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 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 Earthquake | Seismology, magnitude and other units of measurement Earthquakes: seismology & measurement Seismology As we have seen, most earthquakes are essentially the product of tectonic stresses which are generated at the boundaries of the Earth's tectonic plates . The released energy travels as shock waves, called seismic waves , which may be felt and measured. The study of seismic waves is known as seismology, a word derived from a Greek word meaning "to shake." Seismographs are the instruments which record earthquakes. Scientists use these instruments as their principal tool to study seismic waves. They are very sensitive instruments that can detect, measure and record ground vibrations and their intensities during an earthquake.

A seismograph is a simple pendulum. As the ground shakes the base and frame move with the vibrations but inertia keeps the pendulum bob in place. It moves relative to vibrating ground. The pendulum displacement records the changes over time the tracing out the record of the event is called a seismogram. Image: A seismograph The image above shows the seismograph in Sunset Crater Volcano National Monument's visitor centre. A pen at the top of the device records a zig-zag line on the moving, paper-covered cylinder whenever an earthquake is detected. The seismogram (paper record) is removed each day and replaced. The earthquake data from the seismogram is then analyzed by scientists. Seismographs can help us determine the time, epicentre , focus , and the type of faulting which produced an earthquake as well as estimate how much energy was released. Magnitude and other units of measurement 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. The severity of an earthquake is called its magnitude.

Various scales were proposed to measure the magnitude of earthquakes until 1935, when the Richter Scale was developed by a seismologist named Conrad Richter to measure the intensity of the seismic waves. The amplitude (height) of the largest recorded wave of an earthquake at a specific distance is called the Richter magnitude. Under the Richter scale, each order of magnitude is 10 times more intensive than the last one, which means that a two is 10 times more intense than a one and a three is one hundred times greater. But 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. Image: Earthquake Severity - Richter Scale 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. Earthquake size is determined not only by amount of displacement but also area of the ruptured fault plane. 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. There is no limit to the possible magnitude of an earthquake but historically just over magnitude 9 is the record. The earthquake of most recent history to reach 9 on the Richter scale was the Japan quake of March 2011. This was also the largest recorded Japanese earthquake of all time. Image: Earthquake in Japan, March 2011 Unit1: Earthquakes/Measuring Earthquakes The assessment of earthquake intensity on a descriptive scale depends on actual observations of earthquake effects. Observations on the performance of building structures, natural phenomena, and human perceptions are essential for evaluating the earthquake intensity. Intensity of an earthquake depends on the distance from epicenter, and also on the local soil conditions, geology and topography. In a typical case, however, the largest intensity is observed in the vicinity of epicenter and it diminishes with the distance. 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 Intensity (MMI) Scale. 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 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 VIII or above. Detailed specifications of various intensity levels related to the MMI intensity scale are presented in the table below. Modified Mercalli Intensity (MMI) Scale MMI Level Description I Not felt except by a very few under favourable circumstances. Typically marginal and long-period effects of strong earthquakes. II Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing. III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing cars may rock slightly.

Vibration similar to that of passing truck. Duration may be estimated. IV If during the day, felt indoors by many; outdoors by few. If at night, few awakened. Dishes, windows and doors rattle, walls creak. A sensation such as heavy truck shaking the building. Standing cars rock noticeably. In the upper range of IV wooden walls and frames creak. V Felt by nearly everyone, many awakened; direction may be estimated. Some dishes and windows broken, some plaster cracked, unstable objects overturned. Disturbance of trees, poles and other tall objects. Pendulum clocks may stop. VI Felt by all; many people frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Books off shelves; pictures off walls. Weak plaster and masonry D cracked; minor chimney damage. Movement of moderately heavy furniture; some furniture overturned. Small bells ring (school, church). Trees, bushes shaken visibly, or heard to rustle. VII Everybody runs outdoors. Noticed by persons driving cars. Hanging objects quiver. Furniture broken. Damage to masonry D3, including cracks. Some cracks in masonry C. Weak chimneys broken at roof line.

Fall of plaster, loose bricks, stones, tiles, cornices, unbraced parapets and architectural ornaments. Waves on pond; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. VIII Persons driving in cars are disturbed. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Panel walls thrown out of frame structures. Chimneys, factory stacks, monuments, walls, and columns fall. Heavy furniture overturned or damaged. Changes in well water. Sand and mud ejected in small amounts. Cracks in wet ground and on steep slopes. Branches broken from the trees. IX General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Ground noticeably cracked, underground pipes broken. In alluviated areas sand and mud ejected, earthquake fountains and sand craters appear. X Most masonry and frame structures destroyed with their foundations.

