Editing Earthquake (section)

==Occurrence==
[[File:Fault types.svg|thumb|Three types of faults:<br />
A. [[strike-slip fault|Strike-slip]]<br />
B. [[Normal fault|Normal]]<br />
C. [[Reverse fault|Reverse]]
]]

[[Tectonics|Tectonic]] earthquakes occur anywhere on the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a [[Fault (geology)|fault plane]]. The sides of a fault move past each other smoothly and [[Aseismic creep|aseismically]] only if there are no irregularities or [[Asperity (faults)|asperities]] along the fault surface that increases the frictional resistance. Most fault surfaces do have such asperities, which leads to a form of [[Stick-slip phenomenon|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 [[Potential energy|stored energy]].<ref name="Ohnaka">{{cite book | url=https://books.google.com/books?id=Bp0gAwAAQBAJ&pg=PA234 | title=The Physics of Rock Failure and Earthquakes | publisher=Cambridge University Press | author=Ohnaka, M. | year=2013 | page=148 | isbn=978-1-107-35533-0}}</ref> This energy is released as a combination of radiated elastic [[Strain (materials science)|strain]] [[seismic waves]],<ref>{{cite journal | last1 = Vassiliou | first1 = Marius | last2 = Kanamori | first2 = Hiroo | year = 1982 | title = The Energy Release in Earthquakes | journal = Bull. Seismol. Soc. Am. | volume = 72 | pages = 371–387 }}</ref> 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 (geology)|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 [[Structure of the Earth|Earth's deep interior.]]<ref name="USGS1">{{cite web|last=Spence |first=William |author2=S.A. Sipkin |author3=G.L. Choy |title=Measuring the Size of an Earthquake |publisher=United States Geological Survey|year=1989 |url=https://earthquake.usgs.gov/learning/topics/measure.php |access-date=2006-11-03 |url-status=dead |archive-url=https://web.archive.org/web/20090901233601/http://earthquake.usgs.gov/learning/topics/measure.php |archive-date=2009-09-01 }}</ref>

===Fault types===
{{Further|Fault (geology)|Strike and dip}}
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 where movement on them involves a vertical component. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. The topmost, brittle part of the Earth's crust, and the cool slabs of the tectonic plates that are descending into the hot mantle, are the only parts of our planet that can store elastic energy and release it in fault ruptures. Rocks hotter than about {{cvt|300|C||}} flow in response to stress; they do not rupture in earthquakes.<ref>{{cite journal |last1=Sibson |first1=R.H. |year=1982 |title=Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States |journal=Bulletin of the Seismological Society of America |volume=72 |issue=1 |pages=151–163}}</ref><ref>Sibson, R.H. (2002) "Geology of the crustal earthquake source" International handbook of earthquake and engineering seismology, Volume 1, Part 1, p. 455, eds. W H K Lee, H Kanamori, P C Jennings, and C. Kisslinger, Academic Press, {{ISBN|978-0-12-440652-0}}</ref> The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately {{cvt|1000|km|||}}. Examples are the earthquakes in [[1957 Andreanof Islands earthquake|Alaska (1957)]], [[1960 Valdivia earthquake|Chile (1960)]], and [[2004 Indian Ocean earthquake and tsunami|Sumatra (2004)]], all in subduction zones. The longest earthquake ruptures on strike-slip faults, like the [[San Andreas Fault]] ([[1857 Fort Tejon earthquake|1857]], [[1906 San Francisco earthquake|1906]]), the [[North Anatolian Fault]] in Turkey ([[1939 Erzincan earthquake|1939]]), and the [[Denali Fault]] in Alaska ([[2002 Denali earthquake|2002]]), are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

==== Normal faults ====
Normal faults occur mainly in areas where the crust is being [[Extensional tectonics|extended]] such as a [[divergent boundary]]. Earthquakes associated with normal faults are generally less than magnitude 7. 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 {{convert|6|km|spell=in||}}.<ref>Hjaltadóttir S., 2010, "Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland"</ref><ref>{{cite web |title=Reports and publications &#124; Seismicity &#124; Icelandic Meteorological office |url=http://en.vedur.is/earthquakes-and-volcanism/reports-and-publications/ |access-date=2011-07-24 |publisher=En.vedur.is |archive-date=2008-04-14 |archive-url=https://web.archive.org/web/20080414235419/http://en.vedur.is/earthquakes-and-volcanism/reports-and-publications/ |url-status=live }}</ref>

