How Are Earthquake Magnitudes Measured?

Earthquake strength is reported on a special scale that gives us some idea of the “magnitude” of these major events. This measurement method, commonly referred to by the name Richter scale, is based on the same idea that astronomers use to measure the brightness of objects in the sky. That scale also uses the word “magnitude” when referring to brightness. Richter originally sought to provide an idea of the size of a quake.

A measuring instrument called a seismometer records the movement of the Earth during a quake. When the earth shakes the instrument needle moves to show how much movement is taking place. Charles Richter and an associate developed the scale with “0” representing a minor earthquake. On this unique scale, “0” doesn’t mean there is no quake activity. That’s why a quake of 2.0 or a similar “minor” earthquake may be noticed. It’s important to remember that magnitude scales are analog in nature. They give us an idea of how the strength of one quake relates to the strength of another.

Records show that earthquakes of 4.6 magnitude on the Richter scale (or higher) may be recorded on seismographs equipment located thousands of miles from the center of the earthquake activity. A quake of 2.0 or lower on this scale is considered “micro” and is usually not felt by human beings. When an earthquake reaches near 3.0 on the scale it can be recorded but may still not be felt.

The remainder of the scale, with corresponding descriptions, is: 3.0 to 3.9 – no significant damage; 4.0 to 4.9 – noticeable shaking but lower levels of damage; 5.0 to 5.9 – damage to buildings of poor construction; 6.0 to 6.9 – destructive, even up to 100 miles from the center of activity; 7.0 to 7.9 – major damage over larger areas; 8.0 to 8.9 – hundreds of miles of area affected, with major damage; 9.0 to 9.9 – thousands of miles of area affected, with major damage. An earthquake of 10.0 has never been recorded. Larger quakes have the same force as millions of tons of TNT.

Thousands of micro-quakes and minor earthquakes occur each year. Scientists record only a few earthquakes above 7.0 on the Richter scale each year. The Richter scale was developed to assign numbers to the power of earthquakes. The scale is also referred to as the Richter magnitude scale. One form of this measuring method is called the local magnitude scale.

Another man, Giuseppe Mercalli developed a scale to measure earthquakes years before Richter. However, it was never used consistently because the reports were based on individual observation and estimates by those individuals as to the strength and movement. Whether the reports of an earthquake’s strength or intensity come from personal observation by experienced people or from the movement of sensitive instruments, the measurement of earthquakes is an important tool.

These numbers and reports help us understand how many different levels of movement are capable within our Earth. These scientific tools bring us closer to understanding where we live and how we live.