An earthquake’s magnitude is measured by a network of stations with equipment (seismographs) designed to measure the movement of the ground at that site.
In the United States, the seismographic network stretches from a point near the Bearing Sea in Alaska to a point on Disney World property in Florida and includes sites in nearly every state.
How does the seismographic network record and rate an earthquake? It involves several factors.
When sections of the earth slip past each other, energy from that movement is released and that energy causes the ground to vibrate. In turn, that vibration causes the ground adjacent to the area of the slip to vibrate, and this pattern continues across the region where the earthquake happened, causing the shaking felt when an earthquake hits.
That energy is measured in different ways to come up with a measure of the strength of an earthquake.
Magnitude is the most common measure of an earthquake's size, according to the USGS. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like.
While you often hear the measurement of an earthquake in terms of the Richter scale, the USGS no longer uses that scale for large earthquakes, using instead other measurements such as the Moment Magnitude scale, or the Mw scale. The Mw scale is related to the total energy released in an earthquake.
The intensity of an earthquake is another value scientists study. Intensity is a measure of the shaking and damage caused by an earthquake and will be different from location to location.
Earthquakes are classified in categories ranging from minor to great, depending on their magnitude. Here's the list the USGS uses to classify earthquakes:
Great: 8 or higher