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The Richter Scale is not commonly used anymore, as it has been replaced by another scale called the moment magnitude scale which is a more accurate measure of the earthquake size.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. How do scientists measure earthquakes?

How do scientists measure earthquakes? With a really big ruler? No, not quite. There are two ways in which scientists quantify the size of earthquakes: magnitude and intensity. You have probably heard of the Richter scale which is still used for small earthquakes, but most large earthquakes are now commonly reported using the moment magnitude scale (see below). Richter scale Magnitude is a measure of the amount of energy released during an earthquake, and you've probably heard news reports about earthquake magnitudes measured using the Richter scale. Something like, "A magnitude 7.3 earthquake struck Japan today. Details at ten." Did you ever wonder why, if it's that important, they just don't tell you right away? The Richter scale was invented, logically enough, in the 1930s by Dr. Charles Richter , a seismologist at the California Institute of Technology. It is a measure of the largest seismic wave recorded on a particular kind of seismograph located 100 kilometers (about 62 miles) from the epicenter of the earthquake. Think of a seismograph as a kind of sensitive pendulum that records the shaking of the Earth. The output of a seismograph is known as a seismogram.

In the early days, seismograms were produced using ink pens on paper or beams of light on photographic paper, but now it's most often done digitally using computers. The seismograph that Dr. Richter used amplified movements by a factor of 3000, so the waves on the seismograms were much bigger than those that actually occurred in the Earth. The epicenter of an earthquake is the point on the Earth's surface directly above the source, or focus, of the movement that causes the quake. Dr. Richter studied records from many earthquakes in southern California, and realized that some earthquakes made very small waves whereas others produced large waves. So, to make it easier to compare the sizes of the waves he recorded, Richter used the logarithms of the wave heights on seismograms measured in microns (1/1,000,000th of a meter, or 1/1000th of a millimeter). Remember, you have to be using a particular kind of seismograph located 100 km from the epicenter when you make the measurement; otherwise, all sorts of complicated calculations have to be made. That's why seismologists spend so many years in college!

A wave one millimeter (1000 microns) high on a seismogram would have a magnitude of 3 because 1000 is ten raised to the third power. In contrast, a wave ten millimeters high would have a magnitude of 4. For reasons that we won't go into, a factor of 10 change in the wave height corresponds to a factor of 32 change in the amount of energy released during the earthquake. In other words, a magnitude 7 earthquake would produce seismogram waves 10 x 10 = 100 times as high and release energy 32 x 32 = 1024 times as great as a magnitude 5 earthquake. The Richter scale is open-ended, meaning there is no limit to how small or large an earthquake might be. Due to the nature of logarithms, it is even possible to have earthquakes with negative magnitudes, although they are so small that humans would never feel them. At the other end of the spectrum, there should never be an earthquake much above magnitude 9 on the Earth simply because it would require a fault larger than any on the planet.

The largest earthquake ever recorded on Earth was a magnitude 9.5 that occurred in Chile in 1960, followed in size by the 1964 Good Friday earthquake in Alaska (magnitude 9.2), a magnitude 9.1 earthquake in Alaska during 1957, and a magnitude 9.0 earthquake in Russia during 1952. Two large earthquakes, one a magnitude 9.0 and one a magnitude 8.2, occurred on Dec. 26, 2004 and March 28, 2005, respectively, along the same fault zone off the coast of Sumatra, Indonesia. The list of really large earthquakes in the previous paragraph brings up another interesting point. Five earthquakes of magnitude 9 or above have been recorded during the past 45 years, which averages out to one every decade. It turns out that earthquake occurrences seem to follow what is called a power-law distribution, meaning that if there is on average on magnitude 9 earthquake every ten years somewhere in the world, then on average there should be one magnitude 8 earthquake every year, 10 magnitude 7 earthquakes every year, and 100 magnitude 6 earthquakes every year.

