An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth, resulting

from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range
in size from those that are so weak that they cannot be felt to those violent enough to toss people around and
destroy whole cities. The seismicity, or seismic activity, of an area is the frequency, type and size of
earthquakes experienced over a period of time. The word tremoris also used for non-earthquake seismic
rumbling.
At the Earth's surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground.
When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause
a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
In its most general sense, the word earthquake is used to describe any seismic event—whether natural or
caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of
geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An
earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level
directly above the hypocenter.

Contents

 1Naturally occurring earthquakes

o 1.1Earthquake fault types

o 1.2Earthquakes away from plate boundaries

o 1.3Shallow-focus and deep-focus earthquakes

o 1.4Earthquakes and volcanic activity

o 1.5Rupture dynamics

o 1.6Tidal forces

o 1.7Earthquake clusters

 1.7.1Aftershocks

 1.7.2Earthquake swarms

 2Intensity of earth quaking and magnitude of earthquakes

 3Frequency of occurrence

 4Induced seismicity

 5Measuring and locating earthquakes

 6Effects of earthquakes

o 6.1Shaking and ground rupture

o 6.2Landslides

o 6.3Fires

o 6.4Soil liquefaction

o 6.5Tsunami
 o 6.6Floods

o 6.7Human impacts

 7Major earthquakes

 8Prediction

 9Forecasting

 10Preparedness

 11Historical views

 12Recent studies

 13In culture

o 13.1Mythology and religion

o 13.2In popular culture

 14See also

 15References

 16Sources

 17External links

Naturally occurring earthquakes

Three types of faults:

A. Strike-slip.
B. Normal.
C. Reverse.

Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive
fracture propagation along a fault plane. The sides of a fault move past each other smoothly
and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional
resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior. Once the
fault 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.
[1]
This energy is released as a combination of radiated elastic strain seismic waves[2], 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.[3]

Earthquake fault types

Main article: Fault (geology)
There are three main types of fault, all of which may cause an interplate 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.
Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful
earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults,
particularly continental transforms, can produce major earthquakes up to about magnitude 8. Earthquakes
associated with normal faults are generally less than magnitude 7. For every unit increase in magnitude, there is
a roughly thirtyfold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases
approximately 30 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake
releases 900 times (30 × 30) more energy than a 5.0 magnitude of earthquake. An 8.6 magnitude earthquake
releases the same amount of energy as 10,000 atomic bombs like those used in World War II.[4]
This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of
the fault that ruptures[5] and the stress drop. Therefore, the longer the length and the wider the width of the
faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth's crust, and the cool slabs
of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can
store elastic energy and release it in fault ruptures. Rocks hotter than about 300 °C (572 °F) flow in response to
stress; they do not rupture in earthquakes.[6][7] The maximum observed lengths of ruptures and mapped faults
(which may break in a single rupture) are approximately 1,000 km (620 mi). Examples are the earthquakes
in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strike-
slip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali
Fault in Alaska (2002), are about half to one third as long as the lengths along subducting plate margins, and
those along normal faults are even shorter.

Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles
The most important parameter controlling the maximum earthquake magnitude on a fault is however not the
maximum available length, but the available width because the latter varies by a factor of 20. Along converging
plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees. [8] Thus the width of
the plane within the top brittle crust of the Earth can become 50–100 km (31–62 mi) (Japan, 2011; Alaska,
1964), making the most powerful earthquakes possible.
Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km (6.2 mi) within the
brittle crust,[9] thus earthquakes with magnitudes much larger than 8 are not possible. Maximum magnitudes
along many normal faults are even more limited because many of them are located along spreading centers, as
in Iceland, where the thickness of the brittle layer is only about six kilometres (3.7 mi).[10][11]
In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the
highest, strike slip by intermediate, and normal faults by the lowest stress levels. [12] This can easily be understood
by considering the direction of the greatest principal stress, the direction of the force that 'pushes' the rock mass
during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the
pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the
rock mass 'escapes' in the direction of the least principal stress, namely upward, lifting the rock mass up, thus
the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types
described above. This difference in stress regime in the three faulting environments can contribute to differences
in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault
dimensions.

Earthquakes away from plate boundaries

Main article: Intraplate earthquake

Comparison of the 1985 and 2017earthquakes on Mexico City, Puebla and Michoacán/Guerrero

Where plate boundaries occur within the continental lithosphere, deformation is spread out 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.[13]
All tectonic plates have internal stress fields caused by their interactions with neighboring plates and
sedimentary loading or unloading (e.g. deglaciation). [14] These stresses may be sufficient to cause failure along
existing fault planes, giving rise to intraplate earthquakes.[15]

Shallow-focus and deep-focus earthquakes

Main article: Depth of focus (tectonics)
Collapsed Gran Hotel building in the San Salvador metropolis, after the shallow 1986 San Salvador earthquake.

