14.5 Earthquake Magnitude Earthquake magnitude is the amount of energy released by the earthquake. The intensity of an earthquake refers to the effects (damage) observed at the surface. There are two measures of the energy released, or magnitude , of an earthquake: 1. The Richter magnitude of an earthquake is determined from the maximum amplitude of ground motion as recorded by seismic waves registering on a seismograph along with the distance to the epicenter (also determined from the seismograph). An earthquake with maximum amplitude of 1 mm on a seismograph 100 km from the epicenter is assigned a Richter magnitude of 3.0. The Richter magnitude is a logarithmic scale meaning that an increase of 1 on the Richter scale is a tenfold increase in magnitude. A graphical device called a nomogram is used to relate the maximum amplitude of seismic waves and epicenter distance to the Richter magnitude.
The Richter scale is a logarithmic scale. Increasing the magnitude by one corresponds to a tenfold increase in amplitude. The seismic waves are 10 times as large and 30 times as much seismic energy was released by the earthquake. In other words a 7.0 earthquake is 10 times stronger than a 6.0 earthquake; a 8.0 earthquake is 100 times stronger than a 6.0 earthquake, etc.
The maximum amplitude of the seismic waves and distance to the earthquake epicenter (as indicated by the S-p interval) is determined from an earthquake seismograph. A graphical device called a nomogram relates distance and amplitude to Richter magnitude. A straight line drawn from the left (distance) axis to the right (amplitude) axis will cross the middle axis at the earthquake’s Richter magnitude.
Note that a seismograph closer to a given earthquake records a greater amplitude of seismic waves than a seismograph further away. Because the amplitude of ground shaking depends on the material the seismic waves are passing through, the Richter magnitude is only an estimate of the energy released by an earthquake. The moment magnitude is a true measure of the energy released by an earthquake and is based on geophysical measurements of the amount of displacement along the fault, the strength of the rocks that ruptured, and the area of the fault that ruptured. Determining the moment magnitude of an earthquake can take months of field measurements and laboratory work, while the Richter magnitude can be determined from seismographs within minutes of an earthquake. 14.6. Earthquake Intensity Earthquake intensity is a measure of the effects of an earthquake and is based on ground observations, instead of instrument measurement, including people’s perceptions and reactions to the earthquake and the damage caused by the earthquake. Damage depends on earthquake’s magnitude, duration, distance from the epicenter, geological conditions, and the type and condition of buildings, highways, and other infrastructure, so the intensity of the same earthquake will vary from place to place. The effects of an earthquake in a given area are assigned an intensity from one to twelve using the Modified Mercalli Intensity Scale.
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- Earthquakes, Seismic Waves