Procedure for Finding Richter Magnitude

  1. Determine the S-P interval and an amplitude from the seismogram for the earthquake.
  2. Locate this S-P interval time on the S-P graph and find the distance to the epicenter.
  3. On the Richter nomogram, locate the distance to the epicenter on the left-hand column and the amplitude on the right-hand column.
  4. Use a ruler to draw a line between the distance and amplitude column. The line will cross the center column at the Richter magnitude for that earthquake.

Typically, since three seismograms from three seismographs are needed to find the epicenter of an earthquake, all three S-P intervals and distances are plotted on the same Richter nomogram. This allows cross-checking of the S-P intervals and distances to the epicenters found for the three seismograms.

s-p graph richter nomogram


Location Amplitude (mm) S-P Interval (seconds) Distance (km)
Reno, NV


Salt Lake City, UT


Flagstaff, AZ



Using the amplitude and distance you determined in the SW US Seismogram resource, plot that data on the Richter nomograph above

Richter Magnitude = __________
The Richter magnitude =
  cube root of 343 - (the density of Au / 9.65)