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66-484: Magnitude scales measure the inherent force or strength of an earthquake – an event occurring at greater or lesser depth. (The " M w " scale is widely used.) The MM scale measures intensity of shaking, at any particular location, on the surface. It was developed from Giuseppe Mercalli 's Mercalli intensity scale of 1902. While shaking experienced at the surface is caused by the seismic energy released by an earthquake, earthquakes differ in how much of their energy

132-602: A peak ground acceleration for each degree. This became known as the "Mercalli–Cancani scale, formulated by Sieberg", or the "Mercalli–Cancani–Sieberg scale", or simply "MCS", and was used extensively in Europe and remains in use in Italy by the National Institute of Geophysics and Volcanology (INGV). When Harry O. Wood and Frank Neumann translated this into English in 1931 (along with modification and condensation of

198-498: A complex form of the Love wave which, although a surface wave, he found provided a result more closely related to the mb  scale than the M s   scale. Lg waves attenuate quickly along any oceanic path, but propagate well through the granitic continental crust, and Mb Lg is often used in areas of stable continental crust; it is especially useful for detecting underground nuclear explosions. Surface waves propagate along

264-401: A different scaling and zero point. K values in the range of 12 to 15 correspond approximately to M 4.5 to 6. M(K), M (K) , or possibly M K indicates a magnitude M calculated from an energy class K. Earthquakes that generate tsunamis generally rupture relatively slowly, delivering more energy at longer periods (lower frequencies) than generally used for measuring magnitudes. Any skew in

330-450: A given intensity of ground shaking seem weaker. Also, some of the original criteria of the most intense degrees (X and above), such as bent rails, ground fissures, landslides, etc., are "related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure". The categories "catastrophe" and "enormous catastrophe" added by Cancani (XI and XII) are used so infrequently that current USGS practice

396-420: A given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram . Magnitude scales vary based on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used. The Earth's crust

462-701: A lowercase " l ", either M l , or M l . (Not to be confused with the Russian surface-wave MLH scale. ) Whether the values are comparable depends on whether the local conditions have been adequately determined and the formula suitably adjusted. In Japan, for shallow (depth < 60 km) earthquakes within 600 km, the Japanese Meteorological Agency calculates a magnitude labeled MJMA , M JMA , or M J . (These should not be confused with moment magnitudes JMA calculates, which are labeled M w (JMA) or M , nor with

528-432: A wave, such as its timing, orientation, amplitude, frequency, or duration. Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram. The various magnitude scales represent different ways of deriving magnitude from such information as is available. All magnitude scales retain the logarithmic scale as devised by Charles Richter , and are adjusted so

594-399: Is "approximately related to the released seismic energy." Intensity refers to the strength or force of shaking at a given location, and can be related to the peak ground velocity. With an isoseismal map of the observed intensities (see illustration) an earthquake's magnitude can be estimated from both the maximum intensity observed (usually but not always near the epicenter ), and from

660-426: Is a function of the energy liberated by an earthquake, while intensity is the degree of shaking experienced at a point on the surface, and varies from some maximum intensity at or near the epicenter, out to zero at distance. It depends upon many factors, including the depth of the hypocenter , terrain, distance from the epicenter, whether the underlying strata there amplify surface shaking, and any directionality due to

726-401: Is actually a surface-wave magnitude. Other magnitude scales are based on aspects of seismic waves that only indirectly and incompletely reflect the force of an earthquake, involve other factors, and are generally limited in some respect of magnitude, focal depth, or distance. The moment magnitude scale – Mw or M w – developed by seismologists Thomas C. Hanks and Hiroo Kanamori ,

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792-510: Is based on an earthquake's seismic moment , M 0 , a measure of how much work an earthquake does in sliding one patch of rock past another patch of rock. Seismic moment is measured in Newton-meters (Nm or N·m ) in the SI system of measurement, or dyne-centimeters (dyn-cm; 1 dyn-cm = 10 Nm ) in the older CGS system. In the simplest case the moment can be calculated knowing only

858-504: Is dissipated as friction (resulting in heating of the crust). An earthquake's potential to cause strong ground shaking depends on the comparatively small fraction of energy radiated as seismic waves, and is better measured on the energy magnitude scale, M e . The proportion of total energy radiated as seismic waves varies greatly depending on focal mechanism and tectonic environment; M e   and M w   for very similar earthquakes can differ by as much as 1.4 units. Despite