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 (43 mi) are classified as 'shallow-focus' earthquakes, while
those with a focal-depth between 70 and 300 km (43 and 186 mi) are commonly termed 'mid-focus' or
'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crustdescends beneath
another tectonic plate, Deep-focus earthquakes may occur at much greater depths (ranging from 300 to 700 km
(190 to 430 mi)).[16] These seismically active areas of subduction are known as Wadati–Benioff zones. Deep-
focus earthquakes occur at a depth where 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.[17]

Earthquakes and volcanic activity

Main article: Volcano tectonic earthquake
Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement
of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during
the 1980 eruption of Mount St. Helens.[18] Earthquake swarms can serve as markers for the location of the
flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a
device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions. [19]

Rupture dynamics
A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation.
The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the
smallest earthquakes, suggesting that it is smaller than 100 m (330 ft) while other evidence, such as a slow
component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possibility that
the nucleation involves some sort of preparation process is supported by the observation that about 40% of
earthquakes are preceded by foreshocks. Once the rupture has initiated, it begins to propagate along the fault
surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high
sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record
information close to a nucleation zone.[20]
Rupture propagation is generally modeled using a fracture mechanics approach, likening the rupture to a
propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy in the volume
around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of
magnitude faster than the displacement velocity across the fault. Earthquake ruptures typically propagate at
velocities that are in the range 70–90% of the S-wave velocity, and this is independent of earthquake size. A
small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity.
These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone
of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic
boomdeveloped in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are
referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake,
observed where the relatively low felt intensities, caused by the slow propagation speed of some great
earthquakes, fail to alert the population of the neighboring coast, as in the 1896 Sanriku earthquake.[20]

Tidal forces
Tides may induce some seismicity, see tidal triggering of earthquakes for details.

Earthquake clusters
Most earthquakes form part of a sequence, related to each other in terms of location and time. [21] Most
earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that
earthquakes can recur in a regular pattern.[22]
Aftershocks
Main article: Aftershock

Magnitude of the Central Italy earthquakes of August and October 2016, of January 2017 and the aftershocks (which
continued to occur after the period shown here).

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 planeadjusts to the effects of the main shock.[21]
Earthquake swarms
Main article: Earthquake swarm
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.[23] In August 2012, a swarm of
earthquakes shook Southern California's Imperial Valley, showing the most recorded activity in the area since
the 1970s.[24]
Sometimes a series of earthquakes occur in what has been called an 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. [25][26]

Intensity of earth quaking and magnitude of earthquakes

Quaking or shaking of the earth is a common phenomenon undoubtedly known to humans from earliest times.
Prior to the development of strong-motion accelerometers that can measure peak ground speed and
acceleration directly, the intensity of the earth-shaking was estimated on the basis of the observed effects, as
categorized on various seismic intensity scales. Only in the last century has the source of such shaking been
identified as ruptures in the earth's crust, with the intensity of shaking at any locality dependent not only on the
local ground conditions, but also on the strength or magnitude of the rupture, and on its distance.[27]
The first scale for measuring earthquake magnitudes was developed by Charles F. Richter in 1935. Subsequent
scales (see seismic magnitude scales) have retained a key feature, where each unit represents a ten-fold
difference in the amplitude of the ground shaking, and a 32-fold difference in energy. Subsequent scales are also
adjusted to have approximately the same numeric value within the limits of the scale. [28]
Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale",
standard practice by most seismological authorities is to express an earthquake's strength on the moment
magnitude scale, which is based on the actual energy released by an earthquake. [29]

Frequency of occurrence
It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About
100,000 of these can be felt.[30][31] Minor earthquakes occur nearly constantly around the world in places
like California and Alaska in the U.S., as well as in El
Salvador, Mexico, Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New
Zealand, Greece, Italy, India, Nepal and Japan, but earthquakes can occur almost anywhere,
including Downstate New York, England, and Australia.[32]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. [33] 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.[34] This is an
example of the Gutenberg–Richter law.