Some well-built wooden structures destroyed, foundations ruined, ground badly cracked. Serious damage to dams, dikes, embankments. Rails bent slightly. Considerable landslides from steep slopes and river banks. Water splashed over banks. Sand and mud shifted horizontally on beaches and flat land. XI Rails bent greatly. Bridges destroyed. Broad fissures in ground. Underground pipes out of service. Earth slumps and land slips in soft ground. XII Total damage. Waves are seen on the ground surface. Large rock masses displaced. Lines of sight and level are distorted. Objects thrown into the air. A major difference between the earthquake intensity and magnitude lies in the fact that magnitude of an earthquake is determined based on measuring the ground motion with instruments (seismographs), whereas the intensity of an earthquake is determined based on observations of earthquake effects on building structures and human perceptions. Another essential difference between a magnitude and intensity of an earthquake lies in the fact that magnitude is a unique indicator of a size of an earthquake - each earthquake is characterized with a single value which indicates its magnitude. At the same time, each earthquake is characterized with various intensities, depending on the location of a particular site with respect to the epicenter.

For example, Canada's largest historic earthquake, the Queen Charlotte Island earthquake of August 22, 1949 was characterized with magnitude 8.1 on the Richter scale. The same earthquake was characterized with MMI intensities ranging from III to over VII, as illustrated in the figure below. As an illustration of MMI intensity of VII or higher in the area close to the epicenter of this earthquake "cows were knocked off their feet, and a geologist with the Geological Survey of Canada working on the north end of Graham Island could not stand up." In Prince Rupert (MMI intensity VI), "windows were shattered and buildings swayed." For more information about the Queen Charlotte Island earthquake and other Canadian historic earthquakes, refer to the Geological Survey of Canada . Summary In this unit you have been introduced to earthquakes, their nature and causes. We have learned what causes earthquakes, which regions of our planet are most earthquake-prone and why. We have also learned the methods of measuring earthquakes. We have learned what is the magnitude of an earthquake per Richter scale and we are able to determine if an earthquake is considered to be minor, moderate or major based on a number which characterizes its magnitude.

We have also been introduced to earthquake intensity and Modified Mercalli Intensity Scale which is commonly used to measure the intensity of an earthquake. We are also able to explain the difference between the earthquake magnitude and intensity. In the next unit we will introduce you to the effects of earthquakes on buildings. We will learn how earthquake forces are developed and how these forces are resisted by structural elements within a building. How Are Earthquakes Measured? How Are Earthquakes Measured? August 20, 2010 01:57pm ET MORE Credit: Southern California Earthquake Data Center. The familiar Richter scale (which is not a physical device but rather a mathematical formula) is no longer widely used by scientists or the media to report an earthquake's size. Today, an earthquake's size is typically reported simply by its magnitude, which is a measure of the size of the earthquake's source, where the ground began shaking. While there are many modern scales used to calculate the magnitude, the most common is the moment magnitude, which allows for more precise measurements of large earthquakes than the Richter scale.

In the news, however, when an earthquake's magnitude is given , the scale used to calculate the magnitude is not usually specified since the modern scales are all very similar. A network of geological monitoring stations, each with instruments that measure how much the ground shakes over time called seismographs allow scientists to calculate an earthquake's time, location and magnitude. Seismographs record a zigzag trace that shows how the ground shakes beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. From these measurements, a quake's magnitude is usually reported as, for example, a magnitude-7.0 in the case of the earthquake that struck Haiti on Jan. 12. Based on their magnitude, quakes are assigned to a class, according to the U.S. Geological Survey. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows: Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage. Major: Magnitude in the rage of 7.0 to 7.9.

A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage. Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage. Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage. Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage. Minor: Magnitude in the rage of 3.0 to 3.9. Micro: Magnitude less than-3.0. Quakes between 2.5 and 3.0 are the smallest generally felt by people. After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon. 13 Crazy Earthquake Facts Brett Israel is a staff writer for OurAmazingPlanet , a sister site to Life's Little Mysteries. Got a question? Email it to Life's Little Mysteries and we'll try to answer it.

Due to the volume of questions, we unfortunately can't reply individually, but we will publish answers to the most intriguing questions, so check back soon. Author Bio Brett Israel, Brett Israel was a staff writer for Live Science with a focus on environmental issues. He holds a bachelor’s degree in biochemistry and molecular biology from The University of Georgia, a master’s degree in journalism from New York University, and has studied doctorate-level biochemistry at Emory University.

