Measuring the Size of an Earthquake Jump to Navigation Measuring the Size of an Earthquake Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter Scale The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.

Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph. Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On the average, one earthquake of such size occurs somewhere in the world each year.

The Richter Scale is not commonly used anymore, as it has been replaced by another scale called the moment magnitude scale which is a more accurate measure of the earthquake size. Magnitude Modern seismographic systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time. This amplification and recording as a function of time is the source of instrumental amplitude and arrival-time data on near and distant earthquakes. Although similar seismographs have existed since the 1890's, it was only in the 1930's that Charles F. Richter, a California seismologist, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake's inherent size or strength could be made. Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary.

Richter's original magnitude scale (ML) was then extended to observations of earthquakes of any distance and of focal depths ranging between 0 and 700 km. Because earthquakes excite both body waves, which travel into and through the Earth, and surface waves, which are constrained to follow the natural wave guide of the Earth's uppermost layers, two magnitude scales evolved - the mb and MS scales. The standard body-wave magnitude formula is mb = log10(A/T) + Q(D,h) , where A is the amplitude of ground motion (in microns); T is the corresponding period (in seconds); and Q(D,h) is a correction factor that is a function of distance, D (degrees), between epicenter and station and focal depth, h (in kilometers), of the earthquake. The standard surface-wave formula is MS = log10 (A/T) + 1.66 log10 (D) + 3.30 . There are many variations of these formulas that take into account effects of specific geographic regions, so that the final computed magnitude is reasonably consistent with Richter's original definition of ML. Negative magnitude values are permissible.

A rough idea of frequency of occurrence of large earthquakes is given by the following table: MS 6.0 - 6.4 210 This table is based on data for a recent 47 year period. Perhaps the rates of earthquake occurrence are highly variable and some other 47 year period could give quite different results. The original mb scale utilized compressional body P-wave amplitudes with periods of 4-5 s, but recent observations are generally of 1 s-period P waves. The MS scale has consistently used Rayleigh surface waves in the period range from 18 to 22 s. When initially developed, these magnitude scales were considered to be equivalent; in other words, earthquakes of all sizes were thought to radiate fixed proportions of energy at different periods. But it turns out that larger earthquakes, which have larger rupture surfaces, systematically radiate more long-period energy. Thus, for very large earthquakes, body-wave magnitudes badly underestimate true earthquake size; the maximum body-wave magnitudes are about 6.5 - 6.8. In fact, the surface-wave magnitudes underestimate the size of very large earthquakes; the maximum observed values are about 8.3 - 8.7.

Some investigators have suggested that the 100 s mantle Love waves (a type of surface wave) should be used to estimate magnitude of great earthquakes. However, even this approach ignores the fact that damage to structure is often caused by energy at shorter periods. Thus, modern seismologists are increasingly turning to two separate parameters to describe the physical effects of an earthquake: seismic moment and radiated energy. Fault Geometry and Seismic Moment, MO The orientation of the fault, direction of fault movement, and size of an earthquake can be described by the fault geometry and seismic moment. These parameters are determined from waveform analysis of the seismograms produced by an earthquake. The differing shapes and directions of motion of the waveforms recorded at different distances and azimuths from the earthquake are used to determine the fault geometry, and the wave amplitudes are used to compute moment. The seismic moment is related to fundamental parameters of the faulting process. MO = µSd , where µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault.

Because fault geometry and observer azimuth are a part of the computation, moment is a more consistent measure of earthquake size than is magnitude, and more importantly, moment does not have an intrinsic upper bound. These factors have led to the definition of a new magnitude scale MW, based on seismic moment, where MW = 2/3 log10(MO) - 10.7 . The two largest reported moments are 2.5 X 1030 dyn·cm (dyne·centimeters) for the 1960 Chile earthquake (MS 8.5; MW 9.6) and 7.5 X 1029 dyn·cm for the 1964 Alaska earthquake (MS 8.3; MW 9.2). MS approaches its maximum value at a moment between 1028 and 1029 dyn·cm. Energy, E The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures. Theoretically, its computation requires summing the energy flux over a broad suite of frequencies generated by an earthquake as it ruptures a fault.

Because of instrumental limitations, most estimates of energy have historically relied on the empirical relationship developed by Beno Gutenberg and Charles Richter: log10E = 11.8 + 1.5MS where energy, E, is expressed in ergs. The drawback of this method is that MS is computed from an bandwidth between approximately 18 to 22 s. It is now known that the energy radiated by an earthquake is concentrated over a different bandwidth and at higher frequencies. With the worldwide deployment of modern digitally recording seismograph with broad bandwidth response, computerized methods are now able to make accurate and explicit estimates of energy on a routine basis for all major earthquakes. A magnitude based on energy radiated by an earthquake, Me, can now be defined, Me = 2/3 log10E - 2.9. For every increase in magnitude by 1 unit, the associated seismic energy increases by about 32 times. Although Mw and Me are both magnitudes, they describe different physical properites of the earthquake. Mw, computed from low-frequency seismic data, is a measure of the area ruptured by an earthquake. Me, computed from high frequency seismic data, is a measure of seismic potential for damage.

Consequently, Mw and Me often do not have the same numerical value. Intensity The increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity is based on an earthquake's local accelerations and how long these persist. Intensity and magnitude thus both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to a receiver. These factors make it difficult for engineers and others who use earthquake intensity and magnitude data to evaluate the error bounds that may exist for their particular applications. An example of how local soil conditions can greatly influence local intensity is given by catastrophic damage in Mexico City from the 1985, MS 8.1 Mexico earthquake centered some 300 km away. Resonances of the soil-filled basin under parts of Mexico City amplified ground motions for periods of 2 seconds by a factor of 75 times. This shaking led to selective damage to buildings 15 - 25 stories high (same resonant period), resulting in losses to buildings of about $4.0 billion and at least 8,000 fatalities. The occurrence of an earthquake is a complex physical process.