The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.[35]

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 United States Geological
Survey 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.
[36]
In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical
fluctuation rather than a systematic trend.[37] More detailed statistics on the size and frequency of earthquakes is
available from the United States Geological Survey(USGS).[38] A recent increase in the number of major
earthquakes has been noted, which could be explained by a cyclical pattern of periods of intense tectonic
activity, interspersed with longer periods of low-intensity. However, accurate recordings of earthquakes only
began in the early 1900s, so it is too early to categorically state that this is the case. [39]
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-kilometre (25,000 mi)
long, horseshoe-shaped zone called the circum-Pacific seismic belt, known as the Pacific Ring of Fire, which for
the most part bounds the Pacific Plate.[40][41] Massive earthquakes tend to occur along other plate boundaries, too,
such as along the Himalayan Mountains.[42]
With the rapid growth of mega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismic risk, some
seismologists are warning that a single quake may claim the lives of up to three million people. [43]

Induced seismicity
Main article: Induced seismicity
While most earthquakes are caused by movement of the Earth's tectonic plates, human activity can also
produce earthquakes. Four main activities contribute to this phenomenon: storing large amounts of water behind
a dam (and possibly building an extremely heavy building), drilling and injecting liquid into wells, and by coal
mining and oil drilling.[44] Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan
Province in May; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time.
The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure
probably increased the power of the earthquake and accelerated the rate of movement for the fault. [45]
The greatest earthquake in Australia's history is also claimed to be induced by human activity: Newcastle,
Australia was built over a large sector of coal mining areas. The earthquake has been reported to be spawned
from a fault that reactivated due to the millions of tonnes of rock removed in the mining process. [46]

Measuring and locating earthquakes

Main articles: Seismic magnitude scales and Seismology
The instrumental scales used to describe the size of an earthquake began with the Richter magnitude scale in
the 1930s. It is a relatively simple measurement of an event's amplitude, and its use has become minimal in the
21st century. Seismic waves travel through the Earth's interior and can be recorded by seismometers at great
distances. The surface wave magnitude was developed in the 1950s as a means to measure remote
earthquakes and to improve the accuracy for larger events. The moment magnitude scalemeasures the
amplitude of the shock, but also takes into account the seismic moment (total rupture area, average slip of the
fault, and rigidity of the rock). The Japan Meteorological Agency seismic intensity scale, the Medvedev–
Sponheuer–Karnik scale, and the Mercalli intensity scale are based on the observed effects and are related to
the intensity of shaking.
Every tremor produces different types of seismic waves, which travel through rock with different velocities:

 Longitudinal P-waves (shock- or pressure waves)

 Transverse S-waves (both body waves)

 Surface waves – (Rayleigh and Love waves)

Propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/s, depending on
the density and elasticity of the medium. In the Earth's interior the shock- or P waves travel much faster than the
S waves (approx. relation 1.7 : 1). The differences in travel time from the epicenter to the observatory are a
measure of the distance and can be used to image both sources of quakes and structures within the Earth. Also,
the depth of the hypocenter can be computed roughly.
In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to
~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up
to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earthquake arrive at an observatory
via the Earth's mantle.
On average, the kilometer distance to the earthquake is the number of seconds between the P and S
wave times 8.[47] Slight deviations are caused by inhomogeneities of subsurface structure. By such analyses of
seismograms the Earth's core was located in 1913 by Beno Gutenberg.
S waves and later arriving surface waves do main damage compared to P waves. P wave squeezes and
expands material in the same direction it is traveling. S wave shakes the ground up and down and back and
forth.[48]
Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is
divided into 754 Flinn–Engdahl regions (F-E regions), which are based on political and geographical boundaries
as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones
belong to larger F-E regions.
Standard reporting of earthquakes includes its magnitude, date and time of occurrence, geographic
coordinates of its epicenter, depth of the epicenter, geographical region, distances to population centers, location
uncertainty, a number of parameters that are included in USGS earthquake reports (number of stations
reporting, number of observations, etc.), and a unique event ID. [49]
Although relatively slow seismic waves have traditionally been used to detect earthquakes, scientists realized in
2016 that gravitational measurements could provide instantaneous detection of earthquakes, and confirmed this
by analyzing gravitational records associated with the 2011 Tohoku-Oki ("Fukushima") earthquake.[50][51]

Effects of earthquakes

1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake, which killed an estimated
60,000 people. A tsunamioverwhelms the ships in the harbor.

The effects of earthquakes include, but are not limited to, the following:

Shaking and ground rupture

Damaged buildings in Port-au-Prince, Haiti, January 2010.

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less
severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex
combination of the earthquake magnitude, the distance from the epicenter, and the local geological and
geomorphological conditions, which may amplify or reduce wave propagation.[52] 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 sett