102-568: The Richter scale ( / ˈ r ɪ k t ər / ), also called the Richter magnitude scale , Richter's magnitude scale , and the Gutenberg–Richter scale , is a measure of the strength of earthquakes , developed by Charles Richter in collaboration with Beno Gutenberg , and presented in Richter's landmark 1935 paper, where he called it the "magnitude scale". This was later revised and renamed
204-496: A hypocenter or focus , the point where an earthquake or an underground explosion originates. The primary purpose of a seismometer is to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' or magnitude must be calculated after the precise location is known. The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake. The Chinese frog seismograph would have dropped its ball in
306-489: A quake , tremor , or temblor – is the shaking of the Earth 's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves . Earthquakes can range in intensity , from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities. The seismic activity of an area
408-729: A tsunami . Earthquakes can trigger landslides . Earthquakes' occurrence is influenced by tectonic movements along faults, including normal, reverse (thrust), and strike-slip faults, with energy release and rupture dynamics governed by the elastic-rebound theory . Efforts to manage earthquake risks involve prediction, forecasting, and preparedness, including seismic retrofitting and earthquake engineering to design structures that withstand shaking. The cultural impact of earthquakes spans myths, religious beliefs, and modern media, reflecting their profound influence on human societies. Similar seismic phenomena, known as marsquakes and moonquakes , have been observed on other celestial bodies, indicating
510-571: A depth of less than 70 km (43 mi) are classified as "shallow-focus" earthquakes, while those with a focal depth between 70 and 300 km (43 and 186 mi) are commonly termed "mid-focus" or "intermediate-depth" earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 to 700 km (190 to 430 mi)). These seismically active areas of subduction are known as Wadati–Benioff zones . Deep-focus earthquakes occur at
612-488: 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. 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
714-407: A distance of 100 km (62 mi)) a maximum amplitude of 1 micron (1 μm, or 0.001 millimeters) on a seismogram recorded by a Wood-Anderson torsion seismometer. Finally, Richter calculated a table of distance corrections, in that for distances less than 200 kilometers the attenuation is strongly affected by the structure and properties of the regional geology. When Richter presented
816-475: A few exceptions to this: 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 earthquake ruptures travel at unusually low velocities. A particularly dangerous form of slow earthquake
918-470: A few) and can vary widely. Millions of minor earthquakes occur every year worldwide, equating to hundreds every hour every day. On the other hand, earthquakes of magnitude ≥8.0 occur about once a year, on average. The largest recorded earthquake was the Great Chilean earthquake of May 22, 1960, which had a magnitude of 9.5 on the moment magnitude scale . Seismologist Susan Hough has suggested that
1020-439: A future event because intensity and ground effects depend not only on the magnitude but also on (1) the distance to the epicenter, (2) the depth of the earthquake's focus beneath the epicenter, (3) the location of the epicenter, and (4) geological conditions . ( Based on U.S. Geological Survey documents. ) The intensity and death toll depend on several factors (earthquake depth, epicenter location, and population density, to name
1122-487: A macroseismic epicenter can be given. The word is derived from the Neo-Latin noun epicentrum , the latinisation of the ancient Greek adjective ἐπίκεντρος ( epikentros ), "occupying a cardinal point, situated on a centre", from ἐπί ( epi ) "on, upon, at" and κέντρον ( kentron ) " centre ". The term was coined by Irish seismologist Robert Mallet . It is also used to mean "center of activity", as in "Travel
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#17327655615611224-707: A magnitude 10 quake may represent a very approximate upper limit for what the Earth's tectonic zones are capable of, which would be the result of the largest known continuous belt of faults rupturing together (along the Pacific coast of the Americas). A research at the Tohoku University in Japan found that a magnitude 10 earthquake was theoretically possible if a combined 3,000 kilometres (1,900 mi) of faults from
1326-463: A magnitude of zero to be around the limit of human perceptibility. Third, he specified the Wood–Anderson seismograph as the standard instrument for producing seismograms. Magnitude was then defined as "the logarithm of the maximum trace amplitude, expressed in microns ", measured at a distance of 100 km (62 mi). The scale was calibrated by defining a magnitude 0 shock as one that produces (at
1428-413: A minimum of three seismometers. Most likely, there are many, forming a seismic array. The emphasis is on precision since much can be learned about the fault mechanics and seismic hazard , if the locations can be determined to be within a kilometer or two, for small earthquakes. For this, computer programs use an iterative process, involving a 'guess and correction' algorithm. As well, a very good model of
