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Name: Jason C.
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: January 27, 2004


Question:
How do you interpret data collected from a seismograph into the Richter scale?



Replies:
The concept of the Richter scale is relatively straightforward, but the actual application is complicated and depends on the specific characteristics of the seismograph. The original equation used by Professor Richter in 1935 said that the magnitude of an earthquake was equal to the base 10 logarithm of the ground motion in millimeters measured on a certain type of seismograph plus a correction factor related to the distance of the earthquake. The distance is calculated from the difference in arrival time for different types of waves that travel at different speeds.

So, for a constant distance between an earthquake's hypocenter and the seismograph, the ground motion has to increase by a factor of 10 to cause an increase of 1 on the Richter scale. Professor Richter was trying to characterize the energy of a seismic event, not damage. So, going from a 2.0 to a 4.0 magnitude event does not imply twice the damage. In fact, a rule of thumb is that the energy released increases by a factor of 30 for each 1.0 increase in magnitude. So, compared to a magnitude 3 event, a magnitude 8 earthquake releases 30*30*30*30*30 or 24.3 million times more energy and causes the amplitude (size) of the ground motions to be 100,000 times greater. The energy in a magnitude 8 earthquake is equivalent to 1 billion tons of TNT, or 30 jumbo thermonuclear weapons.

The original Richter scale was based on just a few instruments of a certain type and for Southern California geologic conditions. In the seven decades since it was created, many adjustments have been made for different regions, wave types, and instruments. Also, newer measurements, referred to as the "seismic moment" and "moment magnitude", have been developed to address some of the Richter scale's shortcomings.

Andy Johnson



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