==== Reverse faults ====
Reverse faults occur in areas where the crust is being [[Thrust tectonics|shortened]] such as at a [[convergent boundary]]. Reverse faults, particularly those along convergent boundaries, are associated with the most powerful earthquakes (called [[megathrust earthquake]]s) including almost all of those of magnitude 8 or more. Megathrust earthquakes are responsible for about 90% of the total seismic moment released worldwide.<ref>{{citation |last1=Stern |first1=Robert J. |title=Subduction zones |journal=Reviews of Geophysics |volume=40 |issue=4 |page=17 |year=2002 |bibcode=2002RvGeo..40.1012S |doi=10.1029/2001RG000108 |s2cid=247695067|doi-access=free }}</ref>

==== Strike-slip faults ====
[[Strike-slip fault]]s 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. Strike-slip faults, particularly continental [[Transform fault|transforms]], can produce major earthquakes up to about magnitude 8. Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of {{cvt|10|km|||}} within the brittle crust.<ref>{{cite web |title=Instrumental California Earthquake Catalog |url=http://wgcep.org/data-inst_eq_cat |url-status=dead |archive-url=https://web.archive.org/web/20110725021215/http://wgcep.org/data-inst_eq_cat |archive-date=2011-07-25 |access-date=2011-07-24 |publisher=WGCEP}}</ref> Thus, earthquakes with magnitudes much larger than 8 are not possible.

[[File:Kluft-photo-Carrizo-Plain-Nov-2007-Img 0327.jpg|thumb|left|Aerial photo of the San Andreas Fault in the [[Carrizo Plain]], northwest of Los Angeles]]

In addition, there exists a hierarchy of stress levels in the three fault types. Thrust faults are generated by the highest, strike-slip by intermediate, and normal faults by the lowest stress levels.<ref>{{cite journal | last1 = Schorlemmer | first1 = D. | last2 = Wiemer | first2 = S. | last3 = Wyss | first3 = M. | year = 2005 | title = Variations in earthquake-size distribution across different stress regimes | journal = Nature | volume = 437 | issue = 7058| pages = 539–542 |bibcode = 2005Natur.437..539S |doi = 10.1038/nature04094 | pmid = 16177788 | s2cid = 4327471 }}</ref> 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, and 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.

=== Energy released ===
For every unit increase in seismic magnitude, there is a roughly thirty-fold increase in the energy released. For instance, an earthquake of magnitude 6.0 releases approximately 32 times as much energy as an earthquake of magnitude 5.0, and a 7.0 magnitude earthquake releases about 1,000 times as much energy as a 5.0 magnitude earthquake. An 8.6-magnitude earthquake releases the same amount of energy as 10,000 atomic bombs of the size used in [[World War II]].<ref>Geoscience Australia.{{full citation needed|date=December 2022}}</ref>

This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures<ref>{{cite journal |last1=Wyss |first1=M. |year=1979 |title=Estimating expectable maximum magnitude of earthquakes from fault dimensions |journal=Geology |volume=7 |issue=7| pages=336–340 |bibcode=1979Geo.....7..336W |doi=10.1130/0091-7613(1979)7<336:EMEMOE>2.0.CO;2}}</ref> and the stress drop. Therefore, the greater the length and width of the faulted area, the greater the resulting magnitude. The most important parameter controlling the maximum earthquake magnitude on a fault, however, is 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.<ref>{{cite web |url=http://www.globalcmt.org/CMTsearch.html |title=Global Centroid Moment Tensor Catalog |publisher=Globalcmt.org |access-date=2011-07-24 |archive-date=2011-07-19 |archive-url=https://web.archive.org/web/20110719183137/http://www.globalcmt.org/CMTsearch.html |url-status=live }}</ref> Thus, the width of the plane within the top brittle crust of the Earth can reach {{cvt|50–100|km|||}} (such as in [[2011 Tōhoku earthquake and tsunami|Japan, 2011]], or in [[1964 Alaska earthquake|Alaska, 1964]]), making the most powerful earthquakes possible.