When an earthquake occurs, much of the available local stress is used to power the earthquake fracture growth to produce heat rather than to generate seismic waves. Of an earthquake system's total energy, perhaps 10 percent to less that 1 percent is ultimately radiated as seismic energy. So the degree to which an earthquake lowers the Earth's available potential energy is only fractionally observed as radiated seismic energy. Determining the Depth of an Earthquake Earthquakes can occur anywhere between the Earth's surface and about 700 kilometers below the surface. For scientific purposes, this earthquake depth range of 0 - 700 km is divided into three zones: shallow, intermediate, and deep. Shallow earthquakes are between 0 and 70 km deep; intermediate earthquakes, 70 - 300 km deep; and deep earthquakes, 300 - 700 km deep. In general, the term "deep-focus earthquakes" is applied to earthquakes deeper than 70 km. All earthquakes deeper than 70 km are localized within great slabs of shallow lithosphere that are sinking into the Earth's mantle. The evidence for deep-focus earthquakes was discovered in 1922 by H.H.

Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismograms of several earthquakes, which in turn led to the construction of travel-time curves for intermediate and deep earthquakes. The most obvious indication on a seismogram that a large earthquake has a deep focus is the small amplitude, or height, of the recorded surface waves and the uncomplicated character of the P and S waves. Although the surface-wave pattern does generally indicate that an earthquake is either shallow or may have some depth, the most accurate method of determining the focal depth of an earthquake is to read a depth phase recorded on the seismogram. The most characteristic depth phase is pP. This is the P wave that is reflected from the surface of the Earth at a point relatively near the epicenter. At distant seismograph stations, the pP follows the P wave by a time interval that changes slowly with distance but rapidly with depth. This time interval, pP-P (pP minus P), is used to compute depth-of-focus tables.

Using the time difference of pP-P as read from the seismogram and the distance between the epicenter and the seismograph station, the depth of the earthquake can be determined from published travel-time curves or depth tables. Another seismic wave used to determine focal depth is the sP phase - an S wave reflected as a P wave from the Earth's surface at a point near the epicenter. This wave is recorded after the pP by about one-half of the pP-P time interval. The depth of an earthquake can be determined from the sP phase in the same manner as the pP phase by using the appropriate travel-time curves or depth tables for sP. If the pP and sP waves can be identified on the seismogram, an accurate focal depth can be determined. by William Spence, Stuart A. Sipkin, and George L. Choy Earthquakes and Volcanoes How Are Earthquake Magnitudes Measured? How Are Earthquake Magnitudes Measured? The Richter Scale Figure 1 - Charles Richter studying a seismogram. There are a number of ways to measure the magnitude of an earthquake. The first widely-used method, the Richter scale, was developed by Charles F.

Richter in 1934. It used a formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer. That scale was specific to California earthquakes; other scales, based on wave amplitudes and total earthquake duration, were developed for use in other situations and they were designed to be consistent with Richter’s scale. The Moment Magnitude Scale Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally. The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modeling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read ‘magnitude 8’) and greater events accurately. Magnitudes are based on a logarithmic scale (base 10).

What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people. Magnitude scales can be used to desribe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond. Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year.

The Mercalli Scale Figure 2 - Giuseppe Mercalli Another way to measure the strength of an earthquake is to use the Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate its intensity. The Mercalli scale isn't considered as scientific as the Richter scale, though. Some witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. The amount of damage caused by the earthquake may not accurately record how strong it was either. Some things that affect the amount of damage that occurs are: the building designs, the distance from the epicenter, and the type of surface material (rock or dirt) the buildings rest on. Different building designs hold up differently in an earthquake and the further you are from the earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand makes a big difference in how much damage it takes. Solid rock usually shakes less than sand, so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a sandy lot.

Magnitude and Energy | Earth 520: Plate Tectonics and People: Foundations of Solid Earth Science When an earthquake happens, the main quantity of interest is its magnitude. How big was the earthquake? Over the years, scientists have developed various ways to measure earthquake size and strength. Here's an overview of the most common magnitude measurements. Mercalli Intensity Scale This scale is a qualitative measure of the amount of felt shaking caused by an earthquake. This scale goes from I (not much of anything) to XII (total destruction). The amount of felt shaking is generally measured by interviewing witnesses to find out how much shaking they felt. Sometimes the interviews can be supplemented by observing any earthquake damage to buildings. For earthquakes in the historic record that happened before the advent of seismometers, Mercalli intensities are often assigned by checking out old newspaper reports and examining the foundations of old buildings. Mercalli intensities are generally depicted on maps by several concentric rings around the epicenter of the earthquake that give some idea of the severity of the felt shaking at various distances away from the earthquake.

The Mercalli is not a very useful scale for science because it can't tell us much about big earthquakes that are not felt by people--earthquakes that occur at a great depth, or in the ocean, for example. Isoseismal map showing reconstructed Mercalli intensities from the 1895 31 Oct earthquake at Charleston Missouri. Source: Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, USGS, Washington: 1993. Richter Scale A Wood-Anderson torsion short-period seismometer. This is the type of seismometer used by Richter to measure earthquake amplitude and develop his magnitude scale. Source: USGS Charles Richter developed a magnitude scale in the 1930s because he wanted to be able to characterize the seismicity he had been measuring in California with some kind of numbering system that would encompass all the earthquakes, from ones that had hardly been felt at all, up to really big ones.

The way he did this was to pick a reference earthquake and measure its maximum ground motion. Then all the other earthquakes he had recorded could be compared to the reference, after correcting for distance. Each integer increase represented a factor of 10 increase in ground motion amplitude. This scale worked because he always used the same type of seismometer and all his earthquakes were in southern California, so there didn't have to be any extra corrections for different depth or rock type. Today, scientists don't use the Richter scale the way he did because not all earthquakes of interest happen in California, and also because the type of seismometer he used is out of date now. For all measurements recorded on a different type of instrument or that measured amplitudes of different wavetypes, you'd have to make a conversion to get your number to be in line with Richter's scale. The handy thing about the Richter scale is that for most earthquakes of interest, the magnitudes end up being numbers that range from about 3 - 9 and these are numbers easily understood by people. (In fact the scale itself does not set an upper or lower limit; limits of earthquake size are set by the Earth).

