In the first method, they measure the length and depth of the earthquake fault and the movement of rock at the fault. A fault is a fracture in the earth's rocky outer shell where sections of rock repeatedly slide past each other. In the second method, scientists use data from a seismograph, a device that records ground movements. Richter magnitude is based on seismographic data. The main difference between the two scales lies in the frequencies of the movements that are recorded.
Frequency is a measure of how often the ground moves back and forth in one second.
During an earthquake, the ground shakes at several frequencies at the same time. The movements travel together through the ground, just as radio broadcast waves of different frequencies travel through the air. This band does not include the lowest frequencies at which the ground moves. But as earthquakes become larger, their movement grows more at low frequencies. Thus, the Richter scale may underestimate the strength of an extremely large earthquake.