===Focus===
{{Main|Depth of focus (tectonics)}}
[[File:HotelSanSalvador.jpg|thumb|Collapsed Gran Hotel building in the [[San Salvador]] metropolis, after the shallow [[1986 San Salvador earthquake]]]]

The majority of tectonic earthquakes originate in the Ring of Fire at depths not exceeding tens of kilometers. Earthquakes occurring at depths less than {{cvt|70|km|||}} are classified as "shallow-focus" earthquakes, while those with focal depths between {{cvt|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 earthquake]]s may occur at much greater depths (ranging from {{cvt|300|to|700|km|}}).<ref>{{cite web| publisher = [[National Earthquake Information Center]]| title = M7.5 Northern Peru Earthquake of 26 September 2005| date = 17 October 2005| url = ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf| access-date = 2008-08-01| archive-date = 25 May 2017| archive-url = https://wayback.archive-it.org/all/20170525100314/ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf| url-status = live}}</ref> These seismically active areas of subduction are known as [[Wadati–Benioff zone]]s. Deep-focus earthquakes occur at depths 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.<ref name="olivine">{{cite journal| last1 = Greene II | first1 = H.W.| last2 = Burnley | first2 = P.C.| title = A new self-organizing mechanism for deep-focus earthquakes| journal = Nature| volume = 341| issue = 6244| pages = 733–737| date = October 26, 1989| doi = 10.1038/341733a0| bibcode=1989Natur.341..733G| s2cid = 4287597}}</ref>

===Volcanic activity===
{{main|Volcano tectonic earthquake}}

Earthquakes often occur in volcanic regions and are caused there, both by [[tectonic plates|tectonic]] faults and the movement of [[magma]] in [[volcano]]es. Such earthquakes can serve as an early warning of volcanic eruptions, as during the [[1980 eruption of Mount St. Helens]].<ref>{{Cite book|last=Foxworthy and Hill|year=1982|title=Volcanic Eruptions of 1980 at Mount St. Helens, The First 100&nbsp;Days: USGS Professional Paper 1249}}</ref> Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by [[seismometers]] and [[tiltmeter]]s (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.<ref>{{cite web|url=http://pubs.usgs.gov/gip/earthq1/volcano.html|title=Volcanoes and Earthquakes|publisher=United States Geological Survey|date=January 7, 1998|author=Watson, John|author2=Watson, Kathie|access-date=May 9, 2009|archive-date=March 26, 2009|archive-url=https://web.archive.org/web/20090326093352/http://pubs.usgs.gov/gip/earthq1/volcano.html|url-status=live}}</ref>

===Rupture dynamics===
A tectonic earthquake begins as an area of initial slip on the fault surface that forms the focus. Once the rupture has been initiated, it begins to propagate away from the focus, spreading out along the fault surface. Lateral propagation will continue until either the rupture reaches a barrier, such as the end of a fault segment, or a region on the fault where there is insufficient stress to allow continued rupture. For larger earthquakes, the depth extent of rupture will be constrained downwards by the [[brittle-ductile transition zone]] and upwards by the ground surface. The mechanics of this process are poorly understood because it is difficult either to recreate such rapid movements in a laboratory or to record seismic waves close to a nucleation zone due to strong ground motion.<ref name="NRS"/>

In most cases, the rupture speed approaches, but does not exceed, the [[S wave|shear wave]] (S wave) velocity of the surrounding rock. There are a few exceptions to this:

==== Supershear earthquakes ====
[[File:Kahramanmaraş after 7.8 magnitude earthquake in Türkiye 5.jpg|250px|thumb|right|The [[2023 Turkey–Syria earthquakes]] ruptured along segments of the [[East Anatolian Fault]] at supershear speeds; more than 50,000 people died in both countries.<ref name="MelgarEtAl23">{{cite journal |last1=Melgar |first1=Diego |last2=Taymaz |first2=Tuncay |last3=Ganas |first3=Athanassios |last4=Crowell |first4=Brendan |last5=Öcalan |first5=Taylan |last6=Kahraman |first6=Metin |last7=Tsironi |first7=Varvara |last8=Yolsal-Çevikbilen |first8=Seda |last9=Valkaniotis |first9=Sotiris |last10=Irmak |first10=Tahir Serkan |last11=Eken |first11=Tuna |last12=Erman |first12=Ceyhun |last13=Özkan |first13=Berkan |last14=Dogan |first14=Ali Hasan |last15=Altuntaş |first15=Cemali |title=Sub- and super-shear ruptures during the 2023 Mw 7.8 and Mw 7.6 earthquake doublet in SE Türkiye |journal=Seismica |year=2023 |volume=2 |issue=3 |page=387 |doi=10.26443/seismica.v2i3.387|s2cid=257520761 |doi-access=free |bibcode=2023Seism...2..387M }}</ref>]]
[[Supershear earthquake]] ruptures are known to have propagated at speeds greater than the S wave velocity. These have so far all been observed during large strike-slip events. The unusually wide zone of damage caused by the [[2001 Kunlun earthquake]] has been attributed to the effects of the [[sonic boom]] developed in such earthquakes.