So, if someone "predicts" that a magnitude 6 earthquake will occur somewhere in the world during the next week, don't be too impressed if it happens because random probability tells us that there should be a magnitude 6 earthquake somewhere in the world every 365/100 = 3.65 days! In reality, things are a little more complicated. But, you get the picture. Mercalli scale What did people do before the Richter scale was invented? To some degree, one of the same things that we do today. They observed the intensity or effects of an earthquake at different locations. Whereas the magnitude of an earthquake is a single number regardless of where it's felt, intensity will vary from place to place. In general, the intensity will be much greater near the epicenter than at large distances from the epicenter. This decrease in intensity with distance is known as attenuation. Imagine it this way: If I drop a rock into a pool of water, the difference between magnitude and intensity is similar to the difference between the height of the splash exactly where I drop the rock and the height of the waves all over the pool.

Earthquake intensity is most often measured using the modified Mercalli scale , which was invented by the Italian geologist Giuseppi Mercalli in 1902 and uses Roman numerals from I to XII. In the United States, we use the modified Mercalli scale, which was adjusted to account for differences in buildings between Italy and southern California. An earthquake intensity of I is generally not felt, and an intensity of XII represents total destruction of buildings. Some kinds of geologic deposits, most notably water saturated muds, amplify seismic waves and may produce intensities much greater than those for nearby areas underlain by bedrock. Thus, after an earthquake seismologists can interview people and make maps showing the intensity of an earthquake in different areas to better understand the influence of rock or soil type on seismic waves. Moment Magnitude Scale Large earthquakes are not measured very well by the Richter scale, especially if the seimometers used are very far away from an earthquake epicenter. The moment magnitude scale is now most commonly used for medium to large earthquakes. Details of how these scales compare are discussed by Rick Aster on page 8 and 9 of the Spring, 2002 issue of Lite Geology .

Created by Bill Haneberg Earthquake Shaking Intensity The 1968 Southern Illinois Earthquake Shaking Intensity Eventually, shaking intensity scales were developed to standardize the measurements and ease comparison of different earthquakes. Shaking intensity varied from barely perceptible to completely destructive. Since 1933, in the United States we have used what's called the Modified-Mercalli Intensity scale, a twelve-stage scale, numbered from I to XII. The lower numbers represent imperceptible shaking levels, XII represents total destruction. A value of IV indicates a level of shaking that is felt by most people. An abridged version of the scale, based on the table in the back of Bolt's "Earthquakes" book, is located here . Early in the study of earthquakes, intensities were assigned by scientists who traveled to regions that had recently experienced strong earthquakes. Throughout much of the 20th century, postal surveys were conducted to measure intensity. Recently, e-mail surveys have been used to map out the felt areas ( click here to see an example). Intensity Patterns and Maps In general, intensities are largest close to the earthquake and decrease with distance from the earthquake and thus provide a means of locating and comparing the size of shallow earthquakes.

The best way of representing and interpreting intensity observations is to create an intensity map, a geographic representation of the distribution of ground shaking. First, the values of intensity are collected by surveying the region and interviewing people who experienced the earthquake-induced shaking. The descriptions and observations are then assigned the appropriate intensity by consulting the detailed description of the Modified Mercalli Scale. Next, each intensity value is plotted on a map. Once the observations are on the map, lines dividing regions of different intensity can be drawn. We call the lines separating regions of different shaking intensity isoseismals. The general location of the earthquake and regions likely to have experienced the most severe damage are also readily identifiable on the resulting intensity map. Here's an example from the 1966 Parkfield earthquake (Parkfield is located in central California). Intensity distribution observed for the 1966 Parkfield, CA earthquake. This earthquake had a magnitude between 5.0 and 5.3 and was felt over an area of approximately 20,000 square kilometers.