Moment Magnitude Today scientists calculate the seismic moment of an earthquake when they want to talk about how big it was. Seismic moment (M0) is simply the product of the average amount of slip that happened, the area of the fault that slipped, and the rigidity of the rock. The equation is : M0 = rigidity x area x slip The rigidity of rock is a constant number based on the rock type. It has units of pressure. Typical assumptions are on the order of 3 x 1010 N/m2. Slip is a length and it is on the order of centimeters (meters for a great earthquake). Area is in units of length2 and is often on the order of km2. The units for seismic moment are then Nm (newton meters). As an example, the 2004 26 December Sumatra-Andaman earthquake had the following dimensions as reported by Lay et al. (2005): Its slip averaged about 5 m, its rupture length was about 1300 km and the fault width was between 160 - 240 km.

Assuming a rigidity of 3 x 1010 N/m2 gives us a seismic moment of 4.4 x 1022Nm. Now, the problem with seismic moment is that regular people who watch the news or read the papers don't really know what to make of a number like 4.4 x 1022Nm, and even scientists are smart enough to realize this, so we convert seismic moment to a magnitude scale called "moment magnitude," usually symbolized Mw or sometimes just M. This scale is designed purposely to be about equal to Richter's scale so that people will have a feel for what it means. Hiroo Kanamori at Caltech derived the formula for the conversion of seismic moment to moment magnitude as follows: Mw = (2/3)*logM0 - 6.05 Calculate magnitude yourself! Using Kanamori's formula to convert seismic moment to moment magnitude, what was the magnitude of the 2004 Dec 26 Sumatra-Andaman earthquake? Try it yourself and then check your work by clicking below: Click here for my answer for the magnitude of the Sumatra-Andaman earthquake. The answer is 9.05.

Here is how I got that answer: The seismic moment was 4.4 x 10^22 Nm. Take the log (base 10) of that number and get 22.64 Multiply that by 2/3 and get 15.10 Subtract 6.05 and get 9.05 Energy Another way to think about earthquake size is in terms of the energy released by an earthquake. It is actually not terribly easy to measure all the energy released by an earthquake because you have to integrate over time and space and include the broadest possible spectrum of frequencies to make sure you are recording all the energy. Therefore, direct measurements usually underestimate the energy. If they aren't trying to measure it instrumentally, most seismologists just use the following empirical formula, developed by Båth (1966) to relate magnitude to energy (in units of joules): log E = 5.24 + 1.44M This relationship was only meant to work for fairly big (M > 5) earthquakes. Plug magnitude values of 5.0, 6.0, and 7.0 into the equation above.

The energy released by an M5 earthquake is about 2.8 x 1012 joules. An M6 earthquake releases 7.8 x 1013 joules, and an M7 radiates 2.1 x 1015 joules. If you don't have a sense for what these numbers mean, the bomb dropped on Hiroshima released about 7.4 x 1012 joules. Even though all of these are large numbers, what I want you to see here is that the difference between these values is huge. A magnitude 7 releases almost 30 times as much energy as a magnitude 6. Credit: FreePhoto.com Time for the big picture! Using the formula to relate magnitude to energy, how much energy was radiated by the Sumatra-Andaman earthquake? Try it yourself and then check your work by clicking below: Click here for my answer to how much energy was radiated by the Sumatra-Andaman earthquake. The answer is 1.84 x 10^18 joules. Here is how I got that answer: The magnitude is 9.05. We already knew that.

Multiply magnitude by 1.44 and get 13.03. Add 5.24 and get 18.27 Raise 10 to the power of 18.27 and get 1.84 x 10^18 joules A 60-watt incandescent light bulb uses 60 joules of energy per second. The rupture of the Sumatra-Andaman earthquake lasted about 500 seconds. So,this earthquake released enough energy during its rupture to light up how many 60-watt lightbulbs? Try it yourself and then check your work by clicking below: Click here for my answer to how many 60-watt lightbulbs the energy would light up. The answer is 6.14 x 10^13 60-watt lightbulbs. Here is how I got that answer: The earthquake radiated 1.84 x 10^18 joules. We knew that already.

Multiply 60 by 500 and get 30000 joules used by one lightbulb in 500 seconds Divide that into 1.84 x 10^18 joules and get 6.15 x 10^13 lightbulbs Wow, that's a lot of light bulbs! In fact, if you assume they are all about 4 inches in diameter, then if you put this many lightbulbs side by side you would have a trail that goes farther than the distance from the Sun to Neptune. That's a long way. That was a really big earthquake. Richter Scale & Magnitude Richter Scale What is the Richter Scale The Richter magnitude scale (often shortened to Richter scale) is the most common standard of measurement for earthquakes. It was invented in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The Richter scale is used to rate the magnitude of an earthquake, that is the amount of energy released during an earthquake.

Image: What is the Richter Scale The Richter scale doesn't measure quake damage (see: Mercalli Scale ) which is dependent on a variety of factors including population at the epicentre, terrain, depth, etc. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frightening the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans. Image: Richter Scale of Earthquake Energy The magnitude of an earthquake is determined using information gathered by a seismograph . How is the Richter Scale calculated The Richter magnitude involves measuring the amplitude (height) of the largest recorded wave at a specific distance from the seismic source. Adjustments are included for the variation in the distance between the various seismographs and the epicentre of the earthquakes. The Richter scale is a base-10 logarithmic scale, meaning that each order of magnitude is 10 times more intensive than the last one. In other words, a two is 10 times more intense than a one and a three is 100 times greater. In the case of the Richter scale, the increase is in wave amplitude.

