What is the Magnitude Scale?

To the people who suffer through a major earthquake it may not be very important to know how strong the quake was on a scale of 1 to 10. But there is such a scale, developed to assign a number to the power or strength of an earthquake. This is commonly called the Richter scale, though it may be referred to as the Richter magnitude scale or the local magnitude scale.

One of the first things we should know about the Richter magnitude scale is that it is not a simple 1 to 10 measure of strength. Instruments are measuring the amount of energy released by the event. An earthquake that is measured as an 8.0 on the scale is not just twice the strength of a 4.0! The increments are much smaller. If an earthquake measures 8.0 on the scale it is 10 times larger/stronger than one measuring 7.0 on the scale.

There is one other detail that should clear up some of the mystery about measuring earthquakes. The local magnitude scale described above may be slightly different than one called the moment magnitude scale. But the bottom line is that a system exists to help us understand how strong one earthquake is in relation to others. These scale numbers help scientists and government agencies determine how to protect people and property from these massive geological events.

Though it may seem that there have been earthquake measurements for at least a century (or more) the truth is Charles Richter and Beno Gutenberg developed the measurement system in 1935 while they were at the California Institute of Technology. Richter wanted to be able to measure smaller, local earthquakes in California separately from the much stronger quakes reported around the state.

The scale is loosely based on a system used to measure brightness of stars and other objects in the sky. In fact, this astronomical scale also uses the term “magnitude.” The scientists used an instrument called a seismometer and Richter established “0” to represent a very minor earthquake event. Zero on the scale does not mean there is no activity. This is crucial to understanding that even a quake that measures 1.0 or 2.0 will certainly get our attention!

It is generally known among the scientific community that quakes measuring 4.6 or higher can be recorded on seismographs that are thousands of miles from the earthquake’s central location. In basic terms, less than 2.0 is a “micro” earthquake. These are generally not felt by human beings at all. From 2.0 to 2.9, earthquakes may not be felt but can be recorded. Earthquakes measuring 3.0 to 3.9 are felt but don’t usually cause damage. The remainder of the scale, in general increments, is: 4.0 to 4.9 – light, noticeable shaking; 5.0 to 5.9 – some damage, especially to poorly constructed buildings; 6.0 to 6.9 – destructive up to 100 miles; 7.0 to 7.9 – major damage over large areas; 8.0 to 8.9 – serious damage and risk over hundreds of miles; 9.0 to 9.9 – devastating over thousands of miles. Earthquakes measuring 10.0 and above – never recorded but would be catastrophic. It’s interesting to note that there are thousands of earthquakes in the 2.0 to 4.9 range every year. The numbers drop off quite a bit after that. There are only a handful of major earthquakes (above 7.0) each year.