==== Slow earthquakes ====
[[Slow earthquake]] ruptures travel at unusually low velocities. 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]].<ref name="NRS">{{cite book|last=National Research Council (U.S.). Committee on the Science of Earthquakes|title=Living on an Active Earth: Perspectives on Earthquake Science|chapter-url=http://www.nap.edu/openbook.php?record_id=10493&page=282|access-date=8 July 2010|year=2003|publisher=National Academies Press|location=Washington, D.C.|isbn=978-0-309-06562-7|page=[https://archive.org/details/livingonactiveea0000unse/page/418 418]|chapter=5. Earthquake Physics and Fault-System Science|url=https://archive.org/details/livingonactiveea0000unse/page/418}}</ref>

====Co-seismic overpressuring and effect of pore pressure====
During an earthquake, high temperatures can develop at the fault plane, increasing pore pressure and consequently vaporization of the groundwater already contained within the rock.<ref name=Sibson>{{cite journal|last1=Sibson |first1= R.H.|year=1973|title=Interactions between Temperature and Pore-Fluid Pressure during Earthquake Faulting and a Mechanism for Partial or Total Stress Relief|journal= Nat. Phys. Sci. |volume=243|issue= 126|pages=66–68|doi= 10.1038/physci243066a0|bibcode= 1973NPhS..243...66S}}</ref><ref name=Rudnicki>{{cite journal|last1=Rudnicki |first1= J.W.|last2=Rice |first2= J.R.|year=2006|title=Effective normal stress alteration due to pore pressure changes induced by dynamic slip propagation on a plane between dissimilar materials|journal= J. Geophys. Res. |volume= 111, B10308|issue= B10|doi=10.1029/2006JB004396|bibcode= 2006JGRB..11110308R|s2cid= 1333820|url=https://dash.harvard.edu/bitstream/1/2668811/1/Rice_PorePressDynSlip.pdf|url-status=live|archive-url=https://web.archive.org/web/20190502041503/https://dash.harvard.edu/bitstream/handle/1/2668811/Rice_PorePressDynSlip.pdf;jsessionid=071046244FA1B0E26418CE95B726BA0E?sequence=1|archive-date=2019-05-02|archive-format=PDF}}</ref><ref name=Guerriero>{{cite journal|last1=Guerriero |first1= V |last2=Mazzoli |first2= S.|year=2021|title=Theory of Effective Stress in Soil and Rock and Implications for Fracturing Processes: A Review|journal=Geosciences  |volume=11|issue= 3 |pages=119|doi=10.3390/geosciences11030119|bibcode= 2021Geosc..11..119G |doi-access=free}}</ref> In the coseismic phase, such an increase can significantly affect slip evolution and speed, in the post-seismic phase it can control the [[Aftershock]] sequence because, after the main event, pore pressure increase slowly propagates into the surrounding fracture network.<ref name=Nur>{{cite journal|last1=Nur |first1= A |last2=Booker |first2= J.R.|year=1972|title=Aftershocks Caused by Pore Fluid Flow?|journal=Science |volume=175|issue= 4024 |pages=885–887|doi= 10.1126/science.175.4024.885 |pmid= 17781062 |bibcode= 1972Sci...175..885N |s2cid= 19354081 }}</ref><ref name=Guerriero /> From the point of view of the [[Mohr-Coulomb theory|Mohr-Coulomb strength theory]], an increase in fluid pressure reduces the normal stress acting on the fault plane that holds it in place, and fluids can exert a lubricating effect. As thermal overpressurization may provide positive feedback between slip and strength fall at the fault plane, a common opinion is that it may enhance the faulting process instability. After the mainshock, the pressure gradient between the fault plane and the neighboring rock causes a fluid flow that increases pore pressure in the surrounding fracture networks; such an increase may trigger new faulting processes by reactivating adjacent faults, giving rise to aftershocks.<ref name=Nur /><ref name=Guerriero /> Analogously, artificial pore pressure increase, by fluid injection in Earth's crust, may [[Induced seismicity|induce seismicity]].

