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Scientists can now very accruately measure the strength of earthquakes based on precise instruments that measure how much the earth moves. But historical events are only known by descriptions based upon limited and somewhat variable personal observations.
In 1902, Guiseppe Mercalli devised an earthquake intensity scale based upon observed damage, which can be determined from historical descriptions. The U.S. Coast and Geodetic Survey has revised this scale to 12 levels.
While scientists can now measure the strength of modern earthquakes, they often survey survivors of affected areas to rank the event against historical data. Each person would be asked to describe the damage they observed in their own home.
This allows them to assess changes in earthquake intenstity over time.
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