{"id":2018,"date":"2016-06-03T20:10:53","date_gmt":"2016-06-03T20:10:53","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/geologyxwaymakerxmaster\/?post_type=chapter&#038;p=2018"},"modified":"2025-10-13T16:58:46","modified_gmt":"2025-10-13T16:58:46","slug":"reading-causes-of-earthquakes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/geo\/chapter\/reading-causes-of-earthquakes\/","title":{"raw":"Reading: Causes of Earthquakes","rendered":"Reading: Causes of Earthquakes"},"content":{"raw":"The following video explains the cause of earthquakes.\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1280979\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/VSgB1IWr6O4?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Overview of Elastic Rebound Theory<\/h3>\r\n<p id=\"x-ck12-MWUwNTM4ZWVkNjlmNjhlM2FiZGUyOGZhYjYxOWNmMTI.-9nq\">In an earthquake, the initial point where the rocks rupture in the crust is called the\u00a0<strong>focus<\/strong>. The epicenter is the point on the land surface that is directly above the focus. In about 75% of earthquakes, the focus is in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientists and the media (figure 1).<\/p>\r\n\r\n\r\n[caption id=\"attachment_810\" align=\"aligncenter\" width=\"500\"]<img class=\"wp-image-810 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/459\/2015\/06\/21062326\/Fig_9_2_1_2.jpg\" alt=\"Diagram showing the epicenter directly above the focus\" width=\"500\" height=\"388\" \/> Figure 1. In the vertical cross section of crust, there are two features labeled\u2014the focus and the epicenter, which is directly above the focus.[\/caption]\r\n<p id=\"x-ck12-YjMwZjFkYTVkMTY1YWFiNGQ1ZTUxYzFjMGJlNzA1ZGE.-imk\"><\/p>\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_2647\" align=\"alignright\" width=\"144\"]<img class=\"wp-image-2647 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201711\/Fault_types.png\" alt=\"Stirke-slip, normal, and thrust\" width=\"144\" height=\"360\" \/> Figure 2. Fault types[\/caption]\r\n\r\nTectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a <strong>fault plane<\/strong>. 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.[footnote]Ohnaka, M. (2013). <a class=\"external text\" href=\"https:\/\/books.google.com\/books?id=Bp0gAwAAQBAJ&amp;pg=PA234\" target=\"_blank\" rel=\"nofollow noopener\"><i>The Physics of Rock Failure and Earthquakes<\/i><\/a>. Cambridge University Press. p.\u00a0148.[\/footnote]\r\n\r\nThis energy is released as a combination of radiated elastic strain seismic 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.[footnote]Spence, William; S. A. Sipkin; G. L. Choy (1989). \"Measuring the Size of an Earthquake.\" United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2006-11-03<\/span><\/span>.[\/footnote]\r\n<h2>Earthquake Fault Types<\/h2>\r\nThere 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.\r\n\r\nReverse 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 \u00d7 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.\r\n\r\n[caption id=\"attachment_2648\" align=\"alignright\" width=\"400\"]<img class=\"wp-image-2648\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201744\/800px-Kluft-photo-Carrizo-Plain-Nov-2007-Img_0327.jpg\" alt=\"San Andreas Fault\" width=\"400\" height=\"267\" \/> Figure 3. Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles[\/caption]\r\n\r\nThis is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures[footnote]Wyss, M. (1979). \"Estimating expectable maximum magnitude of earthquakes from fault dimensions\". <i>Geology<\/i> <b>7<\/b> (7): 336\u2013340 doi:<a class=\"external text\" href=\"https:\/\/dx.doi.org\/10.1130%2F0091-7613%281979%297%3C336%3AEMEMOE%3E2.0.CO%3B2\" rel=\"nofollow\">10.1130\/0091-7613(1979)7&lt;336:EMEMOE&gt;2.0.CO;2<\/a>.[\/footnote] 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 degrees Celsius flow in response to stress; they do not rupture in earthquakes.[footnote]<span class=\"reference-text\">Sibson R. H. (1982) \"Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States,\" <em>Bulletin of the Seismological Society of America<\/em>, Vol 72, No. 1, pp. 151\u2013163. See also\u00a0<\/span><span class=\"reference-text\">Sibson, R. H. (2002) \"Geology of the crustal earthquake source,\" <em>International handbook of earthquake and engineering seismology<\/em>, Volume 1, Part 1, page 455, eds. W.H.K. Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger, Academic Press.<\/span>[\/footnote] The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1000 km. 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.\r\n\r\nThe 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.