Earthquakes can be measured in several ways, two of which are intensity and magnitude. Intensity measures damage to the surface and the effects on humans. The scale used to measure intensity is called the Modified Mercalli Intensity Scale. Magnitude does not depend on population and effects to ground structures, but rather on wave amplitude and distance from the epicenter. This measure uses formulas and seismograph information to determine the magnitude. The scale used to measure magnitude is the Richter scale. Although each earthquake has a unique magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors. Depending on the size, nature, and location of an earthquake, seismologists use several different methods to estimate magnitude. The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data. Richter magnitudes and earthquake effects: Less than 3.5 Generally not felt, but recorded. 3.5 - 5.4 Often felt, but rarely causes damage. Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings. 6.1 - 6.9 Can be destructive in areas up to about 100 kilometers across where people live. 7.0 - 7.9 Major earthquake. Can cause serious damage over large areas. 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across. For scientific calculation of the Richter scale: http://www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html

WickedWillie's answer is a good one. However, the Richter scale is falling out of favor. The problem is that, when you get into the great quakes, the Richter scale breaks down. The Richter scale is based on the amplitude of the earthquake waves. However, over about magnitude 8 there is not much of an increase in amplitude. What changes is the duration of the earthquake. In recent years another scale has been gaining favor. It is called the moment magnitude scale. Moment magnitude can be determined either seismograms or field observations. So, there are two independent ways to get the magnitude. Therefore each can be compared to the other for increased accuracy and you can more easily estimate pre-siesmograph quakes. The moment magnitude scale is calibrated so that at the lower magnitudes is corresponds to the richter magnitudes. However, at the higher magnitudes, the moment magnitude scale can separate earthquakes. You can goto http://en.wikipedia.org/wiki/Moment_magnitude_scale for more information on this scale.

could u give an example of how logarithms is used in the richter scale?