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ERTH 2415 (21)
Chapter 4

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Department
Earth Sciences
Course
ERTH 2415
Professor
Claire Samson
Semester
Winter

Description
Chapter 4 part 22 97114Magnitude of Earthquakes Magnitude is an estimate of the relative size or energy release of an earthquakeIt is commonly measured from the amplitude of seismic waves on a seismogramRichter ScaleDevised by Charles Richter in 1935 at the California Institute of technologyIt is used to asses moderate size earthquakesIt uses waves with frequencies between 05 to 10 Hz that saturate for distant or truly large earthquakes It was devised to describe the magnitude of Californian earthquakes or events with shallow hypocenters that are located less than 500 km from the seismometersHe hypothesized that the bigger the earthquake the greater the shaking of the earth and thus the greater the amplitude swing of the lines made on the seismogramRichter defined magnitude as the logarithm to the base ten of the maximum seismic wave amplitude in thousandths of a millimeter recorded on a standard seismograph at a distance of 100 km from the earthquake centreHe assigned simple whole numbers to describe magnitudes for every tenfold increase in the amplitude of the recorded seismic wave the Richter magnitude increases one number The energy released by earthquakes increases even more rapidly than the tenfold increase in amplitude of the seismic wave trace For example if the amplitude of the seismic waves increased 10000 times the Richter magnitude would move up from a 4 to an 8The actual shaking in earthquakes above magnitude 6 does not increase very much more maybe three times more for each step up in magnitude The bigger the earthquake means that more people in a larger area and for a longer time will experience the intense shaking Upon learning of an earthquake one can rapidly measure the amplitude of seismic waves and the difference in arrival times of P and S waves 25 Magnitude is the threshold of detection by humansThis scale is now restricted to measuring only local earthquakes with moderate magnitudes Other measures of earthquake size
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