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The **moment magnitude scale** (**MMS**; denoted as ** M_{w}** or

**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 a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary 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.

An **earthquake** is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity, or **seismic activity,** of an area is the frequency, type and size of earthquakes experienced over a period of time. The word *tremor* is also used for non-earthquake seismic rumbling.

- History
- Richter scale: the original measure of earthquake magnitude
- Modified Richter scale
- Correcting weaknesses of the modified Richter scale
- Seismic moment
- Introduction of an energy-motivated magnitude Mw
- Moment magnitude scale
- Current use
- Definition
- Relations between seismic moment, potential energy released and radiated energy
- Comparative energy released by two earthquakes
- Comparison with Richter scale
- Subtypes of Mw
- See also
- Notes
- Sources
- External links

The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (*M*_{L}). Even though the formulas are different, the new scale was designed to produce magnitude values for a given earthquake similar to those produced by the older one. Under suitable assumptions, as with the Richter magnitude scale, an increase of one step on this logarithmic scale corresponds to a 10^{1.5} (about 32) times increase in the amount of energy released, and an increase of two steps corresponds to a 10^{3} (1,000) times increase in energy. Thus, an earthquake of *M*_{w} of 7.0 releases about 32 times as much energy as one of 6.0 and nearly 1,000 times one of 5.0.

The so-called **Richter magnitude scale** – more accurately, *Richter's magnitude scale*, or just *Richter magnitude* – for measuring the strength ("size") of earthquakes refers to the original "magnitude scale" developed by Charles F. Richter and presented in his landmark 1935 paper, and later revised and renamed the **Local magnitude scale**, denoted as "ML" or "M_{L}". Because of various shortcomings of the ML scale most seismological authorities now use other scales, such as the moment magnitude scale (M_{w} ), to report earthquake magnitudes, but much of the news media still refers to these as "Richter" magnitudes. All magnitude scales retain the logarithmic character of the original, and are scaled to have roughly comparable numeric values.

A **logarithmic scale** is a nonlinear scale used when there is a large range of quantities. Common uses include earthquake strength, sound loudness, light intensity, and pH of solutions.

The moment magnitude is based on the seismic moment of the earthquake, which is equal to the shear modulus of the rock near the fault multiplied by the average amount of slip on the fault and the size of the area that slipped.^{ [2] }

**Seismic moment** is a quantity used by seismologists to measure the size of an earthquake. The scalar seismic moment is defined by the equation , where

In materials science, **shear modulus** or **modulus of rigidity**, denoted by *G*, or sometimes *S* or *μ*, is defined as the ratio of shear stress to the shear strain:

In geology, a **fault** is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

While earthquake magnitudes are usually calculated on several scales, the moment magnitude value, being more directly related to the energy of an earthquake, is generally preferred. The generic "M" magnitude the U.S. Geological Survey uses for reporting magnitudes to the public is (for M > 4 events) the moment magnitude, which the press often calls the "Richter magnitude".^{ [3] }

In 1935, Charles Richter and Beno Gutenberg developed the local magnitude (M_{L} ) scale (popularly known as the Richter scale) with the goal of quantifying medium-sized earthquakes (between magnitude 3.0 and 7.0) in Southern California. This scale was based on the ground motion measured by a particular type of seismometer (a Wood-Anderson seismograph) at a distance of 100 kilometres (62 mi) from the earthquake's epicenter. Because of this, there is an upper limit on the highest measurable magnitude, and all large earthquakes will tend to have a local magnitude of around 7.^{ [4] } Further, the magnitude becomes unreliable for measurements taken at a distance of more than about 600 kilometres (370 mi) from the epicenter. Since this M_{L} scale was simple to use and corresponded well with the damage which was observed, it was extremely useful for engineering earthquake-resistant structures, and gained common acceptance.^{ [5] }

**Beno Gutenberg** was a German-American seismologist who made several important contributions to the science. He was a colleague and mentor of Charles Francis Richter at the California Institute of Technology and Richter's collaborator in developing the Richter magnitude scale for measuring an earthquake's magnitude.

**California** is a state in the Pacific Region of the United States. With 39.6 million residents, California is the most populous U.S. state and the third-largest by area. The state capital is Sacramento. The Greater Los Angeles Area and the San Francisco Bay Area are the nation's second- and fifth-most populous urban regions, with 18.7 million and 8.8 million residents respectively. Los Angeles is California's most populous city, and the country's second-most populous, after New York City. California also has the nation's most populous county, Los Angeles County, and its largest county by area, San Bernardino County. The City and County of San Francisco is both the country's second-most densely populated major city after New York City and the fifth-most densely populated county, behind only four of the five New York City boroughs.

