There are a number of ways to measure the magnitude (size) of an earthquake. The most widely used method is the Richter Scale.
The Richter scale, developed in the 1930s, is a base-10 logarithmic scale, which defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude. This means that an earthquake of 6 is ten times more powerful than one with a score of 5 and an earthquake of 7 is 100 times more powerful than an earthquake measuring 5 on the Richter Scale.
The magnitude (size) of an earthquake is measured using a seismometer. This is a machine that measures movements in the earth’s surface.
The Mercalli Scale is also used to measure the size of an earthquake. This is a twelve-point scale for expressing the local intensity of an earthquake, ranging from I (virtually imperceptible) to XII (total destruction). The Mercalli Scale includes a set of definitions based upon what people in the area feel, and their observations of damage to buildings around them. These scales are important to business and property owners in earthquake affected areas.