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An earthquake (also known as a 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 of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, 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 this.







Naturally occurring earthquakes









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 strainseismic 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.




Earthquake fault types




There are three
main types of fault that may cause an earthquake: normal, reverse
(thrust) and strike-slip. Normal and reverse faulting are examples of
dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened
such as at a convergent boundary. Strike-slip faults are steep
structures where the two sides of the fault slip horizontally past each
other ; transform boundaries are a particular type of strike-slip fault.
Many earthquakes are caused by movement on faults that have components
of both dip-slip and strike-slip; this is known as oblique slip.




Earthquakes away from plate boundaries




Where plate
boundaries occur within continental lithosphere, deformation is spread
out a over a much larger area than the plate boundary itself. In the
case of the San Andreas fault
continental transform, many earthquakes occur away from the plate
boundary and are related to strains developed within the broader zone of
deformation caused by major irregularities in the fault trace (e.g. the
“Big bend” region). The Northridge earthquake
was associated with movement on a blind thrust within such a zone.
Another example is the strongly oblique convergent plate boundary
between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros
mountains. The deformation associated with this plate boundary is
partitioned into nearly pure thrust sense movements perpendicular to the
boundary over a wide zone to the southwest and nearly pure strike-slip
motion along the Main Recent Fault close to the actual plate boundary
itself. This is demonstrated by earthquake focal mechanisms.
All tectonic plates have internal
stress fields caused by their interactions with neighbouring plates and
sedimentary loading or unloading (e.g. deglaciation). These stresses may
be sufficient to cause failure along existing fault planes, giving rise
to intraplate earthquakes.




Shallow-focus and deep-focus earthquakes




The majority of
tectonic earthquakes originate at the ring of fire in depths not
exceeding tens of kilometers. Earthquakes occurring at a depth of less
than 70 km are classified as 'shallow-focus' earthquakes, while those
with a focal-depth between 70 and 300 km are commonly termed 'mid-focus'
or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth at which the subducted lithosphere
should no longer be brittle, due to the high temperature and pressure. A
possible mechanism for the generation of deep-focus earthquakes is
faulting caused by olivine undergoing a phase transition into a spinel structure.
Earthquakes and volcanic activity




Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, like during the Mount St. Helenseruption of 1980.




Earthquake clusters




Most earthquakes form part of a sequence, related to each other in terms of location and time.




Aftershocks




An aftershock is an
earthquake that occurs after a previous earthquake, the mainshock. An
aftershock is in the same region of the main shock but always of a
smaller magnitude. If an aftershock is larger than the main shock, the
aftershock is redesignated as the main shock and the original main shock
is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.





Earthquake swarms





February 2008 earthquake swarm near Mexicali










Earthquake swarms
are sequences of earthquakes striking in a specific area within a short
period of time. They are different from earthquakes followed by a series
of aftershocks
by the fact that no single earthquake in the sequence is obviously the
main shock, therefore none have notable higher magnitudes than the
other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.





Earthquake storms




Sometimes a series of earthquakes occur in a sort of earthquake storm,
where the earthquakes strike a fault in clusters, each triggered by the
shaking or stress redistribution of the previous earthquakes. Similar
to aftershocks
but on adjacent segments of fault, these storms occur over the course
of years, and with some of the later earthquakes as damaging as the
early ones. Such a pattern was observed in the sequence of about a dozen
earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East.





Size and frequency of occurrence




Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia. Larger earthquakes occur less frequently, the relationship being exponential;
for example, roughly ten times as many earthquakes larger than
magnitude 4 occur in a particular time period than earthquakes larger
than magnitude 5. In the (low seismicity) United Kingdom, for example,
it has been calculated that the average recurrences are: an earthquake
of 3.7 - 4.6 every year, an earthquake of 4.7 - 5.5 every 10 years, and
an earthquake of 5.6 or larger every 100 years. This is an example of the Gutenberg-Richter law.
The number of seismic stations has
increased from about 350 in 1931 to many thousands today. As a result,
many more earthquakes are reported than in the past, but this is because
of the vast improvement in instrumentation, rather than an increase in
the number of earthquakes. The USGS
estimates that, since 1900, there have been an average of 18 major
earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0
or greater) per year, and that this average has been relatively stable. In recent years, the number of major earthquakes per year has decreased, although this is thought likely to be a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%,
and 81% of the largest) take place in the 40,000-km-long,
horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains. Humans can cause earthquakes for example by constructing large dams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling.
With the rapid growth of mega-cities such as Mexico City, Tokyo or Tehran,
in areas of high seismic risk, some seismologists are warning that a
single quake may claim the lives of up to 3 million people.





Effects/impacts of earthquakes





1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake. A tsunami overwhelms the ships in the harbor.