Limitations of Earthquake Intensity Measures Although important and a successful approach to the early study of earthquake size, intensity has problems when we expand our study to earthquakes that are not located near reference structures or earthquakes that occur deep beneath Earth's surface. Since intensity decreases with distance from the earthquake, a large deep earthquake, which solely because of its depth is far from Earth's surface, produces small shaking intensity. Thus we cannot use intensity to accurately compare the relative size of shallow and deep earthquakes. Another problem associated with intensity scales is it's dependence on building shape, construction, foundation type. Building practices and materials vary from place to place and thus tying a measure of earthquake size to buildings has some drawbacks if you want to compare earthquakes in different localities. And finally, the intensity of shaking depends on the regional and near-surface geologic conditions. Regional Geology For a fixed earthquake size, the region of strong shaking can be an indication of the regional geologic structure. A particularly illuminating illustration of the effects of regional geology on earthquake shaking is presented in Figure 0-2. The region of damaging shaking is shown for two earthquakes with roughly the same size - the 1811 New Madrid earthquake and the 1906 California earthquake.

The precise magnitude of the 1811 earthquake is uncertain since we had no seismometers available to provide instrumental measures of the shaking but indirect estimates suggest that it was a great earthquake. The great 1906 San Francisco earthquake had a surface-wave magnitude of approximately 8.3. The late Dr. Otto Nuttli of Saint Louis University estimated the intensities for the New Madrid quake by studying the newspapers and personal journals which contained reports of the effects of the quake. Since the land west of the Mississippi was sparsely settled at the time, the region of shaking west of the earthquakes is inferred using the shape of the region in the east. The old, eastern section of the US transmits seismic vibrations very efficiently relative to the relatively young, warm California coastal region. These observations suggest that for an equal-size earthquake, the east is likely to experience a wider extent of damage than coastal California. We are fortunate that earthquakes are less frequent in the eastern half of the continent, but the eastern half of the country has grown dramatically in a time of no large earthquakes. The next large earthquake to occur in the east has potential to produce substantial loss of life and property.

Regions with damaging shaking during the 1811 New Madrid earthquake (estimated magnitude somewhere between 7.0-8.0) and 1906 San Francisco earthquake (estimated magnitude 8.3). For the 1811 event, the shaking to the west of the Mississippi is estimated based on the observations to the east since the region was not yet settled (After Nuttli, 1981). Site Effects One aspect of intensity that is difficult to quantify is the effect of the near-surface geology on the intensity of seismic shaking. We call this effect a site effect because it is local to each site. Several physical parameters can affect shaking intensity. The primary control of the site response is the rock or soil type, and a secondary control is the water content. The "harder" the rock the lower the level of shaking is a good rule of thumb. Igneous rocks such as granite are considered a hard rock, soft rocks are usually sedimentary rocks which include limestone, shale, and sandstone. Relative to a granitic site, a site underlain by sedimentary rocks could experience an increase of one-half to two intensity levels at the same distance from the same earthquake.

Sites underlain by young saturated soil or alluvium usually shake with the highest intensity&endash; relative to granitic sites they may experience up to a three intensity level increase in shaking. Perceptibility Obviously if you are close to a large earthquake, you will know it. But whether or not you feel vibrations from a distant earthquake depends on a number of factors related to where you are and what you are doing. Most people will detect a level of ground vibration equivalent to approximately 2% of the acceleration of gravity (the acceleration of gravity, g = 980 cm/s^2). If you are in the upper stories of a tall building, you are more likely to feel the ground vibrate than if you are on the ground. If you are in a car, you are less likely to detect small vibrations than if you were walking along the same road. And psychological factors can enhance the experience: some people become very sensitive to vibrations after they have experienced a large shock. The aftershocks in an earthquake sequence can be quite nerve-wracking for those individuals. Intensity and Magnitude The complications to the cause of strong shaking and perceptibility make using intensity as a direct measure of earthquake size difficult.

