Seismologists have tried different dimensions leading to several magnitude scales. 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. Mountain 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.