[footnote]\"<a class=\"external text\" href=\"http:\/\/www.globalcmt.org\/CMTsearch.html\" target=\"_blank\" rel=\"nofollow noopener\">Global Centroid Moment Tensor Catalog<\/a>.\" Globalcmt.org<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2011-07-24<\/span><\/span>.[\/footnote] Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Japan, 2011; Alaska, 1964), making the most powerful earthquakes possible.\r\n\r\nStrike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust,[footnote]\"Instrumental California Earthquake Catalog.\" WGCEP. Retrieved 2011-07-24.[\/footnote] 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 6 km.[footnote]Hjaltad\u00f3ttir S., 2010, \"Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland.\"\u00a0See also \"Reports and publications | Seismicity | Icelandic Meteorological office.\" En.vedur.is. Retrieved 2011-07-24.[\/footnote]\r\n\r\nIn 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.[footnote]Schorlemmer, D.; Wiemer, S.; Wyss, M. (2005). \"Variations in earthquake-size distribution across different stress regimes.\"\u00a0<i>Nature\u00a0<\/i><b>437<\/b> (7058): 539\u2013542.\u00a0doi:10.1038\/nature04094.[\/footnote] 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 (<strong>greatest<\/strong> principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass \"escapes\" in the direction of the <strong>least<\/strong> 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.\r\n<h2>Earthquakes away from Plate Boundaries<\/h2>\r\nWhere 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.[footnote]Talebian, M; Jackson, J (2004). \"A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran.\" <i>Geophysical Journal International<\/i> <b>156<\/b> (3): 506\u2013526.\u00a0doi:10.1111\/j.1365-246X.2004.02092.x.[\/footnote]\r\n\r\nAll tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading (e.g. deglaciation).[footnote]Nettles, M.; Ekstr\u00f6m, G. (May 2010). \"Glacial Earthquakes in Greenland and Antarctica.\" <i>Annual Review of Earth and Planetary Sciences<\/i> <b>38<\/b> (1): 467\u2013491.\u00a0doi:10.1146\/annurev-earth-040809-152414.[\/footnote] These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.[footnote]Noson, Qamar, and Thorsen (1988). <i>Washington State Earthquake Hazards: Washington State Department of Natural Resources<\/i>. Washington Division of Geology and Earth Resources Information Circular 85.[\/footnote]\r\n<h2>Shallow-Focus and Deep-Focus Earthquakes<\/h2>\r\n[caption id=\"attachment_2649\" align=\"alignright\" width=\"300\"]<img class=\"wp-image-2649\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201822\/HotelSanSalvador.jpg\" alt=\"collapse building\" width=\"300\" height=\"369\" \/> Figure 4. Collapsed Gran Hotel building in the San Salvador metropolis, after the shallow 1986 San Salvador earthquake.[\/caption]\r\n\r\nThe 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 <strong>shallow-focus<\/strong> earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed <strong>mid-focus<\/strong> or <strong>intermediate-depth<\/strong> earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, <strong>deep-focus<\/strong> earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).[footnote]\"M7.5 Northern Peru Earthquake of 26 September 2005\" (PDF).\u00a0National Earthquake Information Center. 17 October 2005<span class=\"reference-accessdate\">. Retrieved<span class=\"nowrap\">2008-08-01<\/span><\/span>.[\/footnote]\r\n\r\nThese seismically active areas of subduction are known as Wadati\u2013Benioff 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.[footnote]Greene II, H. W.; Burnley, P. C. (October 26, 1989). \"A new self-organizing mechanism for deep-focus earthquakes.\" <i>Nature<\/i> <b>341<\/b>(6244): 733\u2013737. doi:\u00a010.1038\/341733a0.[\/footnote]\r\n<h2>Earthquakes and Volcanic Activity<\/h2>\r\nEarthquakes 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.[footnote]Foxworthy and Hill (1982). <i>Volcanic Eruptions of 1980 at Mount St. Helens, The First 100\u00a0Days: USGS Professional Paper 1249<\/i>.[\/footnote] 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.[footnote]Watson, John; Watson, Kathie (January 7, 1998). \"<a class=\"external text\" href=\"http:\/\/pubs.usgs.gov\/gip\/earthq1\/volcano.html\" target=\"_blank\" rel=\"nofollow noopener\">Volcanoes and Earthquakes<\/a>.\"\u00a0United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">May 9,<\/span>2009<\/span>.[\/footnote]\r\n<h2>Rupture Dynamics<\/h2>\r\nA 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 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.[footnote]National Research Council (U.S.). Committee on the Science of Earthquakes (2003). \"5. Earthquake Physics and Fault-System Science.