1530-560: A particular location in the Earth is the average rate of seismic energy release per unit volume. One of the most devastating earthquakes in recorded history was the 1556 Shaanxi earthquake , which occurred on 23 January 1556 in Shaanxi , China. More than 830,000 people died. Most houses in the area were yaodongs —dwellings carved out of loess hillsides—and many victims were killed when these structures collapsed. The 1976 Tangshan earthquake , which killed between 240,000 and 655,000 people,
1632-765: A single rupture) are approximately 1,000 km (620 mi). 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
1734-625: A wider area, depending on the local geology.) In 1883, John Milne surmised that the shaking of large earthquakes might generate waves detectable around the globe, and in 1899 E. Von Rehbur Paschvitz observed in Germany seismic waves attributable to an earthquake in Tokyo . In the 1920s, Harry O. Wood and John A. Anderson developed the Wood–Anderson seismograph , one of the first practical instruments for recording seismic waves. Wood then built, under
1836-502: A yardstick to measure the extent of the event. The resulting effective upper limit of measurement for M L is about 7 and about 8.5 for M s . New techniques to avoid the saturation problem and to measure magnitudes rapidly for very large earthquakes are being developed. One of these is based on the long-period P-wave; The other is based on a recently discovered channel wave. The energy release of an earthquake, which closely correlates to its destructive power, scales with
1938-525: 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. 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
2040-410: Is called the hypocenter or focus, while the ground level directly above it is the epicenter . Earthquakes are primarily caused by geological faults , but also by volcanic activity , landslides, and other seismic events. The frequency, type, and size of earthquakes in an area define its seismic activity, reflecting the average rate of seismic energy release. Significant historical earthquakes include
2142-473: 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. 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
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#17327655615612244-470: Is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake. The original formula is: where A is the maximum excursion of the Wood-Anderson seismograph , the empirical function A 0 depends only on the epicentral distance of
2346-400: Is divided into 754 Flinn–Engdahl regions (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions. Standard reporting of earthquakes includes its magnitude , date and time of occurrence, geographic coordinates of its epicenter , depth of
2448-545: 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
2550-423: Is proportional to the area of the fault that ruptures and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger 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,
2652-517: Is restricted in the Chinese province thought to be the epicentre of the SARS outbreak." Garner's Modern American Usage gives several examples of use in which "epicenter" is used to mean "center". Garner also refers to a William Safire article in which Safire quotes a geophysicist as attributing the use of the term to "spurious erudition on the part of writers combined with scientific illiteracy on
2754-410: 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 . During an earthquake, high temperatures can develop at the fault plane, increasing pore pressure and consequently vaporization of the groundwater already contained within
2856-467: Is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume. In its most general sense, the word earthquake is used to describe any seismic event that generates seismic waves. Earthquakes can occur naturally or be induced by human activities, such as mining , fracking , and nuclear tests . The initial point of rupture
2958-548: The 3 ⁄ 2 power of the shaking amplitude (see Moment magnitude scale for an explanation). Thus, a difference in magnitude of 1.0 is equivalent to a factor of 31.6 ( = ( 10 1.0 ) ( 3 / 2 ) {\displaystyle =({10^{1.0}})^{(3/2)}} ) in the energy released; a difference in magnitude of 2.0 is equivalent to a factor of 1000 ( = ( 10 2.0 ) ( 3 / 2 ) {\displaystyle =({10^{2.0}})^{(3/2)}} ) in
3060-509: The 1556 Shaanxi earthquake in China, with over 830,000 fatalities, and the 1960 Valdivia earthquake in Chile, the largest ever recorded at 9.5 magnitude. Earthquakes result in various effects, such as ground shaking and soil liquefaction , leading to significant damage and loss of life. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause
3162-400: 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. A tectonic earthquake begins as an area of initial slip on the fault surface that forms
Richter scale - Misplaced Pages Continue
3264-587: The 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history. Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often create tsunamis that can devastate communities thousands of kilometers away. Regions most at risk for great loss of life include those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes. Tectonic earthquakes occur anywhere on
3366-499: 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. 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