A **seismometer** is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a **seismograph**. The output of such a device — formerly recorded on paper or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, and to study the earth's internal structure.

The Richter scale was not effective for characterizing some classes of quakes. As a result, Beno Gutenberg expanded Richter's work to consider earthquakes detected at distant locations. For such large distances the higher frequency vibrations are attenuated and seismic surface waves (Rayleigh and Love waves) are dominated by waves with a period of 20 seconds, corresponding to a wavelength of about 60 km. Their magnitude was assigned a surface wave magnitude scale (M_{s} ). Gutenberg also combined compressional P-waves and the transverse S-waves (which he termed "body waves") to create a body-wave magnitude scale (mb ), measured for periods between 1 and 10 seconds. Ultimately Gutenberg and Richter collaborated to produce a combined scale which was able to estimate the energy released by an earthquake in terms of Gutenberg's surface wave magnitude scale (M_{s} ).^{ [6] }

**Rayleigh waves** are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. Rayleigh waves are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

In elastodynamics, **Love waves**, named after Augustus Edward Hough Love, are horizontally polarized surface waves. The Love wave is a result of the interference of many shear waves (S–waves) guided by an elastic layer, which is *welded* to an elastic half space on one side while bordering a vacuum on the other side. In seismology, **Love waves** are surface seismic waves that cause horizontal shifting of the Earth during an earthquake. Augustus Edward Hough Love predicted the existence of Love waves mathematically in 1911. They form a distinct class, different from other types of seismic waves, such as P-waves and S-waves, or Rayleigh waves. Love waves travel with a lower velocity than P- or S- waves, but faster than Rayleigh waves. These waves are observed only when there is a low velocity layer overlying a high velocity layer/ sub–layers.

In physics, the **wavelength** is the **spatial period** of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is commonly designated by the Greek letter *lambda* (λ). The term *wavelength* is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids.

The Richter scale, as modified, was successfully applied to characterize localities. This enabled local building codes to establish standards for buildings which were earthquake resistant. However a series of quakes were poorly handled by the modified Richter scale. This series of "great earthquakes" included faults that broke along a line of up to 1000 km. Examples include the 1957 Andreanof Islands earthquake and the 1960 Chilean quake, both of which broke faults approaching 1000 km. The M_{s} scale was unable to characterize these "great earthquakes" accurately.^{ [7] }

The difficulties with use of M_{s} in characterizing the quake resulted from the size of these earthquakes. Great quakes produced 20 s waves such that M_{s} was comparable to normal quakes, but also produced very long period waves (more than 200 s) which carried large amounts of energy. As a result, use of the modified Richter scale methodology to estimate earthquake energy was deficient at high energies.^{ [8] }

The concept of seismic moment was introduced in 1966 by Keiiti Aki, a professor of geophysics at the Massachusetts Institute of Technology. Using detailed field studies of the 1964 Niigata earthquake and data from a new generation of seimographs in the * World-Wide Standardized Seismograph Network * (WWSSN), he first confirmed that an earthquake is "a release of accumulated strain energy by a rupture",^{ [9] } and that this can be modeled by a "double couple".^{ [10] } With further analysis he showed how the energy radiated by seismic waves can be used to estimate the energy released by the earthquake.^{ [11] } This was done using *seismic moment*, defined^{ [12] } as

*M*_{0}=*μūS*

with μ being the rigidity (or resistance) of moving a fault with a surface areas of S over an average dislocation (distance) of ū. (Modern formulations replace μūS with the equivalent D̄A, known as the "geometric moment" or "potency".^{ [13] }.) By this equation the *moment* determined from the double couple of the seismic waves can be related to the moment calculated from knowledge of the surface area of fault slippage and the amount of slip. In the case of the Niigata earthquake the dislocation estimated from the seismic moment reasonably approximated the observed dislocation.^{ [14] }

Most earthquake magnitude scales suffered from the fact that they only provided a comparison of the amplitude of waves produced at a standard distance and frequency band; it was difficult to relate these magnitudes to a physical property of the earthquake. Gutenberg and Richter suggested that radiated energy E_{s} could be estimated as