There are many effects of earthquakes including, but not limited to the following:




Shaking and ground rupture




Shaking and ground
rupture are the main effects created by earthquakes, principally
resulting in more or less severe damage to buildings or other rigid
structures. The severity of the local effects depends on the complex
combination of the earthquake magnitude, the distance from epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.
Specific local geological,
geomorphological, and geostructural features can induce high levels of
shaking on the ground surface even from low-intensity earthquakes. This
effect is called site or local amplification. It is principally due to
the transfer of the seismic
motion from hard deep soils to soft superficial soils and to effects of
seismic energy focalization owing to typical geometrical setting of the
deposits.
Ground rupture is a visible breaking
and displacement of the earth's surface along the trace of the fault,
which may be of the order of several metres in the case of major
earthquakes. Ground rupture is a major risk for large engineering
structures such as dams, bridges and nuclear power stations
and requires careful mapping of existing faults to identify any likely
to break the ground surface within the life of the structure.





Landslides and avalanches




Landslides are a
major geologic hazard because they can happen at any place in the world,
much like earthquakes. Severe storms, earthquakes, volcanic activity,
coastal wave attack, and wildfires can all produce slope instability.
Landslide danger may be possible even though emergency personnel are
attempting rescue.





Fires





Fires of the 1906 San Francisco earthquake







Following an earthquake, fires can be generated by break of the electrical power
or gas lines. In the event of water mains rupturing and a loss of
pressure, it may also become difficult to stop the spread of a fire once
it has started. For example, the deaths in the 1906 San Francisco earthquake were caused more by the fires than by the earthquake itself.




Soil liquefaction




Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid.
Soil liquefaction may cause rigid structures, as buildings or bridges,
to tilt or sink into the liquefied deposits. This can be a devastating
effect of earthquakes. For example, in the 1964 Alaska earthquake, many buildings were sunk into the ground by soil liquefaction, eventually collapsing upon themselves.





Tsunami





The tsunami of the 2004 Indian Ocean earthquake










Tsunamis are
long-wavelength, long-period sea waves produced by an sudden or abrupt
movement of large volumes of water. In the open ocean, the distance
between wave crests can surpass 100 kilometers, and the wave periods can
vary from five minutes to one hour. Such tsunamis travel 600-800
kilometers per hour, depending on water depth. Large waves produced by
an earthquake or a submarine landslide can overrun nearby coastal areas
in a matter of minutes. Tsunamis can also travel thousands of kilometers
across open ocean and wreak destruction on far shores hours after the
earthquake that generated them.

Ordinarily, subduction earthquakes
under magnitude 7.5 on the richter scale do not cause tsunamis. However,
there have been recorded instances, yet most destructive tsunamis are
caused by magnitude 7.5 plus earthquakes.
Tsunamis are distinct from tidal
waves, because in a tsunami, water flows straight instead of in a circle
like the typical wave. Earthquake-triggered landslides into the sea can
also cause tsunamis.




Floods




A flood is an overflow of any amount of water that reaches land
Floods usually occur because of the volume of water within a body of
water, such as a river or lake, exceeds the total capacity of the
formation, and as a result some of the water flows or sits outside of
the normal perimeter of the body. However, floods may be secondary
effects of earthquakes, if dams are damaged. Earthquakes may cause
landslips to dam rivers, which then collapse and cause floods.
The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.




Human impacts




Earthquakes may result in disease,
lack of basic necessities, loss of life, higher insurance premiums,
general property damage, road and bridge damage, and collapse of
buildings or destabilization of the base of buildings which may lead to
collapse in future earthquakes. Earthquakes can also lead to volcanic
eruptions, which cause further damages such as substantial crop damage,
like in the "Year Without a Summer" (1816).
Most of civilization agrees that human death is the most significant human impact of earthquakes.




Preparation for earthquakes




Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through the following processes: Earthquake engineering, Earthquake preparedness, Household seismic safety, Seismic retrofit (including special fasteners, materials, and techniques), Seismic hazard, Mitigation of seismic motion, and Earthquake prediction.




Earthquakes in culture







Mythology and religion




In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr,
god of beauty and light, he was punished by being bound in a cave with a
poisonous serpent placed above his head dripping venom. Loki's wife Sigyn
stood by him with a bowl to catch the poison, but whenever she had to
empty the bowl the poison would drip on Loki's face, forcing him to jerk
his head away and thrash against his bonds, causing the earth to
tremble.
In Greek mythology, Poseidon was the god of and cause earthquakes. When he was in a bad mood, he would strike the ground with a trident, causing this and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.




Popular culture




In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.[34] Fictional earthquakes tend to strike suddenly and without warning.[34] For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998). A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection, After the Quake, depicts the consequences of the Kobe earthquake of 1995.
The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works.[34]
Jacob M. Appel's widely-anthologized short story, A Comparative
Seismology, features a con artist who convinces an elderly woman that an
apocalyptic earthquake is imminent. In Pleasure Boating in Lituya Bay, one of the stories in Jim Shepard's Like You'd Understand, Anyway, the "Big One" leads to an even more devastating tsunami.