Seismic (Earthquake) Magnitude

Earthquake size has been measured a number of different ways. The earliest measures were estimates of the number of casualties and the economic costs of events. Later metrics were based on the intensity of shaking measured by the amount of damage caused to human-made structures and the landscape. 

Earthquake magnitude is an instrumental measure of earthquake size. You need a seismometer to measure a magnitude. The earliest magnitude scales were developed in the 1930's by Charles Richter in the United States and Kiyoo Wadati in Japan. Around that time, both scientists gained access to seismograms recorded on seismic networks with relatively uniform seismometers. Richter based his magnitude scale on an analogy with the astronomical brightness scale, a logarithmic measure of celestial object brightness. Richter set up his scale so that magnitudes of most earthquakes would be positive, and he defined a magnitude 0.0 as the event that would produce a ground motion of 1.0 mm on a specific seismic instrument at a distance of 100 kilometers from the source.

Despite the popularity of magnitude scales, measuring an earthquake's size with one number such as a magnitude, is no easy task. As a result, we ended up with quite a few different magnitudes. The different magnitude scales refer to measures of the largest ground motion made on different seismic waves (for example, body waves or surface waves) or using different periods of vibrations. The most common magnitudes that you'll see are mb, body-wave magnitude, Ms, surface-wave magnitude, and Mw, moment magnitude. The "Richter" magnitude described in the press can often by any of a number of different scales. And the original "Local" magnitude ML, varies from place to place depending on variations in the local geology. The different seismic magnitudes do not always agree precisely, and in particular, for large earthquakes there are good reasons that they shouldn't. 

Earthquake Descriptors

Gutenberg and Richter defined a terminology for earthquakes with magnitudes in different ranges. These are often used in the press, and they are often used in scientific papers to specify a particular magnitude range. For example a "great" earthquake is an event with a magnitude equal to or greater than 8.0.

Descriptor Magnitude Range Typical Number Per Year
Great  8.0 or larger 1 or 2
Major 7.0 to 7.9 ~15
Strong 6.0 to 6.9 ~120
Moderate 5.0 to 5.9 ~800
Light 4.0 to 4.9 ~6,000
Minor 3.0 to 3.9 ~50,000
Minor 2.9 or less Thousands per day

 

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.