Felt by nearly everyone many awakened.Sensation like heavy truck striking building. Dishes, windows, doors disturbed walls make cracking sound. Felt indoors by many, outdoors by few during the day.Vibrations similar to the passing of a truck. Many people do not recognize it as an earthquake. Felt quite noticeably by persons indoors, especially on upper floors of buildings.Felt only by a few persons at rest, especially on upper floors of buildings.Not felt except by a very few under especially favorable conditions.MagnitudeĪbbreviated Modified Mercalli Intensity Scale The following table gives intensities that are typically observed at locations near the epicenter of earthquakes of different magnitudes. Great – extensive damage over broad areas, most buildings destroyed Great – extensive damage over broad areas, many buildings destroyed Major – extensive damage, some buildings destroyed Strong – damage variable depending on building construction and substrate Moderate – felt by most people, possible broken plaster and chimneys Light – felt by many people, minor damage possible The table below shows approximately how many earthquakes occur each year in each magnitude range and what the intensity might be at the epicenter for each magnitude range. Refer to the Modified Mercalli Intensity Scale page on the US Geological Survey Earthquake Hazards Program website for an abbreviated version. The most commonly used earthquake intensity scale is the Modified Mercalli earthquake intensity scale. Therefore, each earthquake produces a range of intensity values, ranging from highest in the epicenter area to zero at a distance from the epicenter. Earthquake intensity is a ranking based on the observed effects of an earthquake in each particular place. Ranking Earthquake IntensityĮarthquake intensity is very different from earthquake magnitude. A magnitude 9.0 earthquake, which rarely occurs, releases over a million times as much energy as a magnitude 5.0 earthquake. A magnitude 7.0 releases about 32 × 32 = 1024 times as much energy as a magnitude 5.0 earthquake. For example, a magnitude 6.0 earthquake releases about 32 times as much energy as a magnitude 5.0 earthquake. The magnitude scale portrays energy logarithmically to approximately base 32. Because the moment magnitude scale has replaced the Richter scale, we will assume from here on that we are referring to moment magnitude, not Richter magnitude, when we speak of earthquake magnitude. This produces a magnitude number that is a better indicator of the total amount of energy released by the earthquake. The moment magnitude scale is broadly similar to the Richter scale, but it takes more factors into account, including the total area of the fault that moves during the earthquake, and how much it moves. Therefore, the Richter scale has been replaced by the moment magnitude scale, symbolized as M w.
The Richter scale was found to not transfer very well from the San Andreas fault zone, a transform plate boundary, to the much more powerful earthquakes that occur at convergent plate boundaries, particularly subduction zone earthquakes. There is no upper limit defined for the Richter scale, but after a century of seismograph measurements, it appears that rocks in the earth release their stress before building up enough energy to reach magnitude 10. The effect of distance is factored out of the calculation. The distance to the epicenter must also be taken into account because the greater the distance from the earthquake, the smaller the waves get. The maximum amplitude seismic wave – the height of the tallest one – is measured in mm on a seismogram. Two measurements are factored together to determine the Richter magnitude of an earthquake: the amplitude of the largest waves recorded on a seismogram of the earthquake, and the distance to the epicenter of the earthquake. Richter used seismograms of earthquakes that occurred in the San Andreas fault zone to calibrate his magnitude scale. For several decades, earthquake magnitudes were calculated based on a method first developed by Charles Richter, a seismologist based in California. The magnitude of an earthquake is a number that allows earthquakes to be compared with each other in terms of their relative power. Intensity is determined from effects on people, human structures, and the natural environment. Intensity measures the strength of shaking produced by the earthquake at a certain location. Magnitude is determined from measurements on seismographs. Magnitude measures the energy released at the source of the earthquake. Magnitude and Intensity measure different characteristics of earthquakes.
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