MAGNITUDE
Magnitude is a measurement of the energy released by an earthquake. The first earthquake-magnitude scale was the Richter scale, devised by Charles F. Richter, a seismologist at the California Institute of Technology. The
Richter scale is based on the amplitude of seismic waves – the stronger the earthquake, the stronger the seismic vibrations it causes. The Richter magnitude of an earthquake is expressed as a decimal number, such as 6.7. The most important thing to remember about Richter magnitude is that it is a logarithmic scale, meaning that an increase of one in magnitude corresponds to a factor of ten increase in the amplitude of ground motion. For example, a magnitude 6.7 earthquake causes shaking 10 times greater in amplitude than a magnitude 5.7 earthquake and 100 times greater than a magnitude 4.7 earthquake.
Method for determining the Richter magnitude of an earthquake from a seismogram.
The maximum wave amplitude on the seismogram is connected with the epicenter distance. The intersection of that line with the magnitude axis gives the earthquake magnitude. On the Richter Nomogram, the magnitude is determined by connecting the maximum wave amplitude (85 mm with proper scaling of the seismogram) with the epicenter distance (300 km, or 34 sec S–P lag). The magnitude of the earthquake shown is the intersection of that line with the magnitude axis of the diagram at M=6.0.
Using Figure above, determine the Richter magnitudes for the earthquake data given in the table below.

Amplitude (mm) Distance (km) Richter Magnitude
1
10
100
10
100
100
100
5
10
10
50
500
Looking at the first three earthquakes in the table above, what is the effect on Richter magnitude of a ten-fold increase in maximum seismic-wave amplitude?
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Looking at the last three earthquakes in the table above, what is the effect on Richter magnitude of a ten-fold increase in the distance between the recording station and the epicenter?
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This exercise will expand upon that example. It’s important to note that, although it was the most damaging earthquake in U.S. history, the Northridge earthquake was definitely not the “Big One” that California fears. In fact it was over six times less powerful than the 1992 Landers earthquake that did relatively little damage. The major problem was that the Northridge earthquake struck in the middle of a densely-populated urban area.
Determine the Richter magnitude of the Northridge earthquake. Use the following steps:
A) Measure the S-P lag time for each seismogram.
B) Measure the maximum seismic-wave amplitude for each seismogram.
C) Convert your amplitude measurements into mm using the scaling information provided with each seismogram.
D) Plot lag time and amplitude on Richter Nomogram to determine magnitude.

Three seismograms that record the 1994 Northridge earthquake
Station Lag Time
(seconds)
Max Amplitude
(cm)
GSC
SVD
MLAC
Scale Conversions Magnitude