924-637: Is distinguished from the term zero point in that the latter can also be located in the air, underground, or underwater. The term "ground zero" originally referred to the hypocenter of the Trinity test in Jornada del Muerto desert near Socorro, New Mexico , and the atomic bombings of Hiroshima and Nagasaki in Japan . The United States Strategic Bombing Survey of the atomic attacks, released in June 1946, used

990-512: Is measured at periods of up to 30 seconds. The regional mb Lg scale – also denoted mb_Lg , mbLg , MLg (USGS), Mn , and m N – was developed by Nuttli (1973) for a problem the original M L scale could not handle: all of North America east of the Rocky Mountains . The M L scale was developed in southern California, which lies on blocks of oceanic crust, typically basalt or sedimentary rock, which have been accreted to

1056-522: Is radiated as seismic waves. They also differ in the depth at which they occur; deeper earthquakes have less interaction with the surface, their energy is spread throughout a larger volume, and the energy reaching the surface is spread across a larger area. Shaking intensity is localized. It generally diminishes with distance from the earthquake's epicenter , but it can be amplified in sedimentary basins and in certain kinds of unconsolidated soils. Intensity scales categorize intensity empirically, based on

1122-436: Is stressed by tectonic forces. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking. Magnitude is an estimate of the relative "size" or strength of an earthquake , and thus its potential for causing ground-shaking. It

1188-527: Is the mantle magnitude scale, M m . This is based on Rayleigh waves that penetrate into the Earth's mantle, and can be determined quickly, and without complete knowledge of other parameters such as the earthquake's depth. M d designates various scales that estimate magnitude from the duration or length of some part of the seismic wave-train. This is especially useful for measuring local or regional earthquakes, both powerful earthquakes that might drive

1254-495: Is the point on the Earth 's surface directly below a nuclear explosion , meteor air burst , or other mid-air explosion. In seismology , the hypocenter of an earthquake is its point of origin below ground; a synonym is the focus of an earthquake. Generally, the terms ground zero and surface zero are also used in relation to epidemics , and other disasters to mark the point of the most severe damage or destruction. The term

1320-489: Is to merge them into a single category "Extreme" abbreviated as "X+". The lesser degrees of the MMI scale generally describe the manner in which the earthquake is felt by people. The greater numbers of the scale are based on observed structural damage. This table gives MMIs that are typically observed at locations near the epicenter of the earthquake. Magnitude and intensity, while related, are very different concepts. Magnitude

1386-408: Is uncertainty in such measurements that grows with the wavelength so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock. Computing

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1452-530: The Richter scale he had developed, he proposed calling it the "modified Mercalli scale of 1956" (MM56). In their 1993 compendium of historical seismicity in the United States, Carl Stover and Jerry Coffman ignored Richter's revision, and assigned intensities according to their slightly modified interpretation of Wood and Neumann's 1931 scale, effectively creating a new, but largely undocumented version of

1518-567: The Shindo intensity scale .) JMA magnitudes are based (as typical with local scales) on the maximum amplitude of the ground motion ; they agree "rather well" with the seismic moment magnitude M w   in the range of 4.5 to 7.5, but underestimate larger magnitudes. Body-waves consist of P-waves that are the first to arrive (see seismogram), or S-waves , or reflections of either. Body-waves travel through rock directly. The original "body-wave magnitude" – mB or m B (uppercase "B") –

1584-562: The Earth's surface, and are principally either Rayleigh waves or Love waves . For shallow earthquakes the surface waves carry most of the energy of the earthquake, and are the most destructive. Deeper earthquakes, having less interaction with the surface, produce weaker surface waves. The surface-wave magnitude scale, variously denoted as Ms , M S , and M s , is based on a procedure developed by Beno Gutenberg in 1942 for measuring shallow earthquakes stronger or more distant than Richter's original scale could handle. Notably, it measured

1650-646: The IASPEI in 1967; this is the basis of the standardized M s20 scale (Ms_20, M s (20)). A "broad-band" variant ( Ms_BB , M s (BB) ) measures the largest velocity amplitude in the Rayleigh-wave train for periods up to 60 seconds. The M S7 scale used in China is a variant of M s calibrated for use with the Chinese-made "type 763" long-period seismograph. The MLH scale used in some parts of Russia

1716-560: The advantage of being related more closely to seismic risk than instrumental strong-motion parameters. The MMI scale is not defined in terms of more rigorous, objectively quantifiable measurements such as shake amplitude, shake frequency, peak velocity, or peak acceleration. Human-perceived shaking and building damage are best correlated with peak acceleration for lower-intensity events, and with peak velocity for higher-intensity events. The effects of any one earthquake can vary greatly from place to place, so many MMI values may be measured for

