The determination of the Richter Magnitude

The Richter magnitude of an earthquake is determined by using two pieces of information from the seismogram record: the amplitude (heigh) of the largest wave, and the time between the P and S waves. The latest gives an estimate of the distance from the eartquake focus to the station. Then, a single line can be drawn across a chart called a nonograph(bottom). This line must go from the time value at the left column, to the amplitude value at right one. The Richter magnitude can be read off at the central column.

Each instrument has its own nomograph chart because it has its own specific magnification of the Earth's movements caused by the seismic waves.

A better measure of the size of an earthquake is the amount of energy released by the earthquake. The amount of energy released is related to the Richter Scale by the following equation:

Where Log refers to the logarithm to the base 10, E is the energy released in ergs, and M is the Richter Magnitude. From this equation we can get the results shown on the table. Each increase in 1 in Richter Magnitude represents a 31 fold increase in the amount of energy released. Thus, a magnitude 7 earthquake releases 31 times more energy than a magnitude 6 earthquake. A magnitude 8 earthquake releases 31 x 31 or 961 times more energy than a magnitude 6 earthquake.