Brett Israel, on 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: Earthquakes - General Interest Publication Measuring Earthquakes The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth's surface, and body waves, which travel through the Earth.

Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes. Body waves are of two types, compressional and shear. Both types pass through the Earth's interior from the focus of an earthquake to distant points on the surface, but only compressional waves travel through the Earth's molten core. Because compressional waves travel at great speeds and ordinarily reach the surface first, they are often called "primary waves" or simply "P" waves. P waves push tiny particles of Earth material directly ahead of them or displace the particles directly behind their line of travel. Shear waves do not travel as rapidly through the Earth's crust and mantle as do compressional waves, and because they ordinarily reach the surface later, they are called "secondary" or "S" waves. Instead of affecting material directly behind or ahead of their line of travel, shear waves displace material at right angles to their path and therefore sometimes called "transverse" waves. The first indication of an earthquake is often a sharp thud, signaling the arrival of compressional waves. This is followed by the shear waves and then the "ground roll" caused by the surface waves.

A geologist who was at Valdez, Alaska, during the 1964 earthquake described this sequence: The first tremors were hard enough to stop a moving person, and shock waves were immediately noticeable on the surface of the ground. These shock waves continued with a rather long frequency, which gave the observer an impression of a rolling feeling rather than abrupt hard jolts. After about 1 minute the amplitude or strength of the shock waves increased in intensity and failures in buildings as well as the frozen ground surface began to occur ... After about 3 1/2 minutes the severe shock waves ended and people began to react as could be expected. The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location. The Richter Scale, named after Dr. Charles F.

Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale. The Modified Mercalli Scale expresses the intensity of an earthquake's effects in a given locality in values ranging from I to XII. The most commonly used adaptation covers the range of intensity from the condition of "I -- Not felt except by a very few under especially favorable conditions," to "XII -- Damage total. Lines of sight and level are distorted. Objects thrown upward into the air." Evaluation of earthquake intensity can be made only after eyewitness reports and results of field investigations are studied and interpreted. The maximum intensity experienced in the Alaska earthquake of 1964 was X; damage from the San Francisco and New Madrid earthquakes reached a maximum intensity of XI. Earthquakes of large magnitude do not necessarily cause the most intense surface effects.

The effect in a given region depends to a large degree on local surface and subsurface geologic conditions. An area underlain by unstable ground (sand, clay, or other unconsolidated materials), for example, is likely to experience much more noticeable effects than an area equally distant from an earthquake's epicenter but underlain by firm ground such as granite. In general, earthquakes east of the Rocky Mountains affect a much larger area than earthquakes west of the Rockies. An earthquake's destructiveness depends on many factors. In addition to magnitude and the local geologic conditions, these factors include the focal depth, the distance from the epicenter, and the design of buildings and other structures. The extent of damage also depends on the density of population and construction in the area shaken by the quake. The Loma Prieta earthquake of 1989 demonstrated a wide range of effects. The Santa Cruz mountains suffered little damage from the seismic waves, even though they were close to the epicenter. The central core of the city of Santa Cruz, about 24 kilometers (15 miles) away from the epicenter, was almost competely destroyed.

More than 80 kilometers (50 miles) away, the cities of San Francisco and Oakland suffered selective but severe damage, including the loss of more than 40 lives. The greatest destruction occurred in areas where roads and elevated structures were built on stable ground underlain by loose, unconsolidated soils. The Northridge, California, earthquake of 1994 also produced a wide variety of effects, even over distances of just a few hundred meters. Some buildings collapsed, while adjacent buildings of similar age and construction remained standing. Similarly, some highway spans collapsed, while others nearby did not. What instruments are used to measure earthquakes? | Reference.com What instruments are used to measure earthquakes? A: Quick Answer The seismograph and the seismoscope are the two main instruments used to measure the strength of earthquakes. The seismoscope is a simple instrument that measures the time that an earthquake takes place. The seismograph records the motion of the ground during an earthquake. Full Answer A seismograph's main component is a hanging mass — as heavy as 1,000 pounds — that is connected to a pen, and this large pendulum is situated just above a paper surface.

When the ground moves, so does the paper, which rubs against the pen and marks the movement. Seismographs are isolated and connected to bedrock to ensure that the data they receive is not affected by the movement of surrounding objects. Advanced technology improves the accuracy of the seismograph and makes it sensitive to minute ground movements. 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.

Question: What instrument is used to measure the intesity of an earthquake?
Answer: Seismometer