\"\u00a0<a class=\"external text\" href=\"http:\/\/www.nap.edu\/openbook.php?record_id=10493&amp;page=282\" target=\"_blank\" rel=\"nofollow noopener\"><i>Living on an Active Earth: Perspectives on Earthquake Science<\/i><\/a>. Washington D.C.: National Academies Press. p.\u00a0418.\u00a0<span class=\"reference-accessdate\">Retrieved <span class=\"nowrap\">8 July<\/span> 2010<\/span>.[\/footnote]\r\n\r\nRupture 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\u201390% 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 boom developed 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.[footnote]<em>Ibid.<\/em>[\/footnote]\r\n<h2>Earthquake Clusters<\/h2>\r\nMost earthquakes form part of a sequence, related to each other in terms of location and time.[footnote]\"What are Aftershocks, Foreshocks, and Earthquake Clusters?\"[\/footnote] 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.[footnote]\"<a class=\"external text\" href=\"http:\/\/earthquake.usgs.gov\/research\/parkfield\/repeat.php\" target=\"_blank\" rel=\"nofollow noopener\">Repeating Earthquakes<\/a>.\"\u00a0United States Geological Survey. January 29, 2009<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">May 11,<\/span> 2009<\/span>.[\/footnote]\r\n<h3>Aftershocks<\/h3>\r\nAn 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.[footnote]\"What are Aftershocks, Foreshocks, and Earthquake Clusters?\"[\/footnote]\r\n<h3>Earthquake Swarms<\/h3>\r\nEarthquake 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.[footnote]\"Earthquake Swarms at Yellowstone.\"\u00a0United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2008-09-15<\/span><\/span>.[\/footnote] In August 2012, a swarm of earthquakes shook Southern California's Imperial Valley, showing the most recorded activity in the area since the 1970s.[footnote]Duke, Alan. \"Quake 'swarm' shakes Southern California.\"\u00a0CNN<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">27 August<\/span> 2012<\/span>.[\/footnote]\r\n\r\nSometimes 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.[footnote]Amos Nur; Cline, Eric H. (2000). \"Poseidon's Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean\" (PDF). <i>Journal of Archaeological Science<\/i> <b>27<\/b> (1): 43\u201363.\u00a0See also\u00a0\"<a class=\"external text\" href=\"http:\/\/www.bbc.co.uk\/science\/horizon\/2003\/earthquakestorms.shtml\" target=\"_blank\" rel=\"nofollow noopener\">Earthquake Storms<\/a>.\"\u00a0<i>Horizon<\/i>. 1 April 2003<span class=\"reference-accessdate\">. Retrieved\u00a0<span class=\"nowrap\">2007-05-02<\/span><\/span>.[\/footnote]","rendered":"<p>The following video explains the cause of earthquakes.<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1280979\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/VSgB1IWr6O4?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<div class=\"textbox learning-objectives\">\n<h3>Overview of Elastic Rebound Theory<\/h3>\n<p id=\"x-ck12-MWUwNTM4ZWVkNjlmNjhlM2FiZGUyOGZhYjYxOWNmMTI.-9nq\">In an earthquake, the initial point where the rocks rupture in the crust is called the\u00a0<strong>focus<\/strong>. The epicenter is the point on the land surface that is directly above the focus. In about 75% of earthquakes, the focus is in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage because the focus is near where people live. However, it is the epicenter of an earthquake that is reported by scientists and the media (figure 1).<\/p>\n<div id=\"attachment_810\" style=\"width: 510px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-810\" class=\"wp-image-810 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/459\/2015\/06\/21062326\/Fig_9_2_1_2.jpg\" alt=\"Diagram showing the epicenter directly above the focus\" width=\"500\" height=\"388\" \/><\/p>\n<p id=\"caption-attachment-810\" class=\"wp-caption-text\">Figure 1. In the vertical cross section of crust, there are two features labeled\u2014the focus and the epicenter, which is directly above the focus.<\/p>\n<\/div>\n<p id=\"x-ck12-YjMwZjFkYTVkMTY1YWFiNGQ1ZTUxYzFjMGJlNzA1ZGE.-imk\">\n<\/div>\n<div id=\"attachment_2647\" style=\"width: 154px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2647\" class=\"wp-image-2647 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201711\/Fault_types.png\" alt=\"Stirke-slip, normal, and thrust\" width=\"144\" height=\"360\" \/><\/p>\n<p id=\"caption-attachment-2647\" class=\"wp-caption-text\">Figure 2. Fault types<\/p>\n<\/div>\n<p>Tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a <strong>fault plane<\/strong>. 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.<a class=\"footnote\" title=\"Ohnaka, M. (2013). The Physics of Rock Failure and Earthquakes. Cambridge University Press. p.