===Tidal forces===
{{main|Tidal triggering of earthquakes}}
[[Tides]] may trigger some [[seismicity]].<ref>{{cite journal | last1=Hartzell | first1=Stephen | last2=Heaton | first2=Thomas | title=The fortnightly tide and the tidal triggering of earthquakes | journal=Bulletin of the Seismological Society of America | volume=80 | issue=2 | date=1990-04-01 | issn=1943-3573 | doi=10.1785/BSSA0800020504 | doi-access=free | pages=504–505 | bibcode=1990BuSSA..80..504H | url=https://authors.library.caltech.edu/records/39ffh-nrm98/files/1282.full.pdf?download=1}}</ref>

===Clusters===
Most earthquakes form part of a sequence, related to each other in terms of location and time.<ref name=WAAFEC>{{cite web|url=https://earthquake.usgs.gov/eqcenter/step/explain.php|title=What are Aftershocks, Foreshocks, and Earthquake Clusters?|url-status=dead|archive-url=https://web.archive.org/web/20090511175245/http://earthquake.usgs.gov/eqcenter/step/explain.php|archive-date=2009-05-11}}</ref> 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.<ref>{{cite web|url=https://earthquake.usgs.gov/research/parkfield/repeat.php|title=Repeating Earthquakes|publisher=United States Geological Survey|date=January 29, 2009|access-date=May 11, 2009|archive-date=April 3, 2009|archive-url=https://web.archive.org/web/20090403074132/http://earthquake.usgs.gov/research/parkfield/repeat.php|url-status=live}}</ref> Earthquake clustering has been observed, for example, in Parkfield, California where a long-term research study is being conducted around the [[Parkfield earthquake]] cluster.<ref>{{Cite web |title=The Parkfield, California, Earthquake Experiment |url=https://earthquake.usgs.gov/learn/parkfield/ |access-date=2022-10-24 |publisher=United States Geological Survey |archive-date=2022-10-24 |archive-url=https://web.archive.org/web/20221024200153/https://earthquake.usgs.gov/learn/parkfield/ |url-status=live }}</ref>

====Aftershocks====
{{Main|Aftershock}}
[[File:2016 Central Italy earthquake wide.svg|thumb|upright=1.25|Magnitude of the [[August 2016 Central Italy earthquake|Central Italy earthquakes of August]] and [[October 2016 Central Italy earthquakes|October 2016]] and [[January 2017 Central Italy earthquakes|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. Rapid changes of stress between rocks, and the stress from the original earthquake are the main causes of these aftershocks,<ref name=Britannica>{{Cite web|title=Aftershock {{!}} geology|url=https://www.britannica.com/science/aftershock-geology|access-date=2021-10-13|website=Encyclopædia Britannica|archive-date=2015-08-23|archive-url=https://web.archive.org/web/20150823124854/https://www.britannica.com/science/aftershock-geology|url-status=live}}</ref> along with the crust around the ruptured [[Fault (geology)|fault plane]] as it adjusts to the effects of the mainshock.<ref name="WAAFEC" /> An aftershock is in the same region as the main shock but always of a smaller magnitude, however, they can still be powerful enough to cause even more damage to buildings that were already previously damaged from the mainshock.<ref name=Britannica/> If an aftershock is larger than the mainshock, the aftershock is redesignated as the mainshock and the original main shock is redesignated as a [[foreshock]]. Aftershocks are formed as the crust around the displaced [[Fault (geology)|fault plane]] adjusts to the effects of the mainshock.<ref name=WAAFEC/>

====Swarms====
{{Main|Earthquake swarm}}
Earthquake swarms are sequences of earthquakes striking in a specific area within a short period. They are different from earthquakes followed by a series of [[aftershock]]s by the fact that no single earthquake in the sequence is the main shock, so none has a notably higher magnitude than another. An example of an earthquake swarm is the 2004 activity at [[Yellowstone National Park]].<ref>{{cite web|url=http://volcanoes.usgs.gov/yvo/2004/Apr04Swarm.html|title=Earthquake Swarms at Yellowstone|publisher=United States Geological Survey|access-date=2008-09-15|archive-date=2008-05-13|archive-url=https://web.archive.org/web/20080513060550/http://volcanoes.usgs.gov/yvo/2004/Apr04Swarm.html|url-status=live}}</ref> In August 2012, a swarm of earthquakes shook [[Southern California]]'s [[Imperial Valley]], showing the most recorded activity in the area since the 1970s.<ref>{{cite news|last=Duke|first=Alan|title=Quake 'swarm' shakes Southern California|url=http://www.cnn.com/2012/08/26/us/california-quake-swarm/index.html|publisher=CNN|access-date=27 August 2012|archive-date=27 August 2012|archive-url=https://web.archive.org/web/20120827120248/http://www.cnn.com/2012/08/26/us/california-quake-swarm/index.html|url-status=live}}</ref>