3468-534: The Japan Trench to the Kuril–Kamchatka Trench ruptured together and moved by 60 metres (200 ft) (or if a similar large-scale rupture occurred elsewhere). Such an earthquake would cause ground motions for up to an hour, with tsunamis hitting shores while the ground is still shaking, and if this kind of earthquake occurred, it would probably be a 1-in-10,000-year event. Prior to the development of
3570-448: The S wave . Knowing the relative 'velocities of propagation', it was a simple matter to calculate the distance of the earthquake. One seismograph would give the distance, but that could be plotted as a circle, with an infinite number of possibilities. Two seismographs would give two intersecting circles, with two possible locations. Only with a third seismograph would there be a precise location. Modern earthquake location still requires
3672-426: 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. In most cases, the rupture speed approaches, but does not exceed, the shear wave (S wave) velocity of the surrounding rock. There are
3774-409: The displacements were plotted on a moving graph, driven by a clock mechanism. This was the first seismogram , which allowed precise timing of the first ground motion , and an accurate plot of subsequent motions. From the first seismograms, as seen in the figure, it was noticed that the trace was divided into two major portions. The first seismic wave to arrive was the P wave , followed closely by
3876-413: 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. For every unit increase in magnitude, there is a roughly thirty-fold increase in
3978-422: The local magnitude scale , denoted as ML or M L . Because of various shortcomings of the original M L scale, most seismological authorities now use other similar scales such as the moment magnitude scale (M w ) to report earthquake magnitudes, but much of the news media still erroneously refers to these as "Richter" magnitudes. All magnitude scales retain the logarithmic character of
4080-465: The surface-wave magnitude (M S ) and body wave magnitude (M B ) scales. The Richter scale was defined in 1935 for particular circumstances and instruments; the particular circumstances refer to it being defined for Southern California and "implicitly incorporates the attenuative properties of Southern California crust and mantle." The particular instrument used would become saturated by strong earthquakes and unable to record high values. The scale
4182-573: The "Richter scale",, especially the local magnitude M L and the surface wave M s scale. In addition, the body wave magnitude , mb , and the moment magnitude , M w , abbreviated MMS, have been widely used for decades. A couple of new techniques to measure magnitude are in the development stage by seismologists. All magnitude scales have been designed to give numerically similar results. This goal has been achieved well for M L , M s , and M w . The mb scale gives somewhat different values than
Richter scale - Misplaced Pages Continue
4284-553: The 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East. 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,
4386-433: The 21st century. Seismic waves travel through the Earth's interior and can be recorded by seismometers at great distances. The surface-wave magnitude was developed in the 1950s as a means to measure remote earthquakes and to improve the accuracy for larger events. The moment magnitude scale not only measures the amplitude of the shock but also takes into account the seismic moment (total rupture area, average slip of
4488-399: The Earth's core was located in 1913 by Beno Gutenberg . S waves and later arriving surface waves do most of the damage compared to P waves. P waves squeeze and expand the material in the same direction they are traveling, whereas S waves shake the ground up and down and back and forth. Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world
4590-484: The Earth. Also, the depth of the hypocenter can be computed roughly. P wave speed S waves speed As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle. On average, the kilometer distance to the earthquake is the number of seconds between the P- and S wave times 8. Slight deviations are caused by inhomogeneities of subsurface structure. By such analysis of seismograms,
4692-450: The M s scale. A spectral analysis is required to obtain M 0 . In contrast, the other magnitudes are derived from a simple measurement of the amplitude of a precisely defined wave. All scales, except M w , saturate for large earthquakes, meaning they are based on the amplitudes of waves that have a wavelength shorter than the rupture length of the earthquakes. These short waves (high-frequency waves) are too short
4794-573: 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
4896-453: The amount of energy released, and each increase of 0.2 corresponds to approximately a doubling of the energy released. Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in the world, so long as its sensors are not located in the earthquake's shadow . The following describes the typical effects of earthquakes of various magnitudes near the epicenter. The values are typical and may not be exact in
4998-544: The auspices of the California Institute of Technology and the Carnegie Institute , a network of seismographs stretching across Southern California . He also recruited the young and unknown Charles Richter to measure the seismograms and locate the earthquakes generating the seismic waves. In 1931, Kiyoo Wadati showed how he had measured, for several strong earthquakes in Japan, the amplitude of