\u00a0148.\" id=\"return-footnote-2018-1\" href=\"#footnote-2018-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a><\/p>\n<p>This energy is released as a combination of radiated elastic strain seismic 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&#8217;s total energy is radiated as seismic energy. Most of the earthquake&#8217;s energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth&#8217;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&#8217;s deep interior.<a class=\"footnote\" title=\"Spence, William; S. A. Sipkin; G. L. Choy (1989). &quot;Measuring the Size of an Earthquake.&quot; United States Geological Survey. Retrieved 2006-11-03.\" id=\"return-footnote-2018-2\" href=\"#footnote-2018-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a><\/p>\n<h2>Earthquake Fault Types<\/h2>\n<p>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.<\/p>\n<p>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 \u00d7 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.<\/p>\n<div id=\"attachment_2648\" style=\"width: 410px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2648\" class=\"wp-image-2648\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201744\/800px-Kluft-photo-Carrizo-Plain-Nov-2007-Img_0327.jpg\" alt=\"San Andreas Fault\" width=\"400\" height=\"267\" \/><\/p>\n<p id=\"caption-attachment-2648\" class=\"wp-caption-text\">Figure 3. Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles<\/p>\n<\/div>\n<p>This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures<a class=\"footnote\" title=\"Wyss, M. (1979). &quot;Estimating expectable maximum magnitude of earthquakes from fault dimensions&quot;. Geology 7 (7): 336\u2013340 doi:10.1130\/0091-7613(1979)7&lt;336:EMEMOE&gt;2.0.CO;2.\" id=\"return-footnote-2018-3\" href=\"#footnote-2018-3\" aria-label=\"Footnote 3\"><sup class=\"footnote\">[3]<\/sup><\/a> 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&#8217;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 degrees Celsius flow in response to stress; they do not rupture in earthquakes.<a class=\"footnote\" title=\"Sibson R. H. (1982) &quot;Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States,&quot; Bulletin of the Seismological Society of America, Vol 72, No. 1, pp. 151\u2013163. See also\u00a0Sibson, R. H. (2002) &quot;Geology of the crustal earthquake source,&quot; International handbook of earthquake and engineering seismology, Volume 1, Part 1, page 455, eds. W.H.K. Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger, Academic Press.\" id=\"return-footnote-2018-4\" href=\"#footnote-2018-4\" aria-label=\"Footnote 4\"><sup class=\"footnote\">[4]<\/sup><\/a> The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately 1000 km. 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.<\/p>\n<p>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.<a class=\"footnote\" title=\"&quot;Global Centroid Moment Tensor Catalog.&quot; Globalcmt.org. Retrieved 2011-07-24.\" id=\"return-footnote-2018-5\" href=\"#footnote-2018-5\" aria-label=\"Footnote 5\"><sup class=\"footnote\">[5]<\/sup><\/a> Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Japan, 2011; Alaska, 1964), making the most powerful earthquakes possible.<\/p>\n<p>Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust,<a class=\"footnote\" title=\"&quot;Instrumental California Earthquake Catalog.&quot; WGCEP. Retrieved 2011-07-24.\" id=\"return-footnote-2018-6\" href=\"#footnote-2018-6\" aria-label=\"Footnote 6\"><sup class=\"footnote\">[6]<\/sup><\/a> 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 6 km.<a class=\"footnote\" title=\"Hjaltad\u00f3ttir S., 2010, &quot;Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland.&quot;\u00a0See also &quot;Reports and publications | Seismicity | Icelandic Meteorological office.&quot; En.vedur.is. Retrieved 2011-07-24.\" id=\"return-footnote-2018-7\" href=\"#footnote-2018-7\" aria-label=\"Footnote 7\"><sup class=\"footnote\">[7]<\/sup><\/a><\/p>\n<p>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.<a class=\"footnote\" title=\"Schorlemmer, D.; Wiemer, S.; Wyss, M. (2005). &quot;Variations in earthquake-size distribution across different stress regimes.&quot;\u00a0Nature\u00a0437 (7058): 539\u2013542.\u00a0doi:10.1038\/nature04094.\" id=\"return-footnote-2018-8\" href=\"#footnote-2018-8\" aria-label=\"Footnote 8\"><sup class=\"footnote\">[8]<\/sup><\/a> This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that &#8220;pushes&#8221; 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 (<strong>greatest<\/strong> principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass &#8220;escapes&#8221; in the direction of the <strong>least<\/strong> 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.