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 [[coulomb stress transfer|stress redistribution]] of the previous earthquakes. Similar to [[aftershock]]s but on adjacent segments of fault, these storms occur over the course of years, 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.<ref>{{cite journal |title=Poseidon's Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean |journal=Journal of Archaeological Science |year=2000 |author=Amos Nur |issn=0305-4403 |volume=27 |issue=1 |pages=43–63 |url=http://water.stanford.edu/nur/EndBronzeage.pdf |doi=10.1006/jasc.1999.0431 |last2=Cline |first2=Eric H. |bibcode=2000JArSc..27...43N |url-status=dead |archive-date=2009-03-25 |archive-url=https://web.archive.org/web/20090325050459/http://water.stanford.edu/nur/EndBronzeage.pdf}}</ref><ref>{{cite web |url=http://www.bbc.co.uk/science/horizon/2003/earthquakestorms.shtml |title=Earthquake Storms |work=[[Horizon (BBC TV series)|Horizon]] |date=1 April 2003 |access-date=2007-05-02 |archive-date=2019-10-16 |archive-url=https://web.archive.org/web/20191016045550/http://www.bbc.co.uk/science/horizon/2003/earthquakestorms.shtml |url-status=live }}</ref>

===Frequency===
[[File:Comerio, Luca (1878-1940) - Vittime del terremoto di Messina (dicembre 1908).jpg|thumb|The [[1908 Messina earthquake|Messina earthquake]] and tsunami took about 80,000 lives on December 28, 1908, in [[Sicily]] and [[Calabria]].<ref name="CFTI5">{{Cite web |url=https://storing.ingv.it/cfti/cfti5/quake.php?21318IT |title=1908 12 28, 04:20:27 Calabria meridionale-Messina (Italy)  |last1=Guidoboni E. |last2= Ferrari G. |website=CFTI5 Catalogue of Strong Earthquakes in Italy (461 BC – 1997) and Mediterranean Area (760 B.C. – 1500) |last3=Mariotti D. |last4=Comastri A. |last5=Tarabusi G. |last6=Sgattoni G. |last7=Valensise G}}</ref>]]

