B.
Normal C.
Reverse Tectonic earthquakes occur anywhere on the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a
fault plane. The sides of a fault move past each other smoothly and
aseismically only if there are no irregularities or
asperities along the fault surface that increases the frictional resistance. Most fault surfaces do have such asperities, which 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. 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'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. Fault types 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 flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults (which may break in a single rupture) are approximately . Examples are the earthquakes in
Alaska (1957),
Chile (1960), and
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.
Normal faults Normal faults occur mainly in areas where the crust is being
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 .
Reverse faults Reverse faults occur in areas where the crust is being
shortened such as at a
convergent boundary. Reverse faults, particularly those along convergent boundaries, are associated with the most powerful earthquakes (called
megathrust earthquakes) including almost all of those of magnitude 8 or more. Megathrust earthquakes are responsible for about 90% of the total seismic moment released worldwide.
Strike-slip faults 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. Strike-slip faults, particularly continental
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 within the brittle crust. Thus, earthquakes with magnitudes much larger than 8 are not possible. , 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. 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. This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures 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. Thus, the width of the plane within the top brittle crust of the Earth can reach (such as in
Japan, 2011, or in
Alaska, 1964), making the most powerful earthquakes possible.
Focus 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 are classified as "shallow-focus" earthquakes, while those with focal depths between are commonly termed "mid-focus" or "intermediate-depth" earthquakes. In
subduction zones, where older and colder
oceanic crust descends beneath another tectonic plate,
deep-focus earthquakes may occur at much greater depths (ranging from ). These seismically active areas of subduction are known as
Wadati–Benioff zones. 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.
Volcanic activity 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. 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.
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.
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.
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. 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.
Clusters Most earthquakes form part of a sequence, related to each other in terms of location and time. 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. Earthquake clustering has been observed, for example, in Parkfield, California where a long-term research study is being conducted around the
Parkfield earthquake cluster. The quantitative analysis of these sequences is the primary focus of
statistical seismology.
Aftershocks and
October 2016 and
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, along with the crust around the ruptured
fault plane as it adjusts to the effects of the mainshock. In August 2012, a swarm of earthquakes shook
Southern California's
Imperial Valley, showing the most recorded activity in the area since the 1970s. 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, 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.
Frequency and tsunami took about 80,000 lives on December 28, 1908, in
Sicily and
Calabria. It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt. 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. Larger earthquakes occur less frequently, the relationship being
exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7–4.6 every year, an earthquake of 4.7–5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years. This is an example of the
Gutenberg–Richter law. The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The
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. In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend. More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey. 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. Most of the world's earthquakes (90%, and 81% of the largest) take place in the , 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. Massive earthquakes tend to occur along other plate boundaries too, such as along the
Himalayan Mountains. With the rapid growth of
mega-cities such as Mexico City, Tokyo, and Tehran in areas of high
seismic risk, some seismologists are warning that a single earthquake may claim the lives of up to three million people.
Induced seismicity While most earthquakes are caused by the movement of the Earth's
tectonic plates, 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. 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, and studies point to the state's oil industry as the cause of other earthquakes in the past century. A
Columbia University paper suggested that the 8.0 magnitude
2008 Sichuan earthquake was induced by loading from the
Zipingpu Dam, though the link has not been conclusively proved. ==Measurement and location==