Earthquake Magnitude and Intensity

Ways to Measure the Size of an Earthquake

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The amount of energy released during an earthquake is measured by its magnitude
Charles Richter took the logarithm of the largest ground motion registered by a seismograph as his emasure of earthquake magnitude and came up with the Richter Magnitude Scale
Two earthquakes at the same distance from a seismograph that dfifer in size of ground motion by a factor of 10 differ in magnitude by 1 Richter unit
Ex. the ground motion of an earthquake of magnitude 3 is 10 times that of an earthquake of magnitude 2
Ex. the ground motion of an earthquake of magnitude 6 is 100 times that of an earthquake of magnitude 4

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Most seismologists today use the Moment Magnitude Scale
A measurement more directly related to the physical properties of the faulting that causes the earthquake
Proportional to the logarithm of the area of the fault break and is approximately proportioned to the logarithm of the seismic energy released during the rupture
Both Richter's method and the Moment Magnitude method give roughly the same number
The Moment Magnitude can be measured more accurately from the seismographs, and it can also be deduced directly from field measurements of faulting

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Measures the amount of damage done to the structures involved
This measurement, called the intensity of an earthquake, is determined using the modified Mercalli scales
This scale rates the types of damage and other effects of an earthquake as noted by observers during and after its occurrence
Scale uses Roman numerals I to XII to designate degree of intensity; the higher the numeral, the worse the damage
Specific effects or damage correspond to specific numerals

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