The subjective nature of intensity measurements, their dependence on building practices, population distribution, earthquake depth, etc. lead seismologists to develop the magnitude scale, which is based on measurements from seismograms. However, in general, the level of shaking is largest for large earthquakes and the maximum intensity does provide useful information on earthquake size. A direct correspondence between the instrumental measure of earthquake size, the magnitude is impossible. However, some general rules of thumb for a crustal depth earthquake are possible even though they are unsuitable for detailed work. Faults and Intensity Occasionally, large earthquakes rupture the surface and produce a complex region of intense deformation. For large events, this zone of "surface rupture" is much longer than it is wide, a direct consequence of the fact that large earthquakes are the result of slip along appreciable portions of a fault. The elongation of the intensity of shaking is also visible in intensity maps of shallow, moderate-size earthquakes. Carefully study the diagram of the 1966 Parkfield earthquake and you will see that the region of the most intense shaking was elongated in the northwest-southeast direction. The shaking was aligned with the trend of the San Andreas fault in that part of California.

The effect is even more dramatic in the geometry of the shaking from the 1906 San Francisco earthquake. The zone of most intense shaking was located in a long, linear trend, parallel to the coast which followed the surface trace of the San Andreas fault from central to northern California. The other region of intense shaking further, located to the east, is the result of site effects in the soil-covered central valley of California (sometimes called the Great Valley). The first clues to the role of faulting in earthquakes was developed in the late 1800's by geologists who studied the fresh surface rupture generated during earthquakes and linked the process with long-term surface deformation such as mountain uplift and stream offsets. We next turn our attention to some details of faults and faulting, which leads to some interesting classification of earthquake activity. Mercalli Scale | Intensity Mercalli Scale What is the Mercalli Scale As we have already noted, the severity of an earthquake can vary from events which are barely detectable even using the most sophisticated devices, to devastating events which can level cities and trigger tsunamis and sometimes even volcanic activity.

Various scales were proposed in the past to measure the magnitude of earthquakes until 1935, when the Richter Magnitude Scale was developed by Conrad Richter to measure the intensity of the seismic waves. Image: Modified Mercalli Scale VS Richter Scale However, Richter ratings only give you a rough idea of the actual impact of an earthquake. As we've seen, an earthquake's destructive power varies depending on the composition of the ground in an area and the design and placement of man-made structures. The extent of damage is rated on theMercalli Intensity Scale. The Mercalli Intensity Scale was developed by the Italian volcanologist Giuseppe Mercalli in 1884 and expanded to include 12 degrees of intensity in 1902 by Adolfo Cancani. It was modified again by Harry O. Wood and Frank Neumann in 1931. It is known today as the Modified Mercalli Intensity Scale. Mercalli ratings, which are given as Roman numerals, are based on largely subjective interpretations. A low intensity earthquake, one in which only some people feel the vibration and there is no significant property damage, is rated as a II.

The highest rating, a XII, is applied to earthquakes in which structures are destroyed, the ground is cracked and other natural disasters, such as landslides or tsunamis, are initiated. Richter scale ratings are determined soon after an earthquake, once scientists can compare the data from different seismograph stations . Mercalli ratings, on the other hand, can't be determined until investigators have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating. The Mercalli Intensity Scale measures the intensity of an earthquake by observing its effect on people, the environment and the earth’s surface. While the Mercalli scale describes the intensity of an earthquake based on its observed effects, the Richter scale describes the earthquake's magnitude by measuring the seismic waves generated by an earthquake. The two scales have different applications and measurement techniques. The Mercalli scale is linear and the Richter scale is logarithmic. i.e.

a magnitude 5 earthquake is ten times as intense as a magnitude 4 earthquake Video: Describes Mercalli Scale of intensity, Richter Scale and Moment Magnitude scale of magnitude Other useful resources Earthquakes Earthquakes: Exploring Earthquakes and the Modified Mercalli Scale We can measure an earthquake by: the amount of movement or energy released its intensity or its effect of intensity Magnitude The magnitude (ï¿½sizeï¿½) of an earthquake is based on the measurement of the maximum motion recorded by a seismograph. courtesy of the US Geological Survey Several scales of measurement have been developed. The most familiar of these is the Richter scale - related to the size of seismic waves produced during a quake. However, today seismologists more commonly rely on what is called the Moment Magnitude Scale, which measures the amount of energy released during an earthquake. Intensity The effect of an earthquake on the Earth's surface is referred to as its intensity. Numerous intensity scales have been developed over the last several hundred years. The one currently used in the United States is the Modified Mercalli Intensity Scale developed in 1931.