(in Joules). Unfortunately, the duration of many very large earthquakes was longer than 20 seconds, the period of the surface waves used in the measurement of M_{s} . This meant that giant earthquakes such as the 1960 Chilean earthquake (M 9.5) were only assigned an M_{s} 8.2. Caltech seismologist Hiroo Kanamori ^{ [15] } recognized this deficiency and he took the simple but important step of defining a magnitude based on estimates of radiated energy, M_{w} , where the "w" stood for work (energy):

Kanamori recognized that measurement of radiated energy is technically difficult since it involves integration of wave energy over the entire frequency band. To simplify this calculation, he noted that the lowest frequency parts of the spectrum can often be used to estimate the rest of the spectrum. The lowest frequency asymptote of a seismic spectrum is characterized by the seismic moment, M_{0} . Using an approximate relation between radiated energy and seismic moment (which assumes stress drop is complete and ignores fracture energy),

(where **E** is in Joules and M_{0} is in Nm), Kanamori approximated M_{w} by

The formula above made it much easier to estimate the energy-based magnitude M_{w} , but it changed the fundamental nature of the scale into a moment magnitude scale. Caltech seismologist Thomas C. Hanks noted that Kanamori's M_{w} scale was very similar to a relationship between M_{L} and M_{0} that was reported by Thatcher & Hanks (1973)

Hanks & Kanamori (1979) combined their work to define a new magnitude scale based on estimates of seismic moment

where is defined in newton meters (N·m).

Although the formal definition of moment magnitude is given by this paper and is designated by **M**, it has been common for many authors to refer to M_{w} as moment magnitude. In most of these cases, they are actually referring to moment magnitude **M** as defined above.

Moment magnitude is now the most common measure of earthquake size for medium to large earthquake magnitudes,^{ [16] } but in practice, seismic moment, the seismological parameter it is based on, is not measured routinely for smaller quakes. For example, the United States Geological Survey does not use this scale for earthquakes with a magnitude of less than 3.5, which includes the great majority of quakes.

Current practice in official earthquake reports is to adopt moment magnitude as the preferred magnitude, i.e., M_{w} is the official magnitude reported whenever it can be computed. Because seismic moment (M_{0} , the quantity needed to compute M_{w} ) is not measured if the earthquake is too small, the reported magnitude for earthquakes smaller than M 4 is often Richter's M_{L} .

Popular press reports most often deal with significant earthquakes larger than M ~ 4. For these events, the official magnitude is the moment magnitude M_{w} , not Richter's local magnitude M_{L} .

The symbol for the moment magnitude scale is M_{w} , with the subscript "w" meaning mechanical work accomplished. The moment magnitude M_{w} is a dimensionless value defined by Hiroo Kanamori ^{ [17] } as

where M_{0} is the seismic moment in dyne⋅cm (10^{−7} N⋅m).^{ [18] } The constant values in the equation are chosen to achieve consistency with the magnitude values produced by earlier scales, such as the Local Magnitude and the Surface Wave magnitude.

Seismic moment is not a direct measure of energy changes during an earthquake. The relations between seismic moment and the energies involved in an earthquake depend on parameters that have large uncertainties and that may vary between earthquakes. Potential energy is stored in the crust in the form of elastic energy due to built-up stress and gravitational energy.^{ [19] } During an earthquake, a portion of this stored energy is transformed into

- energy dissipated in frictional weakening and inelastic deformation in rocks by processes such as the creation of cracks
- heat
- radiated seismic energy .

The potential energy drop caused by an earthquake is related approximately to its seismic moment by

where is the average of the *absolute* shear stresses on the fault before and after the earthquake (e.g., equation 3 of Venkataraman & Kanamori 2004) and is the average of the shear moduli of the rocks that constitute the fault. Currently, there is no technology to measure absolute stresses at all depths of interest, nor method to estimate it accurately, and is thus poorly known. It could vary highly from one earthquake to another. Two earthquakes with identical but different would have released different .

The radiated energy caused by an earthquake is approximately related to seismic moment by

where is radiated efficiency and is the static stress drop, i.e., the difference between shear stresses on the fault before and after the earthquake (e.g., from equation 1 of Venkataraman & Kanamori 2004). These two quantities are far from being constants. For instance, depends on rupture speed; it is close to 1 for regular earthquakes but much smaller for slower earthquakes such as tsunami earthquakes and slow earthquakes. Two earthquakes with identical but different or would have radiated different .