It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt.<ref name="usgsfacts">{{cite web|url=https://www.usgs.gov/natural-hazards/earthquake-hazards/science/cool-earthquake-facts|title=Cool Earthquake Facts|publisher=United States Geological Survey|access-date=2021-04-21|archive-date=2021-04-20|archive-url=https://web.archive.org/web/20210420165152/https://www.usgs.gov/natural-hazards/earthquake-hazards/science/cool-earthquake-facts|url-status=live}}</ref><ref name="wp100414">{{Cite news | first=Margaret Webb | last=Pressler | title=More earthquakes than usual? Not really. | department=KidsPost | newspaper= The Washington Post| pages= C10 | date=14 April 2010 }}</ref><!----url does not contain box statistics that print edition does and is included for info only----> Minor earthquakes occur very frequently around the world in places like California and Alaska in the U.S., as well as in El Salvador, Mexico, Guatemala, Chile, Peru, Indonesia, the Philippines, Iran, Pakistan, the [[Azores]] in Portugal, Turkey, New Zealand, Greece, Italy, India, Nepal, and Japan.<ref>{{cite web |url=https://earthquake.usgs.gov/ |title=Earthquake Hazards Program |publisher=United States Geological Survey |access-date=2006-08-14 |archive-date=2011-05-13 |archive-url=https://web.archive.org/web/20110513032733/https://earthquake.usgs.gov/ |url-status=live }}</ref> Larger earthquakes occur less frequently, the relationship being [[Gutenberg–Richter law|exponential]]; for example, roughly ten times as many earthquakes larger than magnitude 4 occur than earthquakes larger than magnitude 5.<ref>{{Cite web|url=https://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php|archiveurl=https://web.archive.org/web/20100524161817/http://earthquake.usgs.gov/earthquakes/eqarchives/year/eqstats.php|url-status=dead|title=USGS Earthquake statistics table based on data since 1900|archivedate=May 24, 2010}}</ref> 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&nbsp;years, and an earthquake of 5.6 or larger every 100&nbsp;years.<ref>{{cite web |url=http://www.quakes.bgs.ac.uk/hazard/Hazard_UK.htm |title=Seismicity and earthquake hazard in the UK |publisher=Quakes.bgs.ac.uk |access-date=2010-08-23 |archive-date=2010-11-06 |archive-url=https://web.archive.org/web/20101106121058/http://quakes.bgs.ac.uk/hazard/Hazard_UK.htm |url-status=live }}</ref> 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 [[United States Geological Survey]] (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.<ref>
{{cite web
 |title       = Common Myths about Earthquakes
 |url         = https://earthquake.usgs.gov/learning/faq.php?categoryID=6&faqID=110
 |publisher   = United States Geological Survey
 |access-date  = 2006-08-14
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20060925135349/http://earthquake.usgs.gov/learning/faq.php?categoryID=6&faqID=110
 |archive-date = 2006-09-25
}}</ref> In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend.<ref>[https://earthquake.usgs.gov/learn/topics/increase_in_earthquakes.php Are Earthquakes Really on the Increase?] {{webarchive|url=https://web.archive.org/web/20140630233346/http://earthquake.usgs.gov/learn/topics/increase_in_earthquakes.php |date=2014-06-30 }}, USGS Science of Changing World. Retrieved 30 May 2014.</ref> More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey.<ref>
{{cite web
 |title=Earthquake Facts and Statistics: Are earthquakes increasing?
 |url=http://neic.usgs.gov/neis/eqlists/eqstats.html
 |publisher=United States Geological Survey
 |access-date=2006-08-14
 |url-status=dead
 |archive-url=https://web.archive.org/web/20060812060818/http://neic.usgs.gov/neis/eqlists/eqstats.html
 |archive-date=2006-08-12
}}</ref> 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.<ref>[http://www.australiangeographic.com.au/journal/the-10-biggest-earthquakes-in-recorded-history.htm/ The 10 biggest earthquakes in history] {{Webarchive|url=https://web.archive.org/web/20130930084024/http://www.australiangeographic.com.au/journal/the-10-biggest-earthquakes-in-recorded-history.htm/ |date=2013-09-30 }}, Australian Geographic, March 14, 2011.</ref>

Most of the world's earthquakes (90%, and 81% of the largest) take place in the {{convert|40000|km|mi|adj=mid|-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]].<ref>
{{cite web
 |title       = Historic Earthquakes and Earthquake Statistics: Where do earthquakes occur?
 |url         = https://earthquake.usgs.gov/learning/faq.php?categoryID=11&faqID=95
 |publisher   = United States Geological Survey
 |access-date  = 2006-08-14
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20060925142008/http://earthquake.usgs.gov/learning/faq.php?categoryID=11&faqID=95
 |archive-date = 2006-09-25
}}</ref><ref>
{{cite web
 |url         = https://earthquake.usgs.gov/learning/glossary.php?termID=150
 |publisher   = United States Geological Survey
 |title       = Visual Glossary – Ring of Fire
 |access-date  = 2006-08-14
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20060828152638/http://earthquake.usgs.gov/learning/glossary.php?termID=150
 |archive-date = 2006-08-28
}}</ref> Massive earthquakes tend to occur along other plate boundaries too, such as along the [[Himalayan Mountains]].<ref>{{cite journal | last1 = Jackson | first1 = James | year = 2006 | title = Fatal attraction: living with earthquakes, the growth of villages into megacities, and earthquake vulnerability in the modern world | url = http://rsta.royalsocietypublishing.org/content/364/1845/1911.full | journal = [[Philosophical Transactions of the Royal Society]] | volume = 364 | issue = 1845 | pages = 1911–1925 | doi = 10.1098/rsta.2006.1805 | pmid = 16844641 | bibcode = 2006RSPTA.364.1911J | s2cid = 40712253 | access-date = 2011-03-09 | archive-date = 2013-09-03 | archive-url = https://web.archive.org/web/20130903085953/http://rsta.royalsocietypublishing.org/content/364/1845/1911.full | url-status = live }}</ref>

