Seismic scale
Encyclopedia
A seismic scale is used to calculate and compare the severity of earthquake
s. (For a quick review, see the table of seismic scales at the end of this article.)
Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a magnitude scale, while the intensity of shaking occurring at any given point on the Earth's surface is measured on an intensity scale.
, and represent the severity of the shaking resulting from an earthquake. Ideally, any given earthquake can be described by only one magnitude, but many intensities since the earthquake effects vary with circumstances such as distance from the epicenter
and local soil conditions. In practise, the same earthquake might have magnitude estimates typically differing by few tenths of a unit, depending on which magnitude scale is used and which data are included in the analysis.
Charles Richter, the creator of the Richter magnitude scale
, distinguished intensity and magnitude as follows: "I like to use the analogy with radio transmissions. It applies in seismology
because seismographs, or the receivers
, record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the broadcasting station
. Magnitude can be compared to the power output in kilowatts of a broadcasting station
. Local intensity on the Mercalli scale
is then comparable to the signal strength on a receiver at a given locality
; in effect, the quality of the signal. Intensity, like signal strength, will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point."
, was introduced in the late 19th century. Since then numerous intensity scales have been developed and are used in different parts of the world: the scale currently used in the United States is the Modified Mercalli scale (MM)
, while the European Macroseismic Scale
(EMS-98) is used in Europe
, the Shindo scale
is used in Japan
, the MSK-64 scale is used in India
, Israel
, Russia
and throughout the CIS
, and the Liedu scale
(GB/T 17742-1999) is used in mainland China
; Hong Kong
, on the other hand, uses the MM scale,
and Taiwan
uses the Shindo scale. Most of these scales have twelve degrees of intensity, which are roughly equivalent to one another in values but vary in the degree of sophistication employed in their formulation.
" was coined by journalists and is not generally used by seismologists in technical literature. ML is obtained by measuring the maximum displacement amplitude of a recording on a Wood-Anderson torsion seismometer
(or one calibrated to it) at a distance of up to 600 km from the epicenter of the earthquake. Other more recent magnitude measurements include: body wave magnitude
(mb), surface wave magnitude
(Ms), and duration magnitude (MD). Each of these is scaled to give values similar to those given by the local magnitude scale; but because each is based on a measurement of one aspect of the seismogram
, they do not always capture the overall power of the source. Specifically, some can be affected by saturation at higher magnitude values—meaning that they systematically underestimate the magnitude of larger events. This problem sets in at around magnitude 6 for local magnitude; surface-wave magnitude saturates above 8. Despite the limitations of older magnitude scales, they are still in wide use, as they can be calculated rapidly, catalogues of them dating back many years are available, they are sufficient for the vast majority of observed events, and the public is familiar with them.
in 1977. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of moment
in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying slip displacement—as well as the amount of energy released. So while seismic moment, too, is calculated from seismogram
s, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for observed earthquakes range over more than 15 orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes.
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s. (For a quick review, see the table of seismic scales at the end of this article.)
Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a magnitude scale, while the intensity of shaking occurring at any given point on the Earth's surface is measured on an intensity scale.
Magnitude and Intensity
The severity of an earthquake is described by both magnitude and intensity. These two frequently-confused terms refer to different, but related, observations. Magnitude, usually expressed as an Arabic numeral that is 1,2,3,4,5,6,7,8,9,0.... characterizes the size of an earthquake by measuring indirectly the energy released. By contrast, intensity indicates the local effects and potential for damage produced by an earthquake on the Earth's surface as it affects humans, animals, structures, and natural objects such as bodies of water. Intensities are usually expressed in Roman numeralsRoman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...
, and represent the severity of the shaking resulting from an earthquake. Ideally, any given earthquake can be described by only one magnitude, but many intensities since the earthquake effects vary with circumstances such as distance from the epicenter
Epicenter
The epicenter or epicentre is the point on the Earth's surface that is directly above the hypocenter or focus, the point where an earthquake or underground explosion originates...
and local soil conditions. In practise, the same earthquake might have magnitude estimates typically differing by few tenths of a unit, depending on which magnitude scale is used and which data are included in the analysis.
Charles Richter, the creator of the Richter magnitude scale
Richter magnitude scale
The expression Richter magnitude scale refers to a number of ways to assign a single number to quantify the energy contained in an earthquake....
, distinguished intensity and magnitude as follows: "I like to use the analogy with radio transmissions. It applies in seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...
because seismographs, or the receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
, record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the broadcasting station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
. Magnitude can be compared to the power output in kilowatts of a broadcasting station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
. Local intensity on the Mercalli scale
Mercalli intensity scale
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake , which is a measure of the energy released...
is then comparable to the signal strength on a receiver at a given locality
Location (geography)
The terms location and place in geography are used to identify a point or an area on the Earth's surface or elsewhere. The term 'location' generally implies a higher degree of can certainty than "place" which often has an ambiguous boundary relying more on human/social attributes of place identity...
; in effect, the quality of the signal. Intensity, like signal strength, will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point."
Seismic intensity scales
The first simple classification of earthquake intensity was devised by Domenico Pignataro in the 1780s. However, the first recognisable intensity scale in the modern sense of the word was drawn up by P.N.G. Egen in 1828; it was ahead of its time. The first widely adopted intensity scale, the Rossi-Forel scaleRossi-Forel scale
The Rossi–Forel scale was one of the first seismic scales to reflect earthquake intensities. Developed by Michele Stefano Conte de Rossi of Italy and François-Alphonse Forel of Switzerland in the late 19th century, it was used for about two decades until the introduction of the Mercalli intensity...
