

Reading a Seismograph
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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Update: June 2012

