- For other uses, see Earthquake (disambiguation).
An earthquake is a trembling or a shaking movement of the Earth's surface.
Earthquakes typically result from the movement of faults, quasi-planar
zones of deformation within its uppermost layers. The word earthquake is also widely used to indicate the source region itself.
The solid earth is in slow but constant motion (see plate tectonics)
and earthquakes occur where the resulting stress exceeds the
capacity of Earth materials to support it. This condition is most often found at (and the resulting frequent occurrence of
earthquakes is used to define) the boundaries of the tectonic plates
into which the Earth's lithosphere can be divided. Events that occur at plate
boundaries are called interplate earthquakes; the less
frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes.
Characteristics
Earthquakes occur every day on Earth, but the majority of them are minor and cause no damage. 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.
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 a smaller one, large
faults 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.
Intensity
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.
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.
Causes
Most earthquakes are powered by the release of the stresses that accumulate over time, typically, at the boundaries of the
plates that make up the Earth's lithosphere. The most severe of these
earthquakes are located along compressional and translational plate boundaries. Deep focus earthquakes are possibly generated as subducted lithospheric material catastrophically
undergoes a phase transition at depths greater than 600 km. Some
earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been
associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can
be modified by fluid pressure. Finally, earthquakes (in a broad sense) can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons
tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as
these, that are caused by human activity, are referred to by the term induced seismicity.
Preparation for earthquakes
Specific fault articles
Specific earthquake articles
- Cascadia Earthquake (1700).
- Lisbon earthquake (1755).
- New Madrid Earthquake (1811).
- San Francisco Earthquake (1906).
- Great Kanto earthquake (1923). On the Japanese
island of Honshu, killing over 140,000 in Tokyo
and environs.
- Great Chilean Earthquake (1960). Biggest
earthquake ever recorded, 9.5 on Moment magnitude scale.
- Good Friday Earthquake (1964) Alaskan
earthquake.
- Sylmar earthquake (1971). Caused great and unexpected
destruction of freeway bridges and flyways in the San Fernando
Valley, leading to the first major seismic retrofits of these
types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
- Tangshan earthquake (1976). The most destructive
earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number
died.
- Loma Prieta earthquake (1989). Severely affecting
Santa Cruz, San Francisco and Oakland in California. Revealed necessity of accelerated seismic retrofit of road and bridge
structures.
- Northridge, California earthquake (1994).
Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
- Great Hanshin earthquake (1995). Killed over
6,400 people in and around Kobe, Japan.
- Chi-Chi earthquake (1999).
- Marmara earthquake
(1990). Killed over 25,000 in northwestern Turkey.
- Gujarat Earthquake (2001).
- Dudley Earthquake (2002).
- Parkfield, California earthquake (2004). Not large
(6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting
future earthquakes elsewhere on similar slip-strike fault structures.
- Chuetsu Earthquake (2004).
- Indian Ocean Earthquake (2004). One of
the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
See also List of earthquakes
Related articles
External links
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