Ask A Scientist General Science Archive

Question:
Our middle school science students would like to know how the Richter
Scale originated, and how the numbers relate to the severity of an 
earthquake.
 carol flintoft

Answer:
The Ricther magnitude was developed by Charles Richter of the
California Institute of Technology in 1935.  It was one of the
first measures of the size of an earthquake to use seismic
instruments that record the ground motion.  The Richter magnitude
is measured from the maximum amplitude of recorded ground motion.
Ground motion is recorded on a seismograph--the larger the
earthquake, the larger is the ground motion.  The ground
motion becomes smaller the farther the seismograph is from
the earthquake, so the distance between the seismograph and
the earthquake must also be taken into account.  For the same
distance between the seismograph and the earthquake, each
unit increase in magnitude corresponds to a factor of ten
increase in ground motion.  For example, a magnitude 6 earthquake
will produce ten times the ground motion that a magntidue 5
earthquake will produce, but ten times less ground motion
than a magnitude 7 earthquake. The amount of energy released
by an earthquake varies by a factor of 30 for each unit increase
in the magnitude--a magnitude 6 earthquake produces 30 times
the energy of a magnitude 5 earthquake and 30 times less energy
than a magnitude 7 earthquake.  Incidentally, a magnitude 5
earthquake produces about as much energy as the nuclear bombs
dropped on Japan during World War II.
-Grant
See e.g. Bruce Bolt's book, "Earthquakes"