5100-404: 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
5202-404: The brittle crust. Thus, earthquakes with magnitudes much larger than 8 are not possible. 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
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#17327655615615304-512: 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. The instrumental scales used to describe the size of an earthquake began with the Richter scale in the 1930s. It is a relatively simple measurement of an event's amplitude, and its use has become minimal in
5406-529: 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 50–100 km (31–62 mi) (such as in Japan, 2011 , or in Alaska, 1964 ), making the most powerful earthquakes possible. The majority of tectonic earthquakes originate in the Ring of Fire at depths not exceeding tens of kilometers. Earthquakes occurring at
5508-678: 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 300 °C (572 °F) flow in response to stress; they do not rupture in earthquakes. The maximum observed lengths of ruptures and mapped faults (which may break in
5610-439: The distance from the earthquake and the underlying rock or soil makeup. The first scale for measuring earthquake magnitudes was developed by Charles Francis Richter in 1935. Subsequent scales ( seismic magnitude scales ) have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking and a 32-fold difference in energy. Subsequent scales are also adjusted to have approximately
5712-554: 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 40,000-kilometre-long (25,000 mi), 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
5814-403: 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
5916-426: 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
6018-533: The energy released by an earthquake; another scale, the Mercalli intensity scale , classifies earthquakes by their effects , from detectable by instruments but not noticeable, to catastrophic. The energy and effects are not necessarily strongly correlated; a shallow earthquake in a populated area with soil of certain types can be far more intense in impact than a much more energetic deep earthquake in an isolated area. Several scales have been historically described as
6120-504: The energy released. For instance, an earthquake of magnitude 6.0 releases approximately 32 times more energy than a 5.0 magnitude earthquake and a 7.0 magnitude earthquake releases 1,000 times more energy than 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,
6222-896: The energy released. The elastic energy radiated is best derived from an integration of the radiated spectrum, but an estimate can be based on mb because most energy is carried by the high-frequency waves. These formulae for Richter magnitude M L {\displaystyle \ M_{\mathsf {L}}\ } are alternatives to using Richter correlation tables based on Richter standard seismic event ( M L = 0 , {\displaystyle {\big (}\ M_{\mathsf {L}}=0\ ,} A = 0.001 m m , {\displaystyle \ A=0.001\ {\mathsf {mm}}\ ,} D = 100 k m ) . {\displaystyle \ D=100\ {\mathsf {km}}\ {\big )}~.} In
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#17327655615616324-602: The entire rupture zone. As an example, in the magnitude 7.9 Denali earthquake of 2002 in Alaska , the epicenter was at the western end of the rupture, but the greatest damage was about 330 km (210 mi) away at the eastern end. Focal depths of earthquakes occurring in continental crust mostly range from 2 to 20 kilometers (1.2 to 12.4 mi). Continental earthquakes below 20 km (12 mi) are rare whereas in subduction zone earthquakes can originate at depths deeper than 600 km (370 mi). During an earthquake, seismic waves propagates in all directions from
6426-470: The epicenter, geographical region, distances to population centers, location uncertainty, several parameters that are included in USGS earthquake reports (number of stations reporting, number of observations, etc.), and a unique event ID. Epicenter The epicenter ( / ˈ ɛ p ɪ ˌ s ɛ n t ər / ), epicentre , or epicentrum in seismology is the point on the Earth 's surface directly above
6528-458: 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 . 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
6630-462: 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
6732-411: 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
6834-418: 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
6936-486: The fault, and rigidity of the rock). The Japan Meteorological Agency seismic intensity scale , the Medvedev–Sponheuer–Karnik scale , and the Mercalli intensity scale are based on the observed effects and are related to the intensity of shaking. The shaking of the earth is a common phenomenon that has been experienced by humans from the earliest of times. Before the development of strong-motion accelerometers,
7038-416: The focus and then expands along the fault surface. The rupture stops where the stresses become insufficient to continue breaking the fault (because the rocks are stronger) or where the rupture enters ductile material. The magnitude of an earthquake is related to the total area of its fault rupture. Most earthquakes are small, with rupture dimensions less than the depth of the focus so the rupture doesn't break
7140-417: 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
7242-406: 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