<\/p>\n<h2>Earthquakes away from Plate Boundaries<\/h2>\n<p>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 &#8220;Big bend&#8221; 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.<a class=\"footnote\" title=\"Talebian, M; Jackson, J (2004). &quot;A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran.&quot; Geophysical Journal International 156 (3): 506\u2013526.\u00a0doi:10.1111\/j.1365-246X.2004.02092.x.\" id=\"return-footnote-2018-9\" href=\"#footnote-2018-9\" aria-label=\"Footnote 9\"><sup class=\"footnote\">[9]<\/sup><\/a><\/p>\n<p>All tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading (e.g. deglaciation).<a class=\"footnote\" title=\"Nettles, M.; Ekstr\u00f6m, G. (May 2010). &quot;Glacial Earthquakes in Greenland and Antarctica.&quot; Annual Review of Earth and Planetary Sciences 38 (1): 467\u2013491.\u00a0doi:10.1146\/annurev-earth-040809-152414.\" id=\"return-footnote-2018-10\" href=\"#footnote-2018-10\" aria-label=\"Footnote 10\"><sup class=\"footnote\">[10]<\/sup><\/a> These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.<a class=\"footnote\" title=\"Noson, Qamar, and Thorsen (1988). Washington State Earthquake Hazards: Washington State Department of Natural Resources. Washington Division of Geology and Earth Resources Information Circular 85.\" id=\"return-footnote-2018-11\" href=\"#footnote-2018-11\" aria-label=\"Footnote 11\"><sup class=\"footnote\">[11]<\/sup><\/a><\/p>\n<h2>Shallow-Focus and Deep-Focus Earthquakes<\/h2>\n<div id=\"attachment_2649\" style=\"width: 310px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2649\" class=\"wp-image-2649\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/115\/2016\/06\/30201822\/HotelSanSalvador.jpg\" alt=\"collapse building\" width=\"300\" height=\"369\" \/><\/p>\n<p id=\"caption-attachment-2649\" class=\"wp-caption-text\">Figure 4. Collapsed Gran Hotel building in the San Salvador metropolis, after the shallow 1986 San Salvador earthquake.<\/p>\n<\/div>\n<p>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 <strong>shallow-focus<\/strong> earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed <strong>mid-focus<\/strong> or <strong>intermediate-depth<\/strong> earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, <strong>deep-focus<\/strong> earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).<a class=\"footnote\" title=\"&quot;M7.5 Northern Peru Earthquake of 26 September 2005&quot; (PDF).\u00a0National Earthquake Information Center. 17 October 2005. Retrieved2008-08-01.\" id=\"return-footnote-2018-12\" href=\"#footnote-2018-12\" aria-label=\"Footnote 12\"><sup class=\"footnote\">[12]<\/sup><\/a><\/p>\n<p>These seismically active areas of subduction are known as Wadati\u2013Benioff 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.<a class=\"footnote\" title=\"Greene II, H. W.; Burnley, P. C. (October 26, 1989). &quot;A new self-organizing mechanism for deep-focus earthquakes.&quot; Nature 341(6244): 733\u2013737. doi:\u00a010.1038\/341733a0.\" id=\"return-footnote-2018-13\" href=\"#footnote-2018-13\" aria-label=\"Footnote 13\"><sup class=\"footnote\">[13]<\/sup><\/a><\/p>\n<h2>Earthquakes and Volcanic Activity<\/h2>\n<p>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.<a class=\"footnote\" title=\"Foxworthy and Hill (1982). Volcanic Eruptions of 1980 at Mount St. Helens, The First 100\u00a0Days: USGS Professional Paper 1249.\" id=\"return-footnote-2018-14\" href=\"#footnote-2018-14\" aria-label=\"Footnote 14\"><sup class=\"footnote\">[14]<\/sup><\/a> 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.<a class=\"footnote\" title=\"Watson, John; Watson, Kathie (January 7, 1998). &quot;Volcanoes and Earthquakes.&quot;\u00a0United States Geological Survey. Retrieved May 9,2009.\" id=\"return-footnote-2018-15\" href=\"#footnote-2018-15\" aria-label=\"Footnote 15\"><sup class=\"footnote\">[15]<\/sup><\/a><\/p>\n<h2>Rupture Dynamics<\/h2>\n<p>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 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.<a class=\"footnote\" title=\"National Research Council (U.S.). Committee on the Science of Earthquakes (2003). &quot;5. Earthquake Physics and Fault-System Science.&quot;\u00a0Living on an Active Earth: Perspectives on Earthquake Science. Washington D.C.: National Academies Press. p.\u00a0418.\u00a0Retrieved 8 July 2010.\" id=\"return-footnote-2018-16\" href=\"#footnote-2018-16\" aria-label=\"Footnote 16\"><sup class=\"footnote\">[16]<\/sup><\/a><\/p>\n<p>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\u201390% 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 boom developed 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.