, was introduced in the late 19th century. Since then numerous intensity scales have been developed and are used in different parts of the world: the scale currently used in the United States is the Modified Mercalli scale (MM)
Mercalli intensity scale
The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake , which is a measure of the energy released...
, while the European Macroseismic Scale
European Macroseismic Scale
The European Macroseismic Scale is the basis for evaluation of seismic intensity in European countries and is also used in a number of countries outside Europe...
(EMS-98) is used in Europe
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
, the Shindo scale
Japan Meteorological Agency seismic intensity scale
The Japan Meteorological Agency seismic intensity scale is a seismic scale used in Japan and Taiwan to measure the intensity of earthquakes. It is measured in units of...
is used in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, the MSK-64 scale is used in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and throughout the CIS
Commonwealth of Independent States
The Commonwealth of Independent States is a regional organization whose participating countries are former Soviet Republics, formed during the breakup of the Soviet Union....
, and the Liedu scale
China Seismic Intensity Scale
The China Seismic Intensity Scale is a national standard in the People's Republic of China used to measure seismic intensity. Similar to EMS-92 on which CSIS drew reference, seismic impacts are classified into 12 degrees of intensity, or liedu in Roman numerals from I for insensible to XII for...
(GB/T 17742-1999) is used in mainland China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
; Hong Kong
Hong Kong
Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
, on the other hand, uses the MM scale,
and Taiwan
Republic of China
The Republic of China , commonly known as Taiwan , is a unitary sovereign state located in East Asia. Originally based in mainland China, the Republic of China currently governs the island of Taiwan , which forms over 99% of its current territory, as well as Penghu, Kinmen, Matsu and other minor...
uses the Shindo scale. Most of these scales have twelve degrees of intensity, which are roughly equivalent to one another in values but vary in the degree of sophistication employed in their formulation.
Magnitude scales
The first attempt to qualitatively define a single, absolute value to describe the size of earthquakes was the magnitude scale (the name being taken from similarly formulated scales used to represent the brightness of stars).Local magnitude scale and related scales
The local magnitude scale (ML), also popularly known as the Richter scale, is a quantitative logarithmic scale. In the 1930s, California seismologist Charles F. Richter devised a simple numerical scale to describe the relative sizes of earthquakes in Southern California. The name "Richter scaleRichter magnitude scale
The expression Richter magnitude scale refers to a number of ways to assign a single number to quantify the energy contained in an earthquake....
" was coined by journalists and is not generally used by seismologists in technical literature. ML is obtained by measuring the maximum displacement amplitude of a recording on a Wood-Anderson torsion seismometer
Seismometer
Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
(or one calibrated to it) at a distance of up to 600 km from the epicenter of the earthquake. Other more recent magnitude measurements include: body wave magnitude
Body wave magnitude
Body wave magnitude is a way of determining the size of an earthquake, using the amplitude of the initial P-wave to calculate the magnitude. The P-wave is a type of body wave that is capable of traveling through the earth at a velocity of around 5 to 8 km/s, and is the first wave from an...
(mb), surface wave magnitude
Surface wave 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...
(Ms), and duration magnitude (MD). Each of these is scaled to give values similar to those given by the local magnitude scale; but because each is based on a measurement of one aspect of the seismogram
Seismogram
A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms typically record motions in three cartesian axes , with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface...
, they do not always capture the overall power of the source. Specifically, some can be affected by saturation at higher magnitude values—meaning that they systematically underestimate the magnitude of larger events. This problem sets in at around magnitude 6 for local magnitude; surface-wave magnitude saturates above 8. Despite the limitations of older magnitude scales, they are still in wide use, as they can be calculated rapidly, catalogues of them dating back many years are available, they are sufficient for the vast majority of observed events, and the public is familiar with them.
Moment magnitude scale
Because of the limitations of the magnitude scales, a new, more uniformly applicable extension of them, known as moment magnitude (MW) scale for representing the size of earthquakes, was introduced by Thomas C. Hanks and Hiroo KanamoriHiroo Kanamori
is a Japanese American seismologist who has made fundamental contributions to understanding the physics of earthquakes and the tectonic processes that cause them....
in 1977. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of moment
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying slip displacement—as well as the amount of energy released. So while seismic moment, too, is calculated from seismogram
Seismogram
A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms typically record motions in three cartesian axes , with the z axis perpendicular to the Earth's surface and the x- and y- axes parallel to the surface...
s, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for observed earthquakes range over more than 15 orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes.
See also
- Earthquake duration magnitudeEarthquake duration magnitudeThe 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...
- Earthquake engineeringEarthquake engineeringEarthquake engineering is the scientific field concerned with protecting society, the natural and the man-made environment from earthquakes by limiting the seismic risk to socio-economically acceptable levels...
- Seismic performance
- Spectral accelerationSpectral accelerationSpectral acceleration is a unit measured in g that describes the maximum acceleration in an earthquake on an object – specifically a damped, harmonic oscillator moving in one physical dimension...
- Peak ground accelerationPeak ground accelerationPeak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...