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. Intensity X (10) is the highest value on the MMI.Perhaps no seismic subject is as irksome to seismologists as discussions of earthquake size. As the level rises toward the larger numbers, the amount of damage increases considerably. At this level doors would rattle, dishes break and weak or poor plaster would crack. For example a level I-V on the Mercalli scale would represent a small amount of observable damage. Thus, the magnitude scale is considered scientifically more objective and therefore more accurate. From a scientific standpoint, the magnitude scale is based on seismic records while the Mercalli is based on observable data which can be subjective. The Mercalli Scale is based on observable earthquake damage. In the United States, we use the Modified Mercalli (MMI) Scale. Sometimes earthquakes are referred to by the maximum intensity they produce. An earthquake causes many different intensities of shaking in the area of the epicenter where it occurs. So the intensity of an earthquake will vary depending on where you are. Intensity scales, like the Modified Mercalli Scale and the Rossi-Forel scale, measure the amount of shaking at a particular location. This happens because the relation between the seismic measurements and the magnitude is complex and different procedures will often give slightly different magnitudes for the same earthquake. Often, several slightly different magnitudes are reported for an earthquake. The magnitude does not depend on where the measurement is made. Magnitude scales, like the moment magnitude, measure the size of the earthquake at its source.
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