That is, the wave amplitude in a level 6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude increases 100 times between a level 7 earthquake and a level 9 earthquake. The amount of energy released increases 31.7 times between whole number values. Image: How is the Richter Scale calculated In fact, it is to point out that, while it is correct to say that for each increase in 1 in the Richter magnitude there is a tenfold increase in amplitude of the wave, it is incorrect to say that each increase of 1 in Richter magnitude represents a tenfold increase in the size of the earthquake (as is commonly incorrectly stated by the press). A better measure of the size of an earthquake is the amount of energy released by the earthquake, which is related to the Richter Scale by the following equation: Log E = 11.8 + 1.5 M (where Log refers to the logarithm to the base 10, E is the energy released in ergs and M the Richter magnitude).

Richter Scale formula This above principle is expressed by the original Richter Scale formula as: where A is the maximum excursion of the Wood-Anderson seismograph , the empirical function A0 depends only on the epicentral distance of the station, . In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the ML value. Richter Scale rating explained On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Image: Richter Scale Graphic Representation At first, the Richter scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph . The Richter Scale has no upper limit.

Recently, another scale called the moment magnitude scale MMS has been devised for more precise study of great earthquakes. The Richter Scale Explained in a video Video: Learn about Richter Scale - What is it? Video: Bill Nye the Science Guy- Earthquakes and Richter Scale Video: How things work: How Does the Richter Scale Work? Highest magnitude ever recorded Tectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events (magnitude greater than 9), where fault displacements are on the order of many metres. The size of an earthquake is not only a function of the amount of displacement but also the area of the fault plane that ruptures. Hence the larger the rupture area, the larger is the earthquake. A magnitude 7 earthquake ruptures a fault area of about 1000 km2 or about 50 km long and 20 km wide. Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at various depths within the Earth’s solid core. The deeper the earthquake, the more powerful it is, but it is also far less likely to reach the surface.

That’s why shallow earthquakes are more common and more dangerous, because the shallower an earthquake, the more damage to surface structures it can cause. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Image: Photo from March 2011 earthquake in Japan Fortunately most earthquakes are extremely small. A majority of quakes register less than 3 on the Richter scale; these tremors, called micro-earthquakes, aren’t generally felt by people and are usually recorded only on local seismographs . With every whole point the magnitude rises, the strength of the waves increases tenfold. Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong enough to be recorded by sensitive seismographs all over the world.

Only a tiny portion of earthquakes (15 or so of the 1.4 million quakes that register above 2.0) register at 7 or above, which is the threshold for a quake to be considered major. A temblor with a magnitude of 8.0 or higher is considered a "great earthquake." On the average, one earthquake of such size, such as the 1964 Good Friday Earthquake in Alaska and the 2011 Japanese Earthquake and Tsunami, occurs somewhere in the world each year. The biggest quake in recorded history was the 9.5 quake that struck Chile in 1960 . It killed nearly 1,900 people and caused about $4 billion in damage in 2010 dollars. Generally, you won't see much damage from earthquakes that register below 4 on the Richter scale. Other useful resources Richter Magnitude 7.0-7.9 Major earthquake. Can cause serious damage over larger areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across. Although each earthquake has a unique Magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors.

Seismologists use a different Mercalli Intensity Scale to express the variable effects of an earthquake. Each earthquake has a unique amount of energy, but magnitude values given by different seismological observatories for an event may vary. Depending on the size, nature, and location of an earthquake, seismologists use several different methods to estimate magnitude. The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data. Long answer: One of Dr. Charles F. Richter's most valuable contributions was to recognize that the seismic waves radiated by all earthquakes can provide good estimates of their magnitudes. You can read about seismic waves by clicking here . He collected the recordings of seismic waves from a large number of earthquakes, and developed a calibrated system of measuring them for magnitude. Richter showed that, the larger the intrinsic energy of the earthquake, the larger the amplitude of ground motion at a given distance. He calibrated his scale of magnitudes using measured maximum amplitudes of shear waves on seismometers particularly sensitive to shear waves with periods of about one second.

The records had to be obtained from a specific kind of instrument, called a Wood-Anderson seismograph. Although his work was originally calibrated only for these specific seismometers, and only for earthquakes in southern California, seismologists have developed scale factors to extend Richter's magnitude scale to many other types of measurements on all types of seismometers, all over the world. In fact, magnitude estimates have been made for thousands of Moon-quakes and for two quakes on Mars. The diagram below demonstrates how to use Richter's original method to measure a seismogram for a magnitude estimate in Southern California: The scales in the diagram above form a nomogram that allows you to do the mathematical computation quickly by eye. The equation for Richter Magnitude is: ML = log10A(mm) + (Distance correction factor) Here A is the amplitude, in millimeters, measured directly from the photographic paper record of the Wood-Anderson seismometer, a special type of instrument. The distance factor comes from a table that can be found in Richter's (1958) book Elementary Seismology.

The equation behind this nomogram, used by Richter in Southern California, is: Thus after you measure the wave amplitude you have to take its logarithm, and scale it according to the distance of the seismometer from the earthquake, estimated by the S-P time difference. The S-P time, in seconds, makes . Seismologists will try to get a separate magnitude estimate from every seismograph station that records the earthquake, and then average them. This accounts for the usual spread of around 0.2 magnitude units that you see reported from different seismological labs right after an earthquake. Each lab is averaging in different stations that they have access to. It may be several days before different organizations will come to a consensus on what was the best magnitude estimate. Seismic Moment: Seismologists have more recently developed a standard magnitude scale that is completely independent of the type of instrument. It is called the moment magnitude, and it comes from the seismic moment. To get an idea of the seismic moment, we go back to the elementary physics concept of torque. A torque is a force that changes the angular momentum of a system. It is defined as the force times the distance from the center of rotation.

