Magnitude is the most commonly reported measure of an earthquake's size. It began as a completely empirical measure defined by Beno Gutenberg and Charles Richter in the 1930's. They wanted a quantitative way to compare earthquakes, based on instrumental recordings, independent of the location of the observer. They borrowed the idea of a magnitude scale from astronomers, who used it to classify the brightness of stars. They defined it in terms of the amplitude of ground velocity recorded on a particular seismograph, scaled by the distance from the instrument to the earthquake.
It has since been shown to be proportional to the energy released in the earthquake but the energy goes up with magnitude faster than the ground velocity, by a factor of 32. Thus, a magnitude 6 earthquake has 32 times more energy than a magnitude 5 and almost 1,000 times more energy than a magnitude 4 earthquake. This does not mean there will be 1,000 times more shaking at your house. Bigger earthquakes last longer and release their energy over a much larger area.
"How big was the earthquake? That should be easy. Why do the scientists always seem to have problems coming up with a simple answer to a simple question"?
Many Californians have felt some version of this frustration after each earthquake where one seismologist always seems to be contradicting another. In fact, earthquakes are very complex. Measuring their size is something like trying to determine the "size" of an abstract modern sculpture with only one use of a tape measure. Which dimension do you measure?
Seismologists have tried different dimensions leading to several magnitude scales. These include local (also sometimes called the Richter scale since it was the first one defined by Richter), surface-wave, body-wave, duration and coda. All these scales measure the amplitude of some aspect of ground motion (velocity or acceleration at different distances and in different frequency bands).
In recent years, seismologists have developed a new scale, called moment magnitude to describe the size of an earthquake. Unlike other magnitude scales that measure only one part of the ground motion, moment magnitude is based on a physical quantity, called moment, that can be determined either from the geometry of the fault plane or from the total energy recorded on a seismogram. It is equal to the area of the fault times the amount of slip across the fault times the rigidity of the rock. Several recent earthquakes have confirmed that moment determined by geologists measuring the fault in the field matches the moment determined by seismologists from a seismogram.
Moment magnitude has many advantages over other magnitude scales. First, because it uses the complete seismogram, it doesn't saturate, allowing us to measure the largest earthquakes. Second, because it can be determined either instrumentally or from geology, we can use it to measure the size of old earthquakes and compare them to instrumentally recorded events. Third, estimates tend to be more reliable so differences of 0.2 in moment magnitude do mean something (just don't compare with some other type of magnitude.)
Earthquakes are not all bad. Earthquakes have created most of the mountains in southern California, producing our beautiful scenery and trapping rain clouds to keep us from being a desert. The mountains also form underground traps for oil and natural gas, making oil wells possible and showing us where to look for oil.