<a class=\"footnote\" title=\"Ibid.\" id=\"return-footnote-2018-17\" href=\"#footnote-2018-17\" aria-label=\"Footnote 17\"><sup class=\"footnote\">[17]<\/sup><\/a><\/p>\n<h2>Earthquake Clusters<\/h2>\n<p>Most earthquakes form part of a sequence, related to each other in terms of location and time.<a class=\"footnote\" title=\"&quot;What are Aftershocks, Foreshocks, and Earthquake Clusters?&quot;\" id=\"return-footnote-2018-18\" href=\"#footnote-2018-18\" aria-label=\"Footnote 18\"><sup class=\"footnote\">[18]<\/sup><\/a> 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.<a class=\"footnote\" title=\"&quot;Repeating Earthquakes.&quot;\u00a0United States Geological Survey. January 29, 2009. Retrieved May 11, 2009.\" id=\"return-footnote-2018-19\" href=\"#footnote-2018-19\" aria-label=\"Footnote 19\"><sup class=\"footnote\">[19]<\/sup><\/a><\/p>\n<h3>Aftershocks<\/h3>\n<p>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.<a class=\"footnote\" title=\"&quot;What are Aftershocks, Foreshocks, and Earthquake Clusters?&quot;\" id=\"return-footnote-2018-20\" href=\"#footnote-2018-20\" aria-label=\"Footnote 20\"><sup class=\"footnote\">[20]<\/sup><\/a><\/p>\n<h3>Earthquake Swarms<\/h3>\n<p>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.<a class=\"footnote\" title=\"&quot;Earthquake Swarms at Yellowstone.&quot;\u00a0United States Geological Survey. Retrieved 2008-09-15.\" id=\"return-footnote-2018-21\" href=\"#footnote-2018-21\" aria-label=\"Footnote 21\"><sup class=\"footnote\">[21]<\/sup><\/a> In August 2012, a swarm of earthquakes shook Southern California&#8217;s Imperial Valley, showing the most recorded activity in the area since the 1970s.<a class=\"footnote\" title=\"Duke, Alan. &quot;Quake 'swarm' shakes Southern California.&quot;\u00a0CNN. Retrieved 27 August 2012.\" id=\"return-footnote-2018-22\" href=\"#footnote-2018-22\" aria-label=\"Footnote 22\"><sup class=\"footnote\">[22]<\/sup><\/a><\/p>\n<p>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.<a class=\"footnote\" title=\"Amos Nur; Cline, Eric H. (2000). &quot;Poseidon's Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean&quot; (PDF). Journal of Archaeological Science 27 (1): 43\u201363.\u00a0See also\u00a0&quot;Earthquake Storms.&quot;\u00a0Horizon. 1 April 2003. Retrieved\u00a02007-05-02.\" id=\"return-footnote-2018-23\" href=\"#footnote-2018-23\" aria-label=\"Footnote 23\"><sup class=\"footnote\">[23]<\/sup><\/a><\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2018\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>7.2: The Nature of Earthquakes. <strong>Provided by<\/strong>: CK-12. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/www.ck12.org\/book\/CK-12-Earth-Science-For-High-School\/section\/7.2\/\">http:\/\/www.ck12.org\/book\/CK-12-Earth-Science-For-High-School\/section\/7.2\/<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc\/4.0\/\">CC BY-NC: Attribution-NonCommercial<\/a><\/em><\/li><li>Naturally occurring earthquakes. <strong>Provided by<\/strong>: Wikipedia. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/en.wikipedia.org\/wiki\/Earthquake#Naturally_occurring_earthquakes\">https:\/\/en.wikipedia.org\/wiki\/Earthquake#Naturally_occurring_earthquakes<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>Earthquakes 101. <strong>Authored by<\/strong>: National Geographic. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/VSgB1IWr6O4\">https:\/\/youtu.be\/VSgB1IWr6O4<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-2018-1\">Ohnaka, M. (2013). <a class=\"external text\" href=\"https:\/\/books.google.com\/books?id=Bp0gAwAAQBAJ&amp;pg=PA234\" target=\"_blank\" rel=\"nofollow noopener\"><i>The Physics of Rock Failure and Earthquakes<\/i><\/a>. Cambridge University Press. p.\u00a0148. <a href=\"#return-footnote-2018-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-2018-2\">Spence, William; S. A. Sipkin; G. L. Choy (1989). \"Measuring the Size of an Earthquake.\" United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2006-11-03<\/span><\/span>. <a href=\"#return-footnote-2018-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><li id=\"footnote-2018-3\">Wyss, M. (1979). \"Estimating expectable maximum magnitude of earthquakes from fault dimensions\". <i>Geology<\/i> <b>7<\/b> (7): 336\u2013340 doi:<a class=\"external text\" href=\"https:\/\/dx.doi.org\/10.1130%2F0091-7613%281979%297%3C336%3AEMEMOE%3E2.0.CO%3B2\" rel=\"nofollow\">10.1130\/0091-7613(1979)7&lt;336:EMEMOE&gt;2.0.CO;2<\/a>. <a href=\"#return-footnote-2018-3\" class=\"return-footnote\" aria-label=\"Return to footnote 3\">&crarr;<\/a><\/li><li id=\"footnote-2018-4\"><span class=\"reference-text\">Sibson R. H. (1982) \"Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States,\" <em>Bulletin of the Seismological Society of America<\/em>, Vol 72, No. 1, pp. 151\u2013163. See also\u00a0<\/span><span class=\"reference-text\">Sibson, R. H. (2002) \"Geology of the crustal earthquake source,\" <em>International handbook of earthquake and engineering seismology<\/em>, Volume 1, Part 1, page 455, eds. W.H.K. Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger, Academic Press.<\/span> <a href=\"#return-footnote-2018-4\" class=\"return-footnote\" aria-label=\"Return to footnote 4\">&crarr;<\/a><\/li><li id=\"footnote-2018-5\">\"<a class=\"external text\" href=\"http:\/\/www.globalcmt.org\/CMTsearch.html\" target=\"_blank\" rel=\"nofollow noopener\">Global Centroid Moment Tensor Catalog<\/a>.\" Globalcmt.org<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2011-07-24<\/span><\/span>. <a href=\"#return-footnote-2018-5\" class=\"return-footnote\" aria-label=\"Return to footnote 5\">&crarr;<\/a><\/li><li id=\"footnote-2018-6\">\"Instrumental California Earthquake Catalog.\" WGCEP. Retrieved 2011-07-24. <a href=\"#return-footnote-2018-6\" class=\"return-footnote\" aria-label=\"Return to footnote 6\">&crarr;<\/a><\/li><li id=\"footnote-2018-7\">Hjaltad\u00f3ttir S., 2010, \"Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland.\"\u00a0See also \"Reports and publications | Seismicity | Icelandic Meteorological office.\" En.vedur.is. Retrieved 2011-07-24. <a href=\"#return-footnote-2018-7\" class=\"return-footnote\" aria-label=\"Return to footnote 7\">&crarr;<\/a><\/li><li id=\"footnote-2018-8\">Schorlemmer, D.; Wiemer, S.; Wyss, M. (2005). \"Variations in earthquake-size distribution across different stress regimes.\"\u00a0<i>Nature\u00a0<\/i><b>437<\/b> (7058): 539\u2013542.\u00a0doi:10.1038\/nature04094. <a href=\"#return-footnote-2018-8\" class=\"return-footnote\" aria-label=\"Return to footnote 8\">&crarr;<\/a><\/li><li id=\"footnote-2018-9\">Talebian, M; Jackson, J (2004). \"A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran.\" <i>Geophysical Journal International<\/i> <b>156<\/b> (3): 506\u2013526.\u00a0doi:10.1111\/j.1365-246X.2004.02092.x. <a href=\"#return-footnote-2018-9\" class=\"return-footnote\" aria-label=\"Return to footnote 9\">&crarr;<\/a><\/li><li id=\"footnote-2018-10\">Nettles, M.; Ekstr\u00f6m, G. (May 2010). \"Glacial Earthquakes in Greenland and Antarctica.\" <i>Annual Review of Earth and Planetary Sciences<\/i> <b>38<\/b> (1): 467\u2013491.\u00a0doi:10.1146\/annurev-earth-040809-152414. <a href=\"#return-footnote-2018-10\" class=\"return-footnote\" aria-label=\"Return to footnote 10\">&crarr;<\/a><\/li><li id=\"footnote-2018-11\">Noson, Qamar, and Thorsen (1988). <i>Washington State Earthquake Hazards: Washington State Department of Natural Resources<\/i>. Washington Division of Geology and Earth Resources Information Circular 85. <a href=\"#return-footnote-2018-11\" class=\"return-footnote\" aria-label=\"Return to footnote 11\">&crarr;<\/a><\/li><li id=\"footnote-2018-12\">\"M7.5 Northern Peru Earthquake of 26 September 2005\" (PDF).\u00a0National Earthquake Information Center. 17 October 2005<span class=\"reference-accessdate\">. Retrieved<span class=\"nowrap\">2008-08-01<\/span><\/span>. <a href=\"#return-footnote-2018-12\" class=\"return-footnote\" aria-label=\"Return to footnote 12\">&crarr;<\/a><\/li><li id=\"footnote-2018-13\">Greene II, H. W.; Burnley, P. C. (October 26, 1989). \"A new self-organizing mechanism for deep-focus earthquakes.\" <i>Nature<\/i> <b>341<\/b>(6244): 733\u2013737. doi:\u00a010.1038\/341733a0. <a href=\"#return-footnote-2018-13\" class=\"return-footnote\" aria-label=\"Return to footnote 13\">&crarr;<\/a><\/li><li id=\"footnote-2018-14\">Foxworthy and Hill (1982). <i>Volcanic Eruptions of 1980 at Mount St. Helens, The First 100\u00a0Days: USGS Professional Paper 1249<\/i>. <a href=\"#return-footnote-2018-14\" class=\"return-footnote\" aria-label=\"Return to footnote 14\">&crarr;<\/a><\/li><li id=\"footnote-2018-15\">Watson, John; Watson, Kathie (January 7, 1998). \"<a class=\"external text\" href=\"http:\/\/pubs.usgs.gov\/gip\/earthq1\/volcano.html\" target=\"_blank\" rel=\"nofollow noopener\">Volcanoes and Earthquakes<\/a>.\"\u00a0United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">May 9,<\/span>2009<\/span>. <a href=\"#return-footnote-2018-15\" class=\"return-footnote\" aria-label=\"Return to footnote 15\">&crarr;<\/a><\/li><li id=\"footnote-2018-16\">National Research Council (U.S.). Committee on the Science of Earthquakes (2003). \"5. Earthquake Physics and Fault-System Science.