![]() ![]() Whereas an earthquake will have one magnitude (well, as noted below, there are likely to be several different estimates of the magnitude 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. Measurements of intensity 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. Further refinements for more modern construction were published in 1931 by the American seismologists Harry Wood and Frank Neumann. 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 18 to quantify (somewhat) the earthquake's effects. 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. In the light bulb analogy, it is the brightness with which you perceive the light at a place in a room. This Live Science animation will help you visualize the exponential magnitude scale in terms of energy released.Įarthquake intensity measures how strongly the earthquake impacts a specific location. 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. When referring to the power or energy released in an earthquake this 32 multiplier is used. To achieve this ten fold increase in ground motion requires about 32 to 33 times the energy. An earthquake causing motion at that distance 10 times larger than an M2 is an M3, and so on. ![]() It is easier to choose a particular earthquake recorded at a particular distance as a "standard" earthquake and 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. 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. In the same way, an earthquake's magnitude is an objective measurement of the energy radiated by an earthquake. ![]() The wattage of a bulb tells you about the strength of the light source. A 100-watt bulb is brighter than a 50-watt bulb, but not nearly as bright as a 250-watt bulb. ![]() One measure of the strength of a light bulb is how much energy it uses. And when news announcers mention the "Richter Scale" seismologists the world over begin gnashing their teeth.Ī familiar analogy to help understand earthquake size metrics is to think about a light bulb. There often seems to be no end of confusion, misunderstanding, and over-interpretation of what are really pretty crude metrics. Perhaps no seismic subject is as irksome to seismologists as discussions of earthquake size. ![]()
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