Earthquakes can be measured in several ways. The first way is to describe the earthquake's intensity. Intensity is the measure, in terms of degrees, of damage to the surface and the effects on humans. Intensity records only observations of effects on the crust, not actual ground motion or wave amplitudes which can be recorded by instruments. While intensity helps to determine how large of an area was effected, it is not an accurate measure of the earthquake for many reasons. Two such reasons are: only the effect on an area showing the greatest intensity is reported, which can imply a greater or lesser intensity than what actually occurred, and the way in which seismic waves travel varies as they pass through different types of rocks, so some areas near by may feel nothing because they are built on faulted rock, while other areas quite a distance from the foci will feel the effects because they are built on compact homogenous rocks.
The second type of measurement is the magnitude of the earthquake. Magnitude does not depend on population and effects to ground structures, but rather on wave amplitude and distance. Magnitude is determined using mathematical formulae and information from seismograms. One such magnitude scale is the Richter scale. This magnitude scale is logarithmic, meaning each step in magnitude is exponentially greater than the last.
To determine the Richter magnitude, information collected by seismometers is used. Using a seismogram, the time difference between the recording of the P wave and the S wave is determined and matched to a corresponding distance value. The single maximum amplitude recorded on the seismogram is calculated and a line is drawn between the amplitude scale and the distance scale. The line crosses another scale, which corresponds to the magnitude. While this type of measurement is the most well known, the Richter scale is not as accurate a measurement as believed. Originally designed specifically for California, the Richter magnitude scale becomes an approximation in other states and countries. Also, the type of wave whose amplitude is to be measured is not specified, and it does not distinguish between deep and shallow foci.
Below is a chart that shows how to measure Richter magnitude by an "eyeball" fit. First, the amplitude of the surface wave is measured on a seismogram produced by a Wood-Anderson seismometer (a specfic type of seismometer) and then it is compared with distance from the earthquake or the S-P time (which is the amount of time between the P-wave and S-wave arrival) to yield a magnitude. Click on the image to see a larger version.
There are many other magnitude measurements. In addition to Richter magnitude, there is also body wave magnitude and surface wave magnitude. These magnitude scales differ by the type of wave amplitude that is measured from the seismogram and the mathematical formula used to determine the magnitude. They are all, however, logarithmic scales.
A third type of measurement is called the seismic moment. Using the seismic waves and field measurements that describe the fault area, the moment, a parameter related to the angular leverage of the forces that produce slip on a fault, can be measured. This moment can be related to a corresponding magnitude for easier interpretation, called the moment magnitude. The benefit of this type of measurement is that it gives a consistent and uniform measure of the size of an earthquake of any magnitude anywhere in the world, and because it takes into account fault geometry. Along with this new type of measurement, the individual amplitudes of body and surface waves are being measured as well.
Last modified on 8/13/98 by Maggi Glasscoe (firstname.lastname@example.org)