7344-685: The formulas below, Δ {\displaystyle \ \Delta \ } is the epicentral distance in kilometers , and Δ ∘ {\displaystyle \ \Delta ^{\circ }\ } is the same distance represented as sea level great circle degrees. The Lillie empirical formula is: Lahr's empirical formula proposal is: and The Bisztricsany empirical formula (1958) for epicentre distances between 4° and 160° is: The Tsumura empirical formula is: The Tsuboi (University of Tokyo) empirical formula is: Earthquake An earthquake – also called
7446-400: The general compass direction of the earthquake, assuming a strong positive pulse. We now know that first motions can be in almost any direction depending on the type of initiating rupture ( focal mechanism ). The first refinement that allowed a more precise determination of the location was the use of a time scale. Instead of merely noting, or recording, the absolute motions of a pendulum ,
7548-453: The hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the planet's liquid outer core refracts the longitudinal or compressional ( P waves ) while it absorbs the transverse or shear waves ( S waves ). Outside the seismic shadow zone, both types of wave can be detected, but because of their different velocities and paths through the Earth, they arrive at different times. By measuring
7650-410: The intensity of a seismic event was estimated based on the observed effects. Magnitude and intensity are not directly related and calculated using different methods. The magnitude of an earthquake is a single value that describes the size of the earthquake at its source. Intensity is the measure of shaking at different locations around the earthquake. Intensity values vary from place to place, depending on
7752-476: The lengths along subducting plate margins, and those along normal faults are even shorter. 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
7854-496: The local crustal velocity structure is required: seismic velocities vary with the local geology. For P waves, the relation between velocity and bulk density of the medium has been quantified in Gardner's relation . Before the instrumental period of earthquake observation, the epicenter was thought to be the location where the greatest damage occurred, but the subsurface fault rupture may be long and spread surface damage across
7956-478: The magnitude scale, the only measure of an earthquake's strength or "size" was a subjective assessment of the intensity of shaking observed near the epicenter of the earthquake, categorized by various seismic intensity scales such as the Rossi–Forel scale . ("Size" is used in the sense of the quantity of energy released, not the size of the area affected by shaking, though higher-energy earthquakes do tend to affect
8058-402: The main causes of these aftershocks, along with the crust around the ruptured fault plane as it adjusts to the effects of the mainshock. 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. If an aftershock is larger than
8160-414: 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 plane adjusts to the effects of the mainshock. 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 aftershocks by
8262-407: The moment magnitude scale (MMS) is most common, although M s is also reported frequently. The seismic moment , M 0 , is proportional to the area of the rupture times the average slip that took place in the earthquake, thus it measures the physical size of the event. M w is derived from it empirically as a quantity without units, just a number designed to conform to
8364-426: The original and are scaled to have roughly comparable numeric values (typically in the middle of the scale). Due to the variance in earthquakes, it is essential to understand the Richter scale uses common logarithms simply to make the measurements manageable (i.e., a magnitude 3 quake factors 10³ while a magnitude 5 quake factors 10 and has seismometer readings 100 times larger). The Richter magnitude of an earthquake
8466-433: The other scales. The reason for so many different ways to measure the same thing is that at different distances, for different hypocentral depths, and for different earthquake sizes, the amplitudes of different types of elastic waves must be measured. M L is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories. For large earthquakes worldwide,
8568-425: The range of 2,000−10,000 km. Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using trilateration . Epicentral distance is also used in calculating seismic magnitudes as developed by Richter and Gutenberg . The point at which fault slipping begins is referred to as the focus of the earthquake. The fault rupture begins at
8670-433: The relative magnitudes of different earthquakes. Additional developments were required to produce a practical method of assigning an absolute measure of magnitude. First, to span the wide range of possible values, Richter adopted Gutenberg's suggestion of a logarithmic scale, where each step represents a tenfold increase of magnitude, similar to the magnitude scale used by astronomers for star brightness . Second, he wanted
8772-449: The resulting scale in 1935, he called it (at the suggestion of Harry Wood) simply a "magnitude" scale. "Richter magnitude" appears to have originated when Perry Byerly told the press that the scale was Richter's and "should be referred to as such." In 1956, Gutenberg and Richter, while still referring to "magnitude scale", labelled it "local magnitude", with the symbol M L , to distinguish it from two other scales they had developed,
