A Short Primer on Earthquake Magnitudes

Alan KafkaWeston Observatory

Department of Geology and GeophysicsBoston College

Earthquake Magnitude

Magnitude is a measure of the actual size of an earthquake at the location where it occurred. This is not the same thing as the amplitudes of the seismic waves where you record them, because the amplitudes of those waves decay with distance as they travel from the epicenter to the station. To estimate the magnitude of the earthquake from seismograms recorded at stations around the world, it is necessary to adjust the amplitude you observed at your station for the amount that the waves decayed as they traveled from the earthquake to your station.

Magnitude Scales

A magnitude scale is a scheme for making these amplitude adjustments to determine a number that represents the size of the earthquake. Since exactly how to make these adjustments is a science in itself, there are many formulas that have been developed by seismologists to make the necessary adjustments. The original magnitude formula was developed by Charles Richter (in 1935), which is why the magnitude of an earthquake is often loosely referred to as the size of the earthquake on the “Richter Scale.”

Why are Different Magnitudes Reported for the Same Earthquake?

Since the development of the Richter Scale, there have been many other magnitude formulas that have been developed by other seismologists. This can lead to quite a bit of confusion, but all of these formulas should give roughly the same result. To minimize this confusion, the U.S. Geological Survey (USGS) publishes an official magnitude for each earthquake they report. This official magnitude is the USGS’s estimate of the magnitude that is most appropriate to release to the public given all of the complications discussed above. These official magnitudes reported by the USGS are the magnitudes that we will use for our earthquake tracking exercise.

Three Sumatra Earthquakes Recorded at

Weston ObservatoryBoston College

Magnitude 9.0December 26, 2004

Magnitude 8.7March 28, 2005

Magnitude 6.8April 10, 2005

Seismograms are shown on the same scale.

Same Distance, Different Magnitudes

∆=133°

Seismograms are shown on the same scale.

AS1 Seismograms

El Salvador and India Earthquakes

Recorded at Devlin Hall Boston College

Same Magnitude, Different Distances

IndiaMagnitude 7.7, January 26, 2001

El SalvadorMagnitude 7.7, January 13, 2001

Seismograms are shown on the same scale.

∆=33°

∆=106°

AS1 Seismograms

Phoenix Country Day School, Paradise Valley, AZ∆=7°

Weston High School,Weston, MA∆=38°

Parkfield, CA EarthquakeMagnitude 6.0, 09/28/04

Same Earthquake, Different DistancesSeismograms are shown on the same scale.

Time (sec/102)

AS1 Seismograms