size of earthquakes) are used in some non-Western countries, and by earthquake specialists. For example, the Japanese shindo scale for measuring the force of earthquakes measures horizontal movement. The press sometimes mistakenly reports such values as "Richter magnitude", and this has given rise to public confusion.

A Shakemap recorded by the Pacific Northwest Seismograph Network that shows the instrument recorded intensity of the shaking of the Nisqually earthquake on February 28, 2001.
A Community Internet Intensity Map generated by the USGS that shows the intensity felt by humans by ZIP Code of the shaking of the Nisqually earthquake on February 28, 2001.Earthquake effects are described in terms of intensity, a scale which attempts to quantify the severity of shaking at a given location. A number of intensity scales are in use, and there is a significant degree of consistency amongst them. The best known is the Mercalli (or Modified Mercalli, MM) scale, but the more consistent and analytical European Macroseismic Scale (EMS) is now increasingly widely used. In Japan the Japan Meterological Agency seismic intensity scale (JMA) is used.

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