8874-530: 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. From the point of view of the Mohr-Coulomb strength theory , an increase in fluid pressure reduces the normal stress acting on
8976-437: The rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, fracking and nuclear tests . An earthquake's point of initial rupture is called its hypocenter or focus. The epicenter is the point at ground level directly above the hypocenter. The seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time. The seismicity at
9078-522: The same numeric value within the limits of the scale. Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale", standard practice by most seismological authorities is to express an earthquake's strength on the moment magnitude scale, which is based on the actual energy released by an earthquake, the static seismic moment. Every earthquake produces different types of seismic waves, which travel through rock with different velocities: Propagation velocity of
9180-464: The seismic waves through solid rock ranges from approx. 3 km/s (1.9 mi/s) up to 13 km/s (8.1 mi/s), depending on the density and elasticity of the medium. In the Earth's interior, the shock- or P waves travel much faster than the S waves (approx. relation 1.7:1). The differences in travel time from the epicenter to the observatory are a measure of the distance and can be used to image both sources of earthquakes and structures within
9282-432: The shaking observed at various distances from the epicenter. He then plotted the logarithm of the amplitude against the distance and found a series of curves that showed a rough correlation with the estimated magnitudes of the earthquakes. Richter resolved some difficulties with this method and then, using data collected by his colleague Beno Gutenberg , he produced similar curves, confirming that they could be used to compare
9384-459: The station, δ {\displaystyle \delta } . In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the M L value. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude. In terms of energy, each whole number increase corresponds to an increase of about 31.6 times
9486-422: 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
9588-704: The surface, but in high magnitude, destructive earthquakes, surface breaks are common. Fault ruptures in large earthquakes can extend for more than 100 km (62 mi). When a fault ruptures unilaterally (with the epicenter at or near the end of the fault break) the waves are stronger in one direction along the fault. The macroseismic epicenter is the best estimate of the location of the epicenter derived without instrumental data. This may be estimated using intensity data, information about foreshocks and aftershocks, knowledge of local fault systems or extrapolations from data regarding similar earthquakes. For historical earthquakes that have not been instrumentally recorded, only
9690-490: The surrounding fracture networks; such an increase may trigger new faulting processes by reactivating adjacent faults, giving rise to aftershocks. Analogously, artificial pore pressure increase, by fluid injection in Earth's crust, may induce seismicity . Tides may trigger some seismicity . 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
9792-443: The thickness of the brittle layer is only about six kilometres (3.7 mi). 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
9894-406: The time difference on any seismograph and the distance on a travel-time graph on which the P wave and S wave have the same separation, geologists can calculate the distance to the quake's epicenter. This distance is called the epicentral distance , commonly measured in ° (degrees) and denoted as Δ (delta) in seismology. The Láska's empirical rule provides an approximation of epicentral distance in
9996-461: The total seismic moment released worldwide. 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 10 km (6.2 mi) within
10098-505: The universality of such events beyond Earth. An earthquake is the shaking of the surface of Earth resulting from a sudden release of energy in the lithosphere that creates seismic waves . Earthquakes may also be referred to as quakes , tremors , or temblors . The word tremor is also used for non-earthquake seismic rumbling . In its most general sense, an earthquake is any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused mostly by
10200-436: 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
10302-437: Was replaced in the 1970s by the moment magnitude scale (MMS, symbol M w ); for earthquakes adequately measured by the Richter scale, numerical values are approximately the same. Although values measured for earthquakes now are M w , they are frequently reported by the press as Richter values, even for earthquakes of magnitude over 8, when the Richter scale becomes meaningless. The Richter and MMS scales measure
10404-641: Was the deadliest of the 20th century. The 1960 Chilean earthquake is the largest earthquake that has been measured on a seismograph, reaching 9.5 magnitude on 22 May 1960. Its epicenter was near Cañete, Chile. The energy released was approximately twice that of the next most powerful earthquake, the Good Friday earthquake (27 March 1964), which was centered in Prince William Sound , Alaska. The ten largest recorded earthquakes have all been megathrust earthquakes ; however, of these ten, only
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