Earthquakes are caused by internal torques, from the interactions of different blocks of the earth on opposite sides of faults. After some rather complicated mathematics, it can be shown that the moment of an earthquake is simply expressed by: The formula above, for the moment of an earthquake, is fundamental to seismologists' understanding of how dangerous faults of a certain size can be. Now, let's imagine a chunk of rock on a lab bench, the rigidity, or resistance to shearing, of the rock is a pressure in the neighborhood of a few hundred billion dynes per square centimeter. (Scientific notation makes this easier to write.) The pressure acts over an area to produce a force, and you can see that the cm-squared units cancel. Now if we guess that the distance the two parts grind together before they fly apart is about a centimeter, then we can calculate the moment, in dyne-cm: Again it is helpful to use scientific notation, since a dyne-cm is really a puny amount of moment. Now let's consider a second case, the Sept. 12, 1994 Double Spring Flat earthquake, which occurred about 25 km southeast of Gardnerville.

The first thing we have to do, since we're working in centimeters, is figure out how to convert the 15 kilometer length and 10 km depth of that fault to centimeters. We know that 100 thousand centimeters equal one kilometer, so we can write that equation and divide both sides by "km" to get a factor equal to one. Of course we can multiply anything by one without changing it, so we use it to cancel the kilometer units and put in the right centimeter units: Of course this result needs scientific notation even more desperately. We can see that this earthquake, the largest in Nevada in 28 years, had two times ten raised to the twelfth power, or 2 trillion, times as much moment as breaking the rock on the lab table. There is a standard way to convert a seismic moment to a magnitude. The equation is: Now let's use this equation (meant for energies expressed in dyne-cm units) to estimate the magnitude of the tiny earthquake we can make on a lab table: Negative magnitudes are allowed on Richter's scale, although such earthquakes are certainly very small.

Next let's take the energy we found for the Double Spring Flat earthquake and estimate its magnitude: The magnitude 6.1 value we get is about equal to the magnitude reported by the UNR Seismological Lab, and by other observers. Seismic Energy: Both the magnitude and the seismic moment are related to the amount of energy that is radiated by an earthquake. Richter, working with Dr. Beno Gutenberg, early on developed a relationship between magnitude and energy. Their relationship is: logES = 11.8 + 1.5M giving the energy ES in ergs from the magnitude M. Note that ES is not the total ``intrinsic'' energy of the earthquake, transferred from sources such as gravitational energy or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which ought to be a small fraction of the total energy transferred during the earthquake process. More recently, Dr. Hiroo Kanamori came up with a relationship between seismic moment and seismic wave energy. It gives: Energy = (Moment)/20,000 For this moment is in units of dyne-cm, and energy is in units of ergs.

dyne-cm and ergs are unit equivalents, but have different physical meaning. Let's take a look at the seismic wave energy yielded by our two examples, in comparison to that of a number of earthquakes and other phenomena.

For this we'll use a larger unit of energy, the seismic energy yield of quantities of the explosive TNT (We assume one ounce of TNT exploded below ground yields 640 million ergs of seismic wave energy): Richter TNT for Seismic Example Magnitude Energy Yield (approximate) 5.5 80,000 tons Little Skull Mtn., NV Quake, 1992 6.0 1 million tons Double Spring Flat, NV Quake, 1994 6.5 5 million tons Northridge, CA Quake, 1994 7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon 7.5 160 million tons Landers, CA Quake, 1992 8.0 1 billion tons San Francisco, CA Quake, 1906 8.5 5 billion tons Anchorage, AK Quake, 1964 9.0 32 billion tons Chilean Quake, 1960 10.0 1 trillion tons (San-Andreas type fault circling

 Earth) 12.0 160 trillion tons (Fault Earth in half through center, OR Earth's daily receipt of solar energy) 160 trillion tons of dynamite is a frightening yield of energy.

Consider, however, that the Earth receives that amount in sunlight every day. Practical ways of estimating magnitude Most seismologists prefer to use the seismic moment to estimate earthquake magnitudes. Finding an earthquake fault's length, depth, and its slip can take several days, weeks, or even months after a big earthquake. Geologists' mapping of the earthquake's fault breaks, or seismologists' plotting of the spatial distribution of aftershocks, can give these parameters after a substantial effort. But some large earthquakes, and most small earthquakes, show neither surface fault breaks nor enough aftershocks to estimate magnitudes the way we have above. However, seismologists have developed ways to estimate the seismic moment directly from seismograms using computer processing methods. The Centroid Moment Tensor Project at Harvard University has been routinely estimating moments of large earthquakes around the world by seismogram inversion since 1982. Another measure of an earthquake Seismologists use a separate method to estimate the effects of an earthquake, called its intensity. Intensity should not be confused with magnitude. Although each earthquake has a single magnitude value, its effects will vary from place to place, and there will be many different intensity estimates.

You can read about the Mercalli Intensity Scale , one popular way to characterize earthquake effects. Magnitude/Intensity | Pacific Northwest Seismic Network Magnitude/Intensity Magnitude/Intensity Perhaps no seismic subject is as irksome to seismologists as discussions of earthquake size. There often seems to be no end of confusion, misunderstanding, and over-interpretation of what are really pretty crude metrics. And when news announcers mention the "Richter Scale" seismologists the world over begin gnashing their teeth. Magnitude A familiar analogy to help understand earthquake size metrics is to think about a light bulb. One measure of the strength of a light bulb is how much energy it uses. A 100-watt bulb is brighter than a 50-watt bulb, but not nearly as bright as a 250-watt bulb. The wattage of a bulb tells you about the strength of the light source. In the same way, earthquakes magnitude is an objective measurement of the energy radiated by an earthquake. However, earthquake magnitude has no physical units, nor a meaningful 0. This is because we can't easily measure the energy the way we can with an electric circuit, so seismologists commonly use a relative measure.