Because and are fundamentally independent properties of an earthquake source, and since can now be computed more directly and robustly than in the 1970s, introducing a separate magnitude associated to radiated energy was warranted. Choy and Boatwright defined in 1995 the *energy magnitude*^{ [20] }

where is in J (N·m).

Assuming the values of σ̄/μ are the same for all earthquakes, one can consider M_{w} as a measure of the potential energy change Δ*W* caused by earthquakes. Similarly, if one assumes is the same for all earthquakes, one can consider M_{w} as a measure of the energy *E*_{s} radiated by earthquakes.

Under these assumptions, the following formula, obtained by solving for M_{0} the equation defining M_{w} , allows one to assess the ratio of energy release (potential or radiated) between two earthquakes of different moment magnitudes, and :

As with the Richter scale, an increase of one step on the logarithmic scale of moment magnitude corresponds to a 10^{1.5} ≈ 32 times increase in the amount of energy released, and an increase of two steps corresponds to a 10^{3} = 1000 times increase in energy. Thus, an earthquake of M_{w} of 7.0 contains 1000 times as much energy as one of 5.0 and about 32 times that of 6.0.

The moment magnitude (M_{w} >) scale was introduced to address the shortcomings of the Richter scale (detailed above) while maintaining consistency. Thus, for medium-sized earthquakes, the moment magnitude values should be similar to Richter values. That is, a magnitude 5.0 earthquake will be about a 5.0 on both scales. Unlike other scales, the moment magnitude scale does not saturate at the upper end; there is no upper limit to the possible measurable magnitudes. However, this has the side-effect that the scales diverge for smaller earthquakes.^{ [21] }

Various ways of determining moment magnitude have been developed, and several subtypes of the M_{w} scale can be used to indicate the basis used.^{ [22] }

**Mwb**– Based on*moment tensor inversion*of long-period (~10 – 100 s) body-waves.**Mwr**– From a*moment tensor inversion*of complete waveforms at regional distances (~ 1,000 miles). Sometimes called RMT.**Mwc**– Derived from a*centroid moment tensor inversion*of intermediate- and long-period body- and surface-waves.**Mww**– Derived from a*centroid moment tensor inversion*of the W-phase.**Mwp**(**Mi**) – Developed by Seiji Tsuboi^{ [23] }for quick estimation of the tsunami potential of large near-coastal earthquakes from measurements of the P-waves, and later extended to teleseismic earthquakes in general.^{ [24] }**Mwpd**– A duration-amplitude procedure which takes into account the duration of the rupture, providing a fuller picture of the energy released by longer lasting ("slow") ruptures than seen with M_{w}.^{ [25] }

- ↑ Hanks & Kanamori 1979.
- ↑ "Glossary of Terms on Earthquake Maps". USGS. Archived from the original on 2009-02-27. Retrieved 2009-03-21.
- ↑ The "USGS Earthquake Magnitude Policy" for reporting earthquake magnitudes to the public as formulated by the
*USGS Earthquake Magnitude Working Group*was implemented January 18, 2002, and posted at https://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php. That page was removed following a web redesign; a copy is archived at the Wayback Machine. - ↑ "On Earthquake Magnitudes".
- ↑ Kanamori 1978.
- ↑ Kanamori 1978.
- ↑ Kanamori 1978.
- ↑ Kanamori 1978.
- ↑ Aki 1966a , p. 25.
- ↑ Aki 1966a , p. 36.
- ↑ Aki 1966b , p. 87.
- ↑ Aki 1966b , p. 84, equation 12.
- ↑ Bormann, Wendt & Di Giacomo 2013 , p. 12, equation 3.1.
- ↑ Aki 1966b , p. 84.
- ↑ Kanamori 1977.
- ↑ Boyle 2008.
- ↑ Kanamori 1977.
- ↑ Hanks & Kanamori 1979.
- ↑ Kostrov 1974; Dahlen 1977.
- ↑ Choy & Boatwright 1995
- ↑ Hanks & Kanamori 1979.
- ↑ USGS Technical Terms used on Event Pages.
- ↑ Tsuboi et al. 1995.
- ↑ Bormann, Wendt & Di Giacomo 2013 , §3.2.8.2, p. 135.
- ↑ Bormann, Wendt & Di Giacomo 2013 , §3.2.8.3, pp. 137–128.