Here is a photograph of Giuseppe Mercalli, the inventor of the Mercalli Scale (From Walker, 1982). courtesy of the US Geological Survey Modified Mercalli Intensity Scale This is a semi-quantitative scale used to evaluate ground shaking and damage. This scale is composed of 12 increasing levels of intensity based on observed effects. These range from imperceptible shaking to catastrophic destruction, designated by Roman numerals. It is influenced by the amount of kinetic energy released, the substrate, the buildings, etc. Although it confounds many variables, it is valuable in evaluating ground shaking in modern quakes evaluating historic earthquakes, where written records exist. This postcard shows the damage effect of different earthquakes in Los Angeles. Resources: Modified Mercalli Intensity Scale - A copy of the scale composed of 12 increasing levels of intensity based on observed effects. Did You Feel It? This is a web site through which you can report your own experiences during an earthquake, so that the USGA can use the Mercalli Scale of Intensity to map out ground shaking.

For further information visit Tremor Troop Earthquakes, Federal Emergency Management Agency, FEMA 159, April 1999, published by the National Science Teachers Association. Single copies of this publication may be obtained from FEMA at no cost by calling 1-800-480-2520. This is a complete binder of activities for Grades K-6. A fun and creative classroom activity to illustrate earthquake intensity can be found on pg. 110 ï¿½ The Mercalli Scale ï¿½ Calling Station KWAT. 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 Measuring Earthquakes Measuring Earthquakes ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Seismologists use two main devices to measure an earthquake: a seismograph and a seismoscope.ï¿½ The seismograph is an instrument that measures seismic waves caused by an earthquake.ï¿½ The seismograph has three main devices, the Richter Magnitude Scale, the Modified Mercalli Intensity Scale, and the Moment-Magnitude Scale.ï¿½ The seismoscope is an instrument that measures the occurrence or the time of an occurrence of an earthquake (ï¿½Inventorsï¿½).ï¿½ Unlike other measuring devices, the seismoscope is a simple device without any technological background.ï¿½ The seismoscope is the oldest and most accurate instrument for measuring direction. ï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ First invented in 132 AD, the Dragon Jar was the first instrument for determining the direction of an earthquake.

ï¿½(Photo by Peter Bormann).ï¿½ Chang Heng, a Chinese scientist, developed the Dragon Jar.ï¿½ The Dragon Jar consists of a large jar with eight dragons protruding around the top.ï¿½ Each dragonhead holds a ball in its mouth while a frog sits with its mouth open directly underneath.ï¿½ Behind each dragonhead lies a trigger.ï¿½ Down the center of the jar is a thin stick that is loosely secured.ï¿½ The tremors of an earthquake cause the stick to fall on one of the eight triggers.ï¿½ When the trigger is set off, the dragonhead linked to the trigger drops the ball into the frogï¿½s mouth.ï¿½ The sound of the ball dropping into the frogï¿½s mouth indicated an earthquake had just occurred.ï¿½ By looking at which ball dropped, the direction of the earthquake could be determined.ï¿½ Hengï¿½s seismoscope was not only the first seismoscope but also very accurate and precise.ï¿½ In 138 AD, Hengï¿½s seismoscope detected an earthquake 1,000 miles away (ï¿½Chineseï¿½

).ï¿½ Wang Zhenduo recreated Chang Hengï¿½s seismoscope in 1951.ï¿½ Instead of a thin stick loosely secured in the center of the jar, Zhenduo replaced it with a copper pendulum shaft that connected to eight copper arms (ï¿½Zhang Hengï¿½).ï¿½ Like the seismoscope, other devices used to gather information on earthquakes were also first developed outside the United States .

ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Nicholas Cirillo developed the first mechanical device used to study earthquakes in 1731.ï¿½ A series of earthquakes in Naples inspired Cirillo to design this mechanical device.ï¿½ Cirilloï¿½s device consisted of a pendulum that swung freely.ï¿½ The tremors of the earthquake caused the pendulum to swing.ï¿½ The lines caused by the pendulum indicated the amplitude of the ground motion.ï¿½ In Calabria , Italy , a series of earthquakes in 1783 that killed over 50,000 civilians.ï¿½ Because of these earthquakes, more mechanical devices were created (Kauffman and Judson 181). ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ ï¿½ D.

Domenico Salsano, a clockmaker and mechanic, invented a similar device to Nicholas Cirillo.ï¿½ Salsanoï¿½s device was a long pendulum with a brush connected at the tip.ï¿½ The brush would then trace the motions of the earthquakeï¿½s tremors with slow-drying ink onto an ivory slab.ï¿½ Salsanoï¿½s device had a bell attached that rang when the tremors were large enough (Kauffman and Judson 181).ï¿½ James Forbes also created a similar mechanical device.ï¿½ His device was called the ï¿½Inverted-pendulum Seismometer.ï¿½ï¿½ This device was a metal rid with a movable base (Kauffman and Judson 181).ï¿½ Later in 1855, Luigi Palmier of Italy designed a mercury seismometer.

The mercury seismometer had U-shaped tubes filled with mercury and arranged along the compass points.ï¿½ When an earthquake occurred, the movement of the mercury made an electrical contact that stopped a clock and started the recording drum.ï¿½ The motion of a float on the surface of mercury was recorded on the drum.ï¿½ Palmierï¿½s seismometer was the first device to accurately record the time and of an earthquake while also recording the duration and the intensity of the earthquake (ï¿½Inventorsï¿½).ï¿½ The instruments used to gather information on earthquakes are seismographs.

ï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ In 1880 John Milne, an English seismologist and geologist, is credited for the development of the first modern seismograph in 1880.ï¿½ Milne called his seismograph the ï¿½Horizontal seismographï¿½ (ï¿½Inventorsï¿½).ï¿½ Milneï¿½s seismograph consists of three parts: the inertia member, the transducer, and a recorder.ï¿½ ï¿½(Drawing by Professor Stephen A.

Nelson)ï¿½ The inertia member is a weight suspended by a wire or a spring.ï¿½ It is similar to a pendulum; however, it can only swing in one direction.ï¿½ The transducer is a device that detected the motion between the mass and the ground.ï¿½ This motion is then converted into a form that can be recorded.ï¿½ The transducer can be a mechanical lever or an electrodynamic system.ï¿½ In an electrodynamic system, a coil of wire moved back and forth in a magnetic field.ï¿½ This movement created an electric current that passed through a galvanometer then recorded on a sheet of paper (Kauffman and Judson 182).ï¿½ Sir James Alfred Ewing, Thomas Grey and John Milne founded the Seismological Society of Japan .ï¿½ The society funded the invention of seismographs (ï¿½Inventorsï¿½).

ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ The Horizontal Pendulum seismograph was improved after World War II.ï¿½ The new device is called the Press-Ewing seismograph.ï¿½ The Press-Ewing seismograph is widely used throughout the United States to record long period waves.ï¿½ The difference between the Press-Ewing and the Horizontal pendulum seismograph is that the Press-Ewing seismograph has a pivot that hold the pendulum was replaced with elastic wire to avoid friction (ï¿½Inventorsï¿½).ï¿½ The design of a seismograph is a weight freely suspended from a support that is attached to bedrock.ï¿½ When the seismic waves reached the seismograph, the inertia of the weight kept the device stable while the ground and support shake.ï¿½ The movement of the ground in relation to the movement of the weight it recorded onto a piece of paper that is wrapped around a rotating drum (Lutgens and Edwards 306).ï¿½ To record motion in all directions, three seismographs are required.ï¿½ One

 seismograph is needed to measure vertical motion, and two to record horizontal motion.ï¿½ The two seismographs recording horizontal directions, record in 90-degree angles (Kauffman and Judson 182).