1782-452: The amount of slip, the area of the surface ruptured or slipped, and a factor for the resistance or friction encountered. These factors can be estimated for an existing fault to determine the magnitude of past earthquakes, or what might be anticipated for the future. An earthquake's seismic moment can be estimated in various ways, which are the bases of the M wb , M wr , M wc , M ww , M wp , M i , and M wpd scales, all subtypes of

1848-426: The amplitude of surface waves (which generally produce the largest amplitudes) for a period of "about 20 seconds". The M s   scale approximately agrees with M L   at ~6, then diverges by as much as half a magnitude. A revision by Nuttli (1983) , sometimes labeled M Sn , measures only waves of the first second. A modification – the "Moscow-Prague formula" – was proposed in 1962, and recommended by

1914-537: The atmosphere rather than strike the surface, is the closest point on the surface to the explosion. The Tunguska event occurred in Siberia in 1908 and flattened an estimated 80 million trees over an area of 2,150 km (830 sq mi) of forest. The trees at the hypocenter of the blast were left standing, but all their limbs had been blown off by the shockwave. The 2013 Chelyabinsk meteor 's hypocenter in Russia

1980-556: The body-wave (mb ) or the seismic energy (M e  ) is there a difference comparable to the difference in damage. Rearranged and adapted from Table 1 in Choy, Boatwright & Kirby 2001 , p. 13. Seen also in IS 3.6 2012 , p. 7. K (from the Russian word класс, 'class', in the sense of a category ) is a measure of earthquake magnitude in the energy class or K-class system, developed in 1955 by Soviet seismologists in

2046-703: The center of this open space was nicknamed "Cafe Ground Zero". During the September 11 attacks in 2001, two aircraft were hijacked by 10 al-Qaeda terrorists and were flown into the Twin Towers of the World Trade Center in New York City , causing massive damage and starting fires that caused the weakened 110-story skyscrapers to collapse . The destroyed World Trade Center site soon became known as "ground zero". Rescue workers also used

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2112-534: The continent. East of the Rockies the continent is a craton , a thick and largely stable mass of continental crust that is largely granite , a harder rock with different seismic characteristics. In this area the M L scale gives anomalous results for earthquakes which by other measures seemed equivalent to quakes in California. Nuttli resolved this by measuring the amplitude of short-period (~1 sec.) Lg waves,

2178-485: The continental crust. All these problems prompted the development of other scales. Most seismological authorities, such as the United States Geological Survey , report earthquake magnitudes above 4.0 as moment magnitude (below), which the press describes as "Richter magnitude". Richter's original "local" scale has been adapted for other localities. These may be labelled "ML", or with

2244-399: The correlation can be reversed to predict tidal height from earthquake magnitude. (Not to be confused with the height of a tidal wave, or run-up , which is an intensity effect controlled by local topography.) Under low-noise conditions, tsunami waves as little as 5 cm can be predicted, corresponding to an earthquake of M ~6.5. Another scale of particular importance for tsunami warnings

2310-509: The descriptions of each degree. This version "found favour with the users", and was adopted by the Italian Central Office of Meteorology and Geodynamics. In 1904, Adolfo Cancani proposed adding two additional degrees for very strong earthquakes, "catastrophe" and "enormous catastrophe", thus creating a 12-degree scale. His descriptions being deficient, August Heinrich Sieberg augmented them during 1912 and 1923, and indicated

2376-499: The descriptions, and removal of the acceleration criteria), they named it the "modified Mercalli intensity scale of 1931" (MM31). Some seismologists refer to this version the "Wood–Neumann scale". Wood and Neumann also had an abridged version, with fewer criteria for assessing the degree of intensity. The Wood–Neumann scale was revised in 1956 by Charles Francis Richter and published in his influential textbook Elementary Seismology . Not wanting to have this intensity scale confused with

2442-505: The duration of shaking. This is why, in the 1989 Loma Prieta earthquake , the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter. Geological structures were also significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland. A similar effect channeled seismic waves between

2508-407: The duration or amplitude of a part of the seismic wave, the coda . For short distances (less than ~100 km) these can provide a quick estimate of magnitude before the quake's exact location is known. Hypocenter A hypocenter or hypocentre (from Ancient Greek ὑπόκεντρον ( hupókentron )  'below the center'), also called ground zero or surface zero ,