- Aki, Keiiti (1966a), "3. Generation and propagation of G waves from the Niigata earthquake of June 14, 1964. Part 1. A statistical analysis" (PDF),
*Bulletin of the Earthquake Research Institute*,**44**: 23–72.

- Aki, Keiiti (1966b), "4. Generation and propagation of G waves from the Niigata earthquake of June 14, 1964. Part 2. Estimation of earthquake moment, released energy and stress-strain drop from G wave spectrum" (PDF),
*Bulletin of the Earthquake Research Institute*,**44**: 73–88.

- Aki, Keiiti (April 1972), "Earthquake Mechanism",
*Tectonophysics*,**13**(1–4): 423–446, Bibcode:1972Tectp..13..423A, doi:10.1016/0040-1951(72)90032-7 .

- Bormann, Peter; Wendt, Siegfried; Di Giacomo, Dominico (2013), "Chapter 3: Seismic Sources and Source Parameters" (PDF), in Bormann,
*New Manual of Seismological Observatory Practice 2 (NMSOP-2)*, doi:10.2312/GFZ.NMSOP-2_ch3 .

- Boyle, Alan (May 12, 2008),
*Quakes by the numbers*, MSNBC , retrieved 2008-05-12,That original scale has been tweaked through the decades, and nowadays calling it the "Richter scale" is an anachronism. The most common measure is known simply as the moment magnitude scale.

.

- Choy, George L.; Boatwright, John L. (10 September 1995), "Global patterns of radiated seismic energy and apparent stress",
*Journal of Geophysical Research*,**100**(B9): 18205–28, Bibcode:1995JGR...10018205C, doi:10.1029/95JB01969 .

- Dahlen, F. A. (February 1977), "The balance of energy in earthquake faulting",
*Geophysical Journal International*,**48**(2): 239–261, Bibcode:1977GeoJ...48..239D, doi:10.1111/j.1365-246X.1977.tb01298.x .

- Dziewonski, Adam M.; Gilbert, Freeman (1976), "The effect of small aspherical perturbations on travel times and a re-examination of the corrections for ellipticity",
*Geophysical Journal of the Royal Astronomical Society*,**44**(1): 7–17, Bibcode:1976GeoJ...44....7D, doi:10.1111/j.1365-246X.1976.tb00271.x .

- Hanks, Thomas C.; Kanamori, Hiroo (May 10, 1979), "A Moment magnitude scale" (PDF),
*Journal of Geophysical Research*,**84**(B5): 2348–50, Bibcode:1979JGR....84.2348H, doi:10.1029/JB084iB05p02348, Archived from the original on August 21, 2010CS1 maint: Unfit url (link).

- Kanamori, Hiroo (July 10, 1977), "The energy release in great earthquakes" (PDF),
*Journal of Geophysical Research*,**82**(20): 2981–2987, Bibcode:1977JGR....82.2981K, doi:10.1029/jb082i020p02981 .

- Kanamori, Hiroo (February 2, 1978), "Quantification of Earthquakes" (PDF),
*Nature*,**271**(5644): 411–414, Bibcode:1978Natur.271..411K, doi:10.1038/271411a0 .

- Kostrov, B. V. (1974), "Seismic moment and energy of earthquakes, and seismic flow of rock [in Russian]",
*Izvestiya, Akademi Nauk, USSR, Physics of the solid earth [Earth Physics]*,**1**: 23–44 (English Trans. 12–21).

- Thatcher, Wayne; Hanks, Thomas C. (December 10, 1973), "Source parameters of southern California earthquakes",
*Journal of Geophysical Research*,**78**(35): 8547–8576, Bibcode:1973JGR....78.8547T, doi:10.1029/JB078i035p08547 .

- Tsuboi, S.; Abe, K.; Takano, K.; Yamanaka, Y. (April 1995), "Rapid Determination of
*M*from Broadband_{w}*P*Waveforms",*Bulletin of the Seismological Society of America*,**85**(2): 606–613

- Utsu, T. (2002), Lee, W.H.K.; Kanamori, H.; Jennings, P.C.; Kisslinger, C., eds., "Relationships between magnitude scales",
*International Handbook of Earthquake and Engineering Seismology*, International Geophysics, Academic Press,**A**(81), pp. 733–46.

- Venkataraman, Anupama; Kanamori, H. (11 May 2004), "Observational constraints on the fracture energy of subduction zone earthquakes" (PDF),
*Journal of Geophysical Research*,**109**(B05302): B05302, Bibcode:2004JGRB..109.5302V, doi:10.1029/2003JB002549 .

