How Quakes are Measured

Earthquakes are commonly measured by their magnitude and intensity.

Magnitude
Magnitude is a number that represents the relative size of an earthquake. It refers to the amount of energy released during a quake. It is calculated from data recorded by seismographs, which record seismic waves traveling through the earth.

There are several methods used to calculate magnitude. The most commonly used scales are the local magnitude (commonly referred to as the Richter Scale), surface-wave magnitude, body-wave magnitude, and moment magnitude. The first 3 methods have limited range and applicability, and are not as reliable for large earthquakes. The moment magnitude scale applies uniformly to earthquakes of all sizes, but it is more difficult to compute.

Moment Magnitude
Moment magnitude is now the preferred method for computing earthquake magnitude. Moment magnitude is more uniformly applicable to all sizes of earthquakes, is the most reliable especially for very large quakes, and takes advantage of information recorded by the increasing number of seismograph stations around the globe.

In 2002, the U.S. Geological Survey issued a policy that the moment magnitude will be the measurement used whenever available. The term "magnitude" now refers to moment magnitude.

Local Magnitude (Richter Scale)
The Richter Scale was developed in 1935 and is the most familiar to the general public. It was originally developed for moderate sized earthquakes (between 3.0 and 7.0) in southern California. It is an open-ended logarithmic scale, meaning each whole-number step represents an approximate 32-fold increase in released energy. For example 6.0 is equal to:
  • 32 times the energy of a 5.0
  • 1,000 times the energy of a 4.0
  • 32,000 times the energy released by a 3.0
"2/10ths Rule"
Every 2/10ths of a unit represents double the energy released at the focus. (The focus is the quake's point of origin within the earth's crust. The epicenter refers to the point on the earth's surface directly above the focus.)
  • 5.0 to a 5.2 is twice as big
  • 5.4 is 4 times as large as a 5.0
  • 5.6 is 8 times as large as a 5.0
The Richter Scale was not reliable when applied to quakes of 7.0 or greater, and was not designed to use data recorded from quakes centered more than 600 kilometers away.