2574-695: The earthquake mechanism. For example, a magnitude 7.0 quake in Salta , Argentina, in 2011, that was 576.8 km deep, had a maximum felt intensity of V, while a magnitude 2.2 event in Barrow in Furness , England, in 1865, about 1 km deep, had a maximum felt intensity of VIII. The small table is a rough guide to the degrees of the MMI scale. The colors and descriptive names shown here differ from those used on certain shake maps in other articles. Dozens of intensity-prediction equations have been published to estimate

2640-490: The effects reported by untrained observers, and are adapted for the effects that might be observed in a particular region. By not requiring instrumental measurements, they are useful for estimating the magnitude and location of historical (preinstrumental) earthquakes: the greatest intensities generally correspond to the epicentral area, and their degree and extent (possibly augmented by knowledge of local geological conditions) can be compared with other local earthquakes to estimate

2706-401: The extent of the area where the earthquake was felt. The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake, one of the most important being soil conditions. For instance, thick layers of soft soil (such as fill) can amplify seismic waves, often at a considerable distance from the source, while sedimentary basins will often resonate, increasing

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2772-470: The generic M w scale. See Moment magnitude scale § Subtypes for details. Seismic moment is considered the most objective measure of an earthquake's "size" in regard of total energy. However, it is based on a simple model of rupture, and on certain simplifying assumptions; it does not account for the fact that the proportion of energy radiated as seismic waves varies among earthquakes. Much of an earthquake's total energy as measured by M w  

2838-627: The hypocenter of the attack was Shima Hospital , approximately 800 ft (240 m) away from the intended aiming point at Aioi Bridge . During the Cold War , the Pentagon (headquarters of United States Department of Defense in Arlington County, Virginia ) was an assured target in the event of nuclear war . The open space in the center of the Pentagon became known informally as ground zero. A snack bar that used to be located at

2904-466: The hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring. The expanding wavefront from the earthquake's rupture propagates at a speed of several kilometers per second; this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it

2970-463: The macroseismic intensity at a location given the magnitude, source-to-site distance, and perhaps other parameters (e.g. local site conditions). These are similar to ground motion-prediction equations for the estimation of instrumental strong-motion parameters such as peak ground acceleration . A summary of intensity prediction equations is available. Such equations can be used to estimate the seismic hazard in terms of macroseismic intensity, which has

3036-462: The magnitude. Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883. It had six degrees or categories, has been described as "merely an adaptation" of the then-standard Rossi–Forel scale of 10 degrees, and is now "more or less forgotten". Mercalli's second scale, published in 1902, was also an adaptation of the Rossi–Forel scale, retaining the 10 degrees and expanding

3102-400: The maximum amplitude of the ground shaking, without distinguishing the different seismic waves. They underestimate the strength: The original "Richter" scale, developed in the geological context of Southern California and Nevada, was later found to be inaccurate for earthquakes in the central and eastern parts of the continent (everywhere east of the Rocky Mountains ) because of differences in

3168-523: The measurement procedures and equations for the principal magnitude scales, M L  , M s  , mb , mB  and mb Lg  . The first scale for measuring earthquake magnitudes, developed in 1935 by Charles F. Richter and popularly known as the "Richter" scale, is actually the Local magnitude scale , label ML or M L . Richter established two features now common to all magnitude scales. All "Local" (ML) magnitudes are based on

3234-513: The mid-range approximately correlates with the original "Richter" scale. Most magnitude scales are based on measurements of only part of an earthquake's seismic wave-train, and therefore are incomplete. This results in systematic underestimation of magnitude in certain cases, a condition called saturation . Since 2005 the International Association of Seismology and Physics of the Earth's Interior (IASPEI) has standardized

3300-402: The other major faults in the area. An earthquake radiates energy in the form of different kinds of seismic waves , whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through. Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram , and then measuring one or more characteristics of

3366-630: The remote Garm ( Tajikistan ) region of Central Asia; in revised form it is still used for local and regional quakes in many states formerly aligned with the Soviet Union (including Cuba). Based on seismic energy (K = log E S , in Joules ), difficulty in implementing it using the technology of the time led to revisions in 1958 and 1960. Adaptation to local conditions has led to various regional K scales, such as K F and K S . K values are logarithmic, similar to Richter-style magnitudes, but have

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3432-468: The same earthquake. These values can be displayed best using a contoured map of equal intensity, known as an isoseismal map . However, each earthquake has only one magnitude. Seismic magnitude scales#Mw Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake . These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at

3498-407: The same location, but twice as deep and on a different kind of fault, was felt over a broad area, injured over 300 people, and destroyed or seriously damaged over 10,000 houses. As can be seen in the table below, this disparity of damage done is not reflected in either the moment magnitude (M w  ) nor the surface-wave magnitude (M s  ). Only when the magnitude is measured on the basis of