The **Modified Mercalli intensity scale**, descended from Giuseppe Mercalli's **Mercalli intensity scale** of 1902, is a seismic intensity scale used for measuring the *intensity of shaking* produced by an earthquake. It measures the *effects* of an earthquake *at a given location*, distinguished from the earthquake's inherent *force* or *strength* as measured by seismic magnitude scales. While shaking is driven by the seismic energy released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. Deeper earthquakes also have less interaction with the surface, and their energy is spread out across a larger area. Shaking intensity is localized, generally diminishing with distance from the earthquake's epicenter, but can be amplified in sedimentary basins and certain kinds of unconsolidated soils.

An **aftershock** is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to known laws. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.

**Hiroo Kanamori** is a Japanese seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them.

**Peak ground acceleration** (**PGA**) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a particular earthquake. Earthquake shaking generally occurs in all three directions. Therefore, PGA is often split into the horizontal and vertical components. Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes. PGA is an important parameter for earthquake engineering, The **design basis earthquake ground motion** (**DBEGM**) is often defined in terms of PGA.

**Induced seismicity** refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth's crust. Most induced seismicity is of a low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant in California which averaged two M4 events and 15 M3 events every year from 2004 to 2009.

**Megathrust earthquakes** occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another. These interplate earthquakes are the planet's most powerful, with moment magnitudes (*M _{w}*) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of known terrestrial source of tectonic activity has produced earthquakes of this scale.

In seismology, the **Gutenberg–Richter law** expresses the relationship between the magnitude and total number of earthquakes in any given region and time period of *at least* that magnitude.

The **surface wave magnitude** scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements in Rayleigh surface waves that travel primarily along the uppermost layers of the Earth. It is currently used in People's Republic of China as a national standard for categorising earthquakes.

A **supershear earthquake** is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity. This causes an effect analogous to a sonic boom.

The **1970 Tonghai earthquake** occurred at 01:00:41 local time on January 5 with a moment magnitude of 7.1 and a maximum Mercalli intensity of X (*Extreme*). The strike-slip rupture originated on the Red River Fault, which had not experienced an earthquake above magnitude 7 since 1700, and affected Tonghai County, Yunnan province, China. At least 10,000 people were killed, making it one of the deadliest in its decade. The tremor caused between US$5 to $25 million in damage, felt over an area of 8,781 km^{2} (3,390 sq mi). In Hanoi, North Vietnam, almost 483 km (300 mi) from the epicenter, victims left their homes as the rupture rumbled through the city.

**Doublet earthquakes** – and more generally, **multiplet earthquakes** – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within four-tenths of a unit of magnitude – distinguishes multiplet events from aftershocks, which decrease in magnitude and frequency according to known laws.

The concept of **Earthquake Duration Magnitude** - originally proposed by Bisztricsany in 1958 using surface waves only - is based on the realization that on a recorded earthquake seismogram, the total length of the seismic wavetrain - sometimes referred to as the CODA - reflects its size. Thus larger earthquakes give longer seismograms [as well as stronger seismic waves] than small ones. The seismic wave interval measured on the time axis of an earthquake record - starting with the first seismic wave onset until the wavetrain amplitude diminishes to at least 10% of its maximum recorded value - is referred to as "earthquake duration". It is this concept that Bisztricsany first used to develop his **Earthquake Duration Magnitude Scale** employing surface wave durations.

A **tsunami earthquake** triggers a tsunami of a magnitude that is very much larger than the magnitude of the earthquake as measured by shorter-period seismic waves. The term was introduced by Hiroo Kanamori in 1972. Such events are a result of relatively slow rupture velocities. They are particularly dangerous as a large tsunami may arrive at a coastline with little or no warning. A tsunami is a sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic islands.

An **earthquake rupture** is the extent of slip that occurs during an earthquake in the Earth's crust. Earthquakes occur for many reasons that include: landslides, movement of magma in a volcano, the formation of a new fault, or, most commonly of all, a slip on an existing fault.

The **1993 Klamath Falls earthquakes** took place in Klamath Falls, Oregon, beginning on Monday, 20 September at 8:28 p.m. The doublet earthquake registered respective magnitudes of 6.0 and 5.9 on the moment magnitude scale. The earthquakes were located at a depth of 5.6 miles (9 km) and tremors continued to be felt more than three months after the initial shocks.

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