With the rapid growth of [[Megacity|mega-cities]] such as Mexico City, Tokyo, and Tehran in areas of high [[seismic risk]], some seismologists are warning that a single earthquake may claim the lives of up to three million people.<ref>"[http://cires.colorado.edu/~bilham/UrbanEarthquakesGlobal.html Global urban seismic risk] {{Webarchive|url=https://web.archive.org/web/20110920015358/http://cires.colorado.edu/~bilham/UrbanEarthquakesGlobal.html |date=2011-09-20 }}." Cooperative Institute for Research in Environmental Science.</ref>

===Induced seismicity===
{{main|Induced seismicity}}
While most earthquakes are caused by the movement of the Earth's [[tectonic plate]]s, human activity can also produce earthquakes. Activities both above ground and below may change the stresses and strains on the crust, including building reservoirs, extracting resources such as coal or oil, and injecting fluids underground for waste disposal or [[fracking]].<ref>{{cite journal |author1=Fougler, Gillian R. |author2=Wilson, Miles |author3=Gluyas, Jon G. |author4=Julian, Bruce R. |author5=Davies, Richard J. |author-link1=Gillian Foulger |title=Global review of human-induced earthquakes |journal=[[Earth-Science Reviews]] |date=2018 |volume=178 |pages=438–514 |doi=10.1016/j.earscirev.2017.07.008 |bibcode=2018ESRv..178..438F |doi-access=free }}</ref> Most of these earthquakes have small magnitudes. The 5.7 magnitude [[2011 Oklahoma earthquake]] is thought to have been caused by disposing wastewater from oil production into [[injection wells]],<ref>{{cite news |last1=Fountain |first1=Henry |title=Study Links 2011 Quake to Technique at Oil Wells |newspaper=The New York Times |url=https://www.nytimes.com/2013/03/29/science/earth/2011-oklahoma-quake-tied-to-wastewater-disposal-at-oil-wells.html |access-date=July 23, 2020 |date=March 28, 2013 |archive-date=July 23, 2020 |archive-url=https://web.archive.org/web/20200723135240/https://www.nytimes.com/2013/03/29/science/earth/2011-oklahoma-quake-tied-to-wastewater-disposal-at-oil-wells.html |url-status=live }}</ref> and studies point to the state's oil industry as the cause of other earthquakes in the past century.<ref>{{cite journal |author1=Hough, Susan E. |author-link1=Susan Hough |author2=Page, Morgan |title=A Century of Induced Earthquakes in Oklahoma? |journal=[[Bulletin of the Seismological Society of America]] |date=2015 |volume=105 |issue=6 |pages=2863–2870 |doi=10.1785/0120150109 |bibcode=2015BuSSA.105.2863H |url=https://pubs.geoscienceworld.org/ssa/bssa/article-abstract/105/6/2863/331910/A-Century-of-Induced-Earthquakes-in-Oklahoma-A?redirectedFrom=fulltext |access-date=July 23, 2020 |archive-date=July 23, 2020 |archive-url=https://web.archive.org/web/20200723210546/https://pubs.geoscienceworld.org/ssa/bssa/article-abstract/105/6/2863/331910/A-Century-of-Induced-Earthquakes-in-Oklahoma-A?redirectedFrom=fulltext |url-status=live }}</ref> A [[Columbia University]] paper suggested that the 8.0 magnitude [[2008 Sichuan earthquake]] was induced by loading from the [[Zipingpu Dam]],<ref>{{cite journal |last1=Klose |first1=Christian D. |title=Evidence for anthropogenic surface loading as trigger mechanism of the 2008 Wenchuan earthquake |journal=Environmental Earth Sciences |date=July 2012 |volume=66 |issue=5 |pages=1439–1447 |doi=10.1007/s12665-011-1355-7|arxiv=1007.2155 |bibcode=2012EES....66.1439K |s2cid=118367859 }}</ref> though the link has not been conclusively proved.<ref>{{cite news |last1=LaFraniere |first1=Sharon |title=Possible Link Between Dam and China Quake |newspaper=The New York Times |url=https://www.nytimes.com/2009/02/06/world/asia/06quake.html |access-date=July 23, 2020 |date=February 5, 2009 |archive-date=January 27, 2018 |archive-url=https://web.archive.org/web/20180127101432/http://www.nytimes.com/2009/02/06/world/asia/06quake.html |url-status=live }}</ref>