The Richter scale is used to measure the magnitude of earthquakes. The scale, more formally known as the Richter magnitude test scale, ranges from 0 to over 8.0. It is a logarithmic scale, meaning each unit of increase on the scale corresponds to an absolute increase by an order of magnitude, or factor of 10. Earthquakes measured at less than about 2 on the Richter scale are not very serious, and can barely even be measured, much less felt. An earthquake is usually considered much more serious once it hits about 5. At magnitudes of 7.0 or 8.0 the damage can be catastrophic.

The magnitude of an earthquake is measured using a seismometer. The original Richter scale was based on a particular sort of seismometer. The device consists of a secure and stable frame, a weight held in place by inertia, vibration-dampening springs to prevent long-term oscillations, and a way of measuring the displacement of the weight from the reference point. The earthquake's strength is measured by the degree to which it causes the weight to be displaced. The seismometer is rarely at the epicenter of an earthquake, so calibration techniques are used to determine what the displacement would have been if it were actually located there.

Earthquakes are caused by tectonic plates deep underneath the earth slipping and grinding against each other. Sometimes tension can be built up for hundreds or thousands of years before it's released in a great quake. Earthquake-prone areas like San Francisco and Los Angeles continue to be at risk from a large quake. Earthquakes cannot be measured very accurately using the Richter scale when the magnitude is above about 8.3, so a more modern measuring metric known as the seismic moment is commonly used.

Great earthquakes, with a magnitude of 8.0 or greater, happen about once a year. Smaller earthquakes happen hundreds or thousands of times a year, some of which can barely be measured. An earthquake smaller than 3.0 on the Richter scale cannot be felt by most people. The largest earthquake ever recorded measured 9.5 on the Richter scale, and occurred in Chile, but caused damage as far away as Hawaii due to tsunamis.