It is easier to choose a particular earthquake recorded at a particular distance as a "standard" earthquake, call it a magnitude 1. An earthquake that causes ground motion at a seismic station (when corrected for distance) 10 times larger than the reference earthquake is M2. An earthquake causing motion at that distance 10 times larger than an M2 is an M3, and so on. To achieve this ten fold increase in ground motion require about 32 to 33 times the energy. When referring to the power or energy released in an earthquake this 32 mutiplier is used. An earthquake that releases about 33 times less energy and causes motion 10 times smaller than an M1 is an M0--and magnitudes can even go negative. This Live Science animation will help you visualize the expodential magnitude scale in terms of energy released. Intensity Earthquake intensity measures how strongly the earthquake impacts a specific location. In the light bulb analogy, it is the brightness with which you perceive the light at a place in a room. Can you read a fine-print book by the lamp? Pick up a needle? Perform delicate surgery?

Depends on the wattage of the bulb, and how far you are from it, right? If you mapped out the brightness in terms of what you could accomplish at the light level in a room, you'd have an intensity map. Well, you can make a map of earthquake impacts using the Modified Mercalli Intensity Scale (MMI), which derived from an earlier ten-degree Rossi-Forel scale , later revised by Italian vulcanologist Giuseppe Mercalli in 1884 and 1906 to quantify (somewhat) the earthquake effects. Further refinements for more modern construction were published in 1931 by the American seismologists Harry Wood and Frank Neumann. Measurements of instensity using the Modified Mercalli scale, are composed of 12 increasing levels that range from imperceptible shaking to catastrophic destruction, usually designated by Roman numerals, which stresses their semi-quantitative nature.

Whereas an earthquake will have one magnitude (well, as noted below, there are likely to be several different estimates of the magntitude of an earthquake depending on the type of estimate etc.), for each individual earthquake there will be a range of intensities depending in part on the magnitude of the source, but also the location of the site at which the intensity was observed. More about Magnitudes - The 3 Approaches If you've had some time to think a bit about the Earthquake/Light bulb analogy you probably have come up with some questions, like: But some bulbs are blue and some white and some yellow, what gives with that? And, flourescent fixtures put out really different quality light than incandescents, and are brighter for the same wattage, eh? And, what about a flash bulb from my camera which is incredibly bright...but just for a fraction of second? And, if my room is filled with steam or smoke, is the intensity still the same? These are all good points; it turns out that similarly for earthquakes the complexities and variability of earthquake rupture processes and of seismic waves as they travel through Earth (and evolving seismometer design and sensitivities) there are different methods for measuring the magnitude of an earthquake.

These may return a slightly different number when used to estimate the energy released in an earthquake. In overview, there are three approaches to using seismograph recordings to quantify earthquake sizes, with numerous flavors of each. One approach is to use some measure of peak amplitudes recorded on seismograms. A second is to use the duration of shaking recorded. A third is to try to match the actual waveform wiggle-for-wiggle with a mathematical model (a "synthetic seismogram"), and report the size of the modeled earthquake. 1. Amplitude based Magnitudes (Local, Body Wave, Surface Wave) The oldest and most famous magnitude measurement method, the Local Magnitude (it used to be referred to as the Richter magnitude), was developed by Dr. Charles Richter and Beno Gutenberg of Cal Tech in 1935. Written as ML, it remains the go-to method for measuring small- to medium-sized earthquakes within 600 km from the recording seismograph. Richter used seismograms recorded on a particular instrument, the Wood-Anderson torsion seismograph. Today, we use more modern instruments and account for their response to mimic what the historical Wood-Anderson seismograph would have produced.

Richter's motivation for creating the local magnitude scale was to measure the ratio of small- to medium-sized earthquakes. It was never intended to measure large or distant earthquakes. All amplitude-based magnitudes rely on a base-10 logarithm of the peak amplitude measured by a seismograph. This is because there are many factors of 10 difference between the smallest and largest amplitudes of observed ground motions. An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger and corresponds to an energy release of 31.6 times greater than one that measures 4.0. Body wave magnitude is a similar concept, but applied usually to teleseisms -- earthquake more than 3000 km from the recording station -- and good for deep and shallow earthquakes. Surface wave magnitudes measure the surface waves that are generated by large regional to teleseismic earthquakes, and that travel long distances without losing much energy from absorption. In general local magnitudes "saturate" (lose resolution) for earthquakes exceeding M5.8 or so, body waves stay on scale to somewhat larger magnitudes, while surface wave magnitudes saturates at about M8 or so.

This is in general because of the frequencies of the seismic waves that each use. The higher-frequency methods (local, duration) can't see the difference between larger earthquakes, while the low frequency measures (surface waves, moment magnitude) characterize the long-wavelength energy that radiate from a bigger rupture surface. 2. Duration Magnitudes (coda magnitude) Another estimate of magnitudes is particularly useful when recordings "clip" and you can't measure the peak amplitude. This is called the coda magnitude, or duration magnitude, and is derived from the observation that the ratio of peak amplitude to the duration of shaking from an earthquake are related. PNSN often calculates the size of PNW earthquakes using the durations averaged from a number of seismograms to obtain "Md" estimates. In the past a seismic analyst picked the duration on each seismogram by eye independently, which was somewhat subjective and variable, as the background noise varies greatly. The modern method, made possible by faster computers, is to model the decaying amplitude of the seismogram in order to automatically and objectively define a duration. 3.

Waveform Modeling (moment magnitude) Modern digital seismic instrumentation, modern seismic theory, and most of all modern high-speed computers have permitted us to be able to model seismograms wiggle-for-wiggle. To do this, we use a standard model of the Earth and of the source to generate synthetic seismograms, and the computer adjusts the location, size, and orientation of the rupture to match the observed waveforms. The magnitude that is derived from waveform modeling is called the Moment magnitude, and it is in some ways the most precise estimate of earthquake size--and the only one applicable to great earthquakes M>8. Of great interest is that since we can observe the deformation of the Earth from such big earthquake ruptures, we have an independent estimate of the energy they released, so that we can (finally!) relate the magnitude to the energy released during rupture. It is worthwhile to note that all of these approaches have been calibrated and defined with respect to each other such that they all agree, on average, in the ranges in which they overlap. So for general purposes an M8 is an M8 is an M8, regardless of the method that generated it.