\"\u00a0<a class=\"external text\" href=\"http:\/\/www.nap.edu\/openbook.php?record_id=10493&amp;page=282\" target=\"_blank\" rel=\"nofollow noopener\"><i>Living on an Active Earth: Perspectives on Earthquake Science<\/i><\/a>. Washington D.C.: National Academies Press. p.\u00a0418.\u00a0<span class=\"reference-accessdate\">Retrieved <span class=\"nowrap\">8 July<\/span> 2010<\/span>. <a href=\"#return-footnote-2018-16\" class=\"return-footnote\" aria-label=\"Return to footnote 16\">&crarr;<\/a><\/li><li id=\"footnote-2018-17\"><em>Ibid.<\/em> <a href=\"#return-footnote-2018-17\" class=\"return-footnote\" aria-label=\"Return to footnote 17\">&crarr;<\/a><\/li><li id=\"footnote-2018-18\">\"What are Aftershocks, Foreshocks, and Earthquake Clusters?\" <a href=\"#return-footnote-2018-18\" class=\"return-footnote\" aria-label=\"Return to footnote 18\">&crarr;<\/a><\/li><li id=\"footnote-2018-19\">\"<a class=\"external text\" href=\"http:\/\/earthquake.usgs.gov\/research\/parkfield\/repeat.php\" target=\"_blank\" rel=\"nofollow noopener\">Repeating Earthquakes<\/a>.\"\u00a0United States Geological Survey. January 29, 2009<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">May 11,<\/span> 2009<\/span>. <a href=\"#return-footnote-2018-19\" class=\"return-footnote\" aria-label=\"Return to footnote 19\">&crarr;<\/a><\/li><li id=\"footnote-2018-20\">\"What are Aftershocks, Foreshocks, and Earthquake Clusters?\" <a href=\"#return-footnote-2018-20\" class=\"return-footnote\" aria-label=\"Return to footnote 20\">&crarr;<\/a><\/li><li id=\"footnote-2018-21\">\"Earthquake Swarms at Yellowstone.\"\u00a0United States Geological Survey<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">2008-09-15<\/span><\/span>. <a href=\"#return-footnote-2018-21\" class=\"return-footnote\" aria-label=\"Return to footnote 21\">&crarr;<\/a><\/li><li id=\"footnote-2018-22\">Duke, Alan. \"Quake 'swarm' shakes Southern California.\"\u00a0CNN<span class=\"reference-accessdate\">. Retrieved <span class=\"nowrap\">27 August<\/span> 2012<\/span>. <a href=\"#return-footnote-2018-22\" class=\"return-footnote\" aria-label=\"Return to footnote 22\">&crarr;<\/a><\/li><li id=\"footnote-2018-23\">Amos Nur; Cline, Eric H. (2000). \"Poseidon's Horses: Plate Tectonics and Earthquake Storms in the Late Bronze Age Aegean and Eastern Mediterranean\" (PDF). <i>Journal of Archaeological Science<\/i> <b>27<\/b> (1): 43\u201363.\u00a0See also\u00a0\"<a class=\"external text\" href=\"http:\/\/www.bbc.co.uk\/science\/horizon\/2003\/earthquakestorms.shtml\" target=\"_blank\" rel=\"nofollow noopener\">Earthquake Storms<\/a>.\"\u00a0<i>Horizon<\/i>. 1 April 2003<span class=\"reference-accessdate\">. Retrieved\u00a0<span class=\"nowrap\">2007-05-02<\/span><\/span>. <a href=\"#return-footnote-2018-23\" class=\"return-footnote\" aria-label=\"Return to footnote 23\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":4,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"7.2: The Nature of Earthquakes\",\"author\":\"\",\"organization\":\"CK-12\",\"url\":\"http:\/\/www.ck12.org\/book\/CK-12-Earth-Science-For-High-School\/section\/7.2\/\",\"project\":\"\",\"license\":\"cc-by-nc\",\"license_terms\":\"\"},{\"type\":\"copyrighted_video\",\"description\":\"Earthquakes 101\",\"author\":\"National Geographic\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/VSgB1IWr6O4\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"},{\"type\":\"cc\",\"description\":\"Naturally occurring earthquakes\",\"author\":\"\",\"organization\":\"Wikipedia\",\"url\":\"https:\/\/en.wikipedia.org\/wiki\/Earthquake#Naturally_occurring_earthquakes\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"8a46da13-6d8c-438a-8758-aa0934768102, 2e3c9e0b-5c7d-423f-9172-9c9236d635eb","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2018","chapter","type-chapter","status-publish","hentry"],"part":29,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapters\/2018","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":14,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapters\/2018\/revisions"}],"predecessor-version":[{"id":3422,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapters\/2018\/revisions\/3422"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/parts\/29"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapters\/2018\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/wp\/v2\/media?parent=2018"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/pressbooks\/v2\/chapter-type?post=2018"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/wp\/v2\/contributor?post=2018"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/geo\/wp-json\/wp\/v2\/license?post=2018"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}