3564-466: The scale. The basis by which the U.S. Geological Survey (and other agencies) assigns intensities is nominally Wood and Neumann's MM31. However, this is generally interpreted with the modifications summarized by Stover and Coffman because in the decades since 1931, "some criteria are more reliable than others as indicators of the level of ground shaking". Also, construction codes and methods have evolved, making much of built environment stronger; these make

3630-507: The seismometer off-scale (a problem with the analog instruments formerly used) and preventing measurement of the maximum wave amplitude, and weak earthquakes, whose maximum amplitude is not accurately measured. Even for distant earthquakes, measuring the duration of the shaking (as well as the amplitude) provides a better measure of the earthquake's total energy. Measurement of duration is incorporated in some modern scales, such as M wpd   and mB c  . M c scales usually measure

3696-501: The site labeled it the " Ground Zero mosque ". In advance of the 10th anniversary of the attacks, New York City mayor Michael Bloomberg urged that the "ground zero" moniker be retired, saying, "…the time has come to call those 16 acres [6.5 hectares] what they are: The World Trade Center and the National September 11th Memorial and Museum." The hypocenter of a meteor air burst , an asteroid or comet that explodes in

3762-429: The spectral distribution can result in larger, or smaller, tsunamis than expected for a nominal magnitude. The tsunami magnitude scale, M t , is based on a correlation by Katsuyuki Abe of earthquake seismic moment (M 0  ) with the amplitude of tsunami waves as measured by tidal gauges. Originally intended for estimating the magnitude of historic earthquakes where seismic data is lacking but tidal data exist,

3828-698: The standardized mB BB   scale. The mb or m b scale (lowercase "m" and "b") is similar to mB , but uses only P-waves measured in the first few seconds on a specific model of short-period seismograph. It was introduced in the 1960s with the establishment of the World-Wide Standardized Seismograph Network (WWSSN); the short period improves detection of smaller events, and better discriminates between tectonic earthquakes and underground nuclear explosions. Measurement of mb  has changed several times. As originally defined by Gutenberg (1945c) m b

3894-532: The term "The Big Momma!", referring to the pile of rubble that was left after the buildings collapsed. Even after the site was cleaned up and construction on the new One World Trade Center and the National September 11 Memorial & Museum were well under way, the term was still frequently used to refer to the site, as when opponents of the Park51 project that was to be located two blocks away from

3960-469: The term liberally, defining it as: For convenience, the term 'ground zero' will be used to designate the point on the ground directly beneath the point of detonation, or 'air zero.' William Laurence , an embedded reporter with the Manhattan Project , reported that "Zero" was "the code name given to the spot chosen for the [Trinity] test" in 1945. The Oxford English Dictionary , citing

4026-487: The use of the term in a 1946 New York Times report on the destroyed city of Hiroshima , defines ground zero as "that part of the ground situated immediately under an exploding bomb, especially an atomic one." The term was military slang, used at the Trinity site where the weapon tower for the first nuclear weapon was at "point zero", and moved into general use very shortly after the end of World War II . At Hiroshima,

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4092-474: The usefulness of the M e   scale, it is not generally used due to difficulties in estimating the radiated seismic energy. Two earthquakes differing greatly in the damage done In 1997 there were two large earthquakes off the coast of Chile. The magnitude of the first, in July, was estimated at M w  6.9, but was barely felt, and only in three places. In October a M w  7.1 quake in nearly

4158-436: Was based on the maximum amplitude of waves in the first 10 seconds or more. However, the length of the period influences the magnitude obtained. Early USGS/NEIC practice was to measure mb  on the first second (just the first few P-waves ), but since 1978 they measure the first twenty seconds. The modern practice is to measure short-period mb  scale at less than three seconds, while the broadband mB BB   scale

4224-496: Was developed by Gutenberg 1945c and Gutenberg & Richter 1956 to overcome the distance and magnitude limitations of the M L   scale inherent in the use of surface waves. mB  is based on the P- and S-waves, measured over a longer period, and does not saturate until around M 8. However, it is not sensitive to events smaller than about M 5.5. Use of mB  as originally defined has been largely abandoned, now replaced by

4290-427: Was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through. With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from

4356-559: Was more populated than that of Tunguska, resulting in civil damage and injury, mostly from flying glass shards from broken windows. An earthquake's hypocenter or focus is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture. This occurs directly beneath the epicenter , at a distance known as the hypocentral depth or focal depth . The focal depth can be calculated from measurements based on seismic wave phenomena. As with all wave phenomena in physics , there

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