But also it is interesting that Richter's orignal local magnitude was the one that all the other techniques generally matched up with (since it was first) so, in a sense, the "Richter Scale" that news reporters often cite (to the chagrin of us seismologists) is really a pretty apt homage. Magnitude and Energy Below is a representation, from the Geological Society of America, of earthquake magnitudes and the equivalent energy release. (For the life of us, we don't know why the geologists used pounds of explosive as a proxy for energy instead of a real physical unit of energy, such as Joules! But the figure makes the point, anyway.) Notice the relationship is not linear? The change in the amount energy released from one magnitude to the next is greater as the earthquake magnitude increases. For example, the difference in amount of energy released from a magnitude 5 to a magnitude 10 is not double, it is 30 million times as much! Need some further practice relating Earthquake Magnitude to Energy?

No problem, the USGS calculates the difference between a 5.8 and 8.7 earthquake and has a calculator where you can input your own numbers to see how much bigger an earthquake can get with different magnitudes. Magnitude Vs. Intensity The chart below claims to compare Richter Scale magnitudes with intensities in a very generalized way--as if a "Richter magnitude" was somehow measuring the same thing as a "Mercalli intensity". Now that you know the basics of earthquake Magnitudes and earthquake Intensities, you know that this chart makes no sense. If you see it or a similar representation, you can be assured the provider of the information is unencumbered by knowledge of the basics. One Final Word - a Plea for Understanding Now that you know how many different approaches there are to measuring an earthquake, and how it depends on the traces that you use, and instrument types that you have available and how far they are from the earthquake, and how many there are.... Perhaps you'll understand why our magnitude estimates change with time immediately after an earthquake as we try to be both as fast and as accurate as possible. And why different organizations will post somewhat different magnitudes for the same earthquake.

A rule of thumb, perhaps is that in the early minutes (or tens of minutes) after an earthquake up to a half a magnitude unit of difference between estimates will generally be shrugged off by seismologists as reasonable scatter. But differences larger than that usually mean that there was a fairly serious problem...like the entirely wrong technique was used, or critical data were omitted. Contact Us Using the Richter Scale to Measure Earthquakes - Video & Lesson Transcript | Study.com Using the Richter Scale to Measure Earthquakes Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching Register for a free trial Are you a student or a teacher? I am a student Start Your Free Trial To Continue Watching As a member, you'll also get unlimited access to over lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Coming up next: Destruction Caused by Earthquakes You're on a roll. Keep up the good work! Your next lesson will play in 10 seconds 0:05 What Is the Richter Scale?

0:49 Measuring Ground Shaking 2:51 The Moment Magnitude Scale 4:01 Lesson Summary Add to Add to Add to Want to watch this again later? Log in or sign up to add this lesson to a Custom Course. Custom Courses are courses that you create from Study.com lessons. Use them just like other courses to track progress, access quizzes and exams, and share content. Teachers Organize and share selected lessons with your class. Make planning easier by creating your own custom course. Students Create a new course from any lesson page or your dashboard. From any lesson page: Click "Add to" located below the video player and follow the prompts to name your course and save your lesson. From your dashboard: Click on the "Custom Courses" tab, then click "Create course". Next, go to any lesson page and begin adding lessons. Edit your Custom Course directly from your dashboard. Personalize: Name your Custom Course and add an optional description or learning objective. Organize: Create chapters to group lesson within your course. Remove and reorder chapters and lessons at any time. Share your Custom Course or assign lessons and chapters.

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In 1935, Charles Richter developed the local magnitude scale, a scale used to measure the magnitude of earthquakes. Now called the Richter scale, these measurements tell us how much the ground shook during a quake and how much energy was released. This information comes from seismographs, which collect data and directly measure the amount of ground shaking. Measuring Ground Shaking The Richter scale runs from 1 - 10 (1 being the least in magnitude and 10 being the greatest), but it is logarithmic. This means that for each 1 point in increase on the scale we get 10 times more ground shaking. Let's look at an example. Say we have a magnitude 1 earthquake on the Richter scale, which is the lowest magnitude earthquake. Compare that with a magnitude 2 earthquake, which is only one step higher (remember, the scale runs from 1 - 10), and you now have 10 times more ground shaking than with the magnitude 1 quake.

Take another step up the scale, so from magnitude 1 to magnitude 3, and this is 10 times more than that first step, so we now have 100 times more ground shaking with just two steps up the scale. That's a lot of seismic activity! Now let's see how this relates to the total energy released during an earthquake. Through measurements of seismic activity, scientists know that the energy released by an earthquake, which is what causes all that shaking and moving in the first place, increases 32 times for each step up the Richter scale. Take our example from before where we have a magnitude 1 and a magnitude 2 quake. The magnitude 2 quake will have 32 times more energy than the magnitude 1 earthquake. Now let's compare our magnitude 1 and magnitude 3 quakes. The magnitude 3 quake is two steps up the Richter scale, so we have 32 times more energy for the first step and then 32 times more energy than that up the second step. 32 times 32 gives us about 1,000 times the energy with just two steps up the scale! This is even more amazing when you compare this with the difference in ground shaking.

A magnitude 3 earthquake has 100 times more ground shaking than a magnitude 1 quake but about 1,000 times the energy. That's a big difference! The Moment Magnitude Scale The Richter scale was originally designed to measure medium-sized earthquakes, those between magnitude 3 and 7, and within a distance of about 400 miles. The moment magnitude scale was created in 1979 to deal with these issues, but it builds on the Richter scale because it was already so accurate for small- to medium-sized quakes. The moment magnitude scale is the currently accepted scale used to measure medium- to large-sized earthquakes. The Richter scale remains the standard for quakes smaller than 3.5 in magnitude, and luckily, this is where most earthquakes fall on the scale. About 1,000,000 magnitude 2 earthquakes occur each year, compared with only about 1,000 magnitude 5 quakes. Magnitude 10 earthquakes are extremely rare, occurring less than once a year.