ï¿½ Seismographs record in a zigzag trace that shoes the varying amplitude of ground oscillations beneath the instrument.ï¿½ Depending on the sensitivity of the seismograph, earthquakes can be detected anywhere in the world.ï¿½ Using data collected by the seismographs, the time, location, and the magnitude can be determined (Bellis).ï¿½ The magnitude of an earthquake can be determined by a mathematical formula called the Richter Magnitude Scale.ï¿½ï¿½ï¿½ Charles F.

Richter developed the Richter Magnitude Scale in 1934.ï¿½ Richter defined the scale as ï¿½The logarithm to base 10 of the maximum seismic wave amplitude recorded on a standard seismograph at a distance of 100 kilometers from the earthquake epicenter.ï¿½ï¿½ The seismic wave used in the calculation is not specified (Bolt 104).ï¿½ï¿½ï¿½ Because it is not specified, S waves or P waves can be used.ï¿½ The Richter scale was originally designed by Richter to differentiate between earthquakes with a low focus point in southern California .ï¿½ The Richter scale is referred to ML, with ï¿½Lï¿½ for local.ï¿½ After many seismograph stations were established, it became clear the formula was only valid for a certain frequency and distance ranges (ï¿½Measuring Earthquakesï¿½).ï¿½ The original scale was modified to measure earthquakes at any distance, focal depth and compensate for geological variations from place to place (Lutgens and Edwards).ï¿½ Adjustments were also included into the formula to compensate for the variation of the distance

 between different seismographs (ï¿½Severityï¿½).ï¿½ Reading the Richter scale is a difficult task to accomplish.ï¿½ Each whole number increase in the scale releases 32 times more energy than the preceding number ( U.S.

Geological Survey).ï¿½ Although this process seems simple, it is actually very complex.ï¿½ Seismologists are given seismograms then had to determine the magnitude at its source.ï¿½ Earthquakes with a magnitude of 2.0 or less are called microearthquakes (Bellis).ï¿½ The Richter scale has no upper limit.ï¿½ Although the largest earthquake is recorded to be an 8.9, it is possible the earth can withstand the stress to produce an earthquake larger than 9.0 (McConnel).ï¿½ According to the chart, large earthquakes will be felt by everyone and even cause serious damage.ï¿½ However, people may not feel some large-magnitude earthquakes beneath the ocean at all (Bellis). Richter magnitude Great earthquake. Can cause serious damage in areas several hundred kilometers across.

(ï¿½Intensityï¿½)ï¿½ Like the Richter scale, the Modified Mercalli Intensity Scale measures an earthquake at its source so where the measurement is made is insufficient (ï¿½Measuring Earthquakesï¿½).ï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ American seismologists Harry Wood and Frank Neumann developed the Modified Mercalli Intensity Scale in 1931.ï¿½ Wood and Neumann extended the Modified Mercalli Intensity Scale to twelve Roman Numerals instead of the original ten.ï¿½ Unlike the Richter scale, the modified Mercalli scale ranks bases on observed effects.ï¿½ The effect an earthquake has on a particular area of land is the intensity of the earthquake.ï¿½ These effects range from waking people up to complete destruction (ï¿½Severityï¿½).ï¿½ I strong People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings. VIII violent Drivers have trouble steering.

Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change. IX disastrous Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage. X most disastrous Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly. XI catastrophic Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII great catastrophe Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

(ï¿½Intensityï¿½)ï¿½ After the occurrence of an earthquake, the Geological Survey mails questionnaires to post offices in that particular area.ï¿½ The questionnaires are distributed to everyone in that area requesting information about the earthquakeï¿½s damage.ï¿½ That information is used to create an isoseismal math that shows the various levels of intensity in that area (ï¿½Severityï¿½).ï¿½ The Modified Mercalli Scale however does have its dependencies: population density, building methods and materials, and distance from the epicenter.ï¿½ The intensity of an earthquake can be underestimated in an area that is densely populated.ï¿½ The amount of damage done by earthquakes depends on how well a building is built.ï¿½ The building methods and materials used to build the structures affect how much damage is caused.ï¿½ This variance in damage could give the same earthquake a different intensity value.ï¿½ An area farther from the epicenter will receive less damage than closer areas.ï¿½ Because each area has a different intensity value, it is difficult to compare individual results (McConnel).ï¿½ Moment magnitude is similar to Modified Mercalli Scale however is based on physical changed of

 the earth.

ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ ï¿½ The moment magnitude is based on the amount of displacement that occurred along a fault zone rather than the measurement of ground motion at a given point (Lutgens and Edwards 314).ï¿½ The Moment magnitude measures energy released by the earthquake more accurately than the Richter scale.ï¿½ The amount of energy released is dependent of a rockï¿½s properties, area of the fault surface, and amount of movement along the fault zone (McConnel).ï¿½ Seismologists calculate Moment magnitude from seismograms after long-period waves are examined (Lutgens and Edwards 314).ï¿½ Moment magnitude is calculated with more accuracy with large earthquakes (McConnel).ï¿½ Moment magnitude gained support and acceptance among many seismologists and engineers.ï¿½ Seismologists and engineers accepted Moment Magnitude because: 1) It is the only magnitude scale that adequately measures the size of large earthquakes 2) It is a measure established from the size of the rupture surface and the amount of displacement, which better determines the amount of

 energy released 3) It can be verified by two different methods, field studies that are based of measurements of fault displacement and by seismograph methods that uses long-period waves (Lutgens and Edwards 314).

Displacement is the measurement of the actual change of location of the ground due to shaking (ï¿½Measuring Earthquakesï¿½).

Earthquakes in occur mainly in the Northeast.ï¿½ Since 1830, the Arizona and Cataract Creek.ï¿½ Many earthquakes occurred in southern Arizona as well.ï¿½ Yuma is recorded to have had dozens of earthquakes in the last 175 years that occurs fairly regularly.ï¿½ AEIC recorded the location of a numerous earthquakes simply as SW Arizona .ï¿½ Because AEIC did not specify which area in SW Arizona, Sierra Vista may have had earthquakes in the past or recently.ï¿½ In the last ten years at least, Sierra Vista has not experienced an earthquake that people have felt.ï¿½ A slammed car door, vehicles colliding with each other, or building being built is the closest event Sierra Vista has experienced to an earth ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿

½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Works Cited Arizona Earthquakes 1830-2005.ï¿½ 10 Jan 2005.ï¿½ Arizona Intensity of the Earthquake.ï¿½ 2005.ï¿½ THEmeter.ï¿½ 24 April 2005 ï¿½ï¿½ Kauffman, Marvin E.

and Sheldon Judson.ï¿½ Physical Geology.ï¿½ 8th ed Prentic Hall: New Jersey .ï¿½ 1990. Lutgens, Fredrick K. and Tarbuck J Edwards.ï¿½ Essentials of Geology.ï¿½ 8th ed.ï¿½ Prentic Hall: New Jersey .ï¿½ 2003. McConnel, David.ï¿½ Earthquakes II.ï¿½ 14 Jan 1997.ï¿½ Natural Science Geology.ï¿½ 258 Mar 2005. < > Measuring Earthquakes.ï¿½ 2001.ï¿½ SKLPC.ï¿½ 21 Apr 2005.ï¿½ Nelson, Stephen A.

Professor.ï¿½ Earthquakes.ï¿½ 23 Oct 2003.ï¿½ Tulane .ï¿½ 24 April 2005 ~sanelson/geol111/earthint.htm The Severity of an Earthquake.ï¿½ 29 Sep 2004.ï¿½ U.S. Geological Survey.ï¿½ 25 Mar 2005. < > Zhang Heng. Albertsons:

Question: What measurement has been used for the intensity of earthquakes?
Answer: Richter scale