Considering that the energy release of a magnitude 2 quake is equal to about 50 kg of explosive while a magnitude 5 releases energy equivalent to 1,800,000 kg, we should be very thankful that they are so infrequent! Lesson Summary The shaking and moving of the ground from earthquakes is not predictable, but it can be measured. Seismic activity, which is the shaking of the ground, is measured by seismographs. The data that scientists collect from these devices can be used to describe the magnitude of an earthquake and the amount of energy it releases from the stress built up underground. × Missouri Geological Survey Missouri Geological Survey Methods of Earthquake Measurement Explained The “size” of earthquakes is commonly expressed in two ways- magnitude and intensity. Magnitude is a measure of the total energy released during an earthquake. It is determined from a seismogram, which plots the ground motion produced by seismic waves. As devised by C.F. Richter in 1935, the magnitude scale allows us to compare earthquakes in relative terms. Though the term is used somewhat indiscriminately, magnitude is a highly technical calculation.

Magnitude is defined as the logarithm (1 micron=.0001 centimeter), as would be recorded by a standard short-period seismograph with specific constants, at an epicentral distance of 100 kilometers (62 miles). The important thing to remember about magnitude is that the scale is logarithmic, which means that each step in magnitude represents a tenfold increase in amplitude of wave motion. Therefore, an earthquake of magnitude 6.0 has ten times the wave amplitude of an earthquake of magnitude 5.0, a hundred times the wave amplitude of a magnitude 4.0 earthquake, and one thousand times the wave amplitude of a magnitude 3.0 earthquake. Because magnitude does not describe the extent of the damage, its usefulness is limited to an approximation of whether the earthquake is large, small, or medium-sized. The destructiveness of an earthquake is a complex matter, related to the geology, population density, and cultural features of a specific area at a specific distance from the epicenter. Seismologists and geologists also describe earthquakes by their intensity. Measured on a numerical scale, intensity is the degree of damage or observable effects caused by an earthquake at a particular location.

An earthquake of a particular magnitude will produce different intensities at different places, according to geology, population density, cultural features, and distance from the epicenter. The most widely used intensity scale, the Modified Mercalli Scale, is divided into 12 degrees, each identified by a Roman numeral. For example, an earthquake intense enough to be felt by a person standing nearby is said to have an intensity of MM III. Earthquake Facts The Severity of an Earthquake The Severity of an Earthquake The severity of an earthquake can be expressed in terms of both intensity and magnitude. However, the two terms are quite different, and they are often confused. Intensity is based on the observed effects of ground shaking on people, buildings, and natural features. It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicenter. Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake. It is based on the amplitude of the earthquake waves recorded on instruments which have a common calibration. The magnitude of an earthquake is thus represented by a single, instrumentally determined value. Earthquakes are the result of forces deep within the Earth's interior that continuously affect the surface of the Earth.

The energy from these forces is stored in a variety of ways within the rocks. When this energy is released suddenly, for example by shearing movements along faults in the crust of the Earth, an earthquake results. The area of the fault where the sudden rupture takes place is called the focus or hypocenter of the earthquake. The point on the Earth's surface directly above the focus is called the epicenter of the earthquake. San Fernando, California, 1971. Highway interchange heavily damaged by the magnitude 6.5 earthquake [Click on image for a larger view] The Richter Magnitude Scale Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes.

The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Van Norman Dam, San Fernando, California, 1971. Earthquake-induced liquefaction of the earth-filled dam resulted in a landslide that caused partial collapse [Click on image for a larger view] At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph.

Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with magnitudes of about 4.5 or greater--there are several thousand such shocks annually--are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On the average, one earthquake of such size occurs somewhere in the world each year. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes. The Richter Scale is not used to express damage. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans. (Top) San Francisco, California, 1906.

Collapse of City Hall after the 8.3 magnitude earthquake. Most of the property destruction was caused by the fire that raged after the earthquake. (Bottom) Anchorage, Alaska, 1964. Much of the damage after this magnitude 8.6 earthquake was due to huge landslides, such as this one under Government Hill elementary School. [Click on image for a larger view] The Modified Mercalli Intensity Scale The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally--total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity value, and to compile isoseismal maps that show the extent of various levels of intensity within the felt area. The maximum observed intensity generally occurs near the epicenter. (Top) Mindanao, Phillippines, 1976. Apartment building destroyed by a magnitude 7.9 earthquake. (Bottom) Long Beach, California, 1933. Exterior walls collapsed onto parked cars after this magnitude 6.3 earthquake (photo by Southern California Earthquake Pictures). [Click on image for a larger view] The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of Vlll or above.

Ninety percent of the world's earthquakes occur in specific areas that are the boundaries of the Earth's major crustal plates. Shown on the map are the epicenter locations of earthquakes of magnitude 4.5 or greater that occurred from 1978 through 1987. [Click on image for a larger view] The following is an abbreviated description of the 12 levels of Modified Mercalli intensity. Not felt except by a very few under especially favorable conditions. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. Felt by all, many frightened.

Some heavy furniture moved; a few instances of fallen plaster. Damage slight. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chmineys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. Damage total. Lines of sight and level are distorted. Objects thrown into the air. Another measure of the relative strength of an earthquake is the size of the area over which the shaking is noticed. This measure has been particularly useful in estimating the relative severity of historic shocks that were not recorded by seismographs or did not occur in populated areas.

The extent of the associated felt areas indicates that some comparatively large earthquakes have occurred in the past in places not considered by the general public to be regions of major earthquake activity. For example, the three shocks in 1811 and 1812 near New Madrid, Mo., were each felt over the entire eastern United States. Because there were so few people in the area west of New Madrid, it is not known how far it was felt in that direction. The 1886 Charleston, S.C., earthquake was also felt over a region of about 2 million square miles, which includes most of the eastern United States. *U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913 This publication is one of a series of general interest publications prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment. To obtain a catalog of additional titles in the series "General Interest Publications of the U.S. Geological Survey," write: U.S. Geological Survey

Question: The energy of what is measured by the Richter magnitude scale?
Answer: Earthquakes