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Richter Magnitude Scale
In 1935, Charles F. Richter devised a scale for quantifying the amount of energy released by an earthquake. His scale, known as the Richter scale, was a logarithmic scale based upon the relationship between an earthquake's peak amplitude, or ground motion, and the distance of the recording device from the quake's epicenter. As a base-ten logarithmic scale, each whole number increase equals a ten-fold increase in ground movement, and around 31 times the energy. The scale was designed for California earthquakes and is not reliable for magnitudes greater than seven, or distances greater than 370 miles. New methods for determining magnitude have since been developed; however, their values are consistent with the Richter scale.
Earthquake Magnitude Classes
Earthquakes with a magnitude of 3.0 to 3.9 are considered minor quakes. Those with magnitudes between 4.0 and 4.9 are referred to as light earthquakes. Quakes registering between 5.0 and 5.9 on the Richter scale are considered moderate earthquakes. Those with magnitudes between 6.0 and 6.9 are referred to as strong quakes. Magnitudes between 7.0 and 7.9 are rated as major earthquakes. All earthquakes with magnitudes over 8.0, and there is no upper limit to the Richter scale, are considered great earthquakes.
Earthquake Damage Scale by Magnitude
There are approximately 900,000 quakes each year that have magnitudes less than 2.5. These earthquakes are rarely felt but are recorded by seismographs. Each year, there are approximately 30,000 quakes with magnitudes between 2.4 and 5.4. These earthquakes typically produce only minor damage. Earthquakes with magnitudes between 5.5 and 6.0 cause slight damage to structures, of which there are an average of 500 each year. An average of 100 quakes with magnitudes between 6.1 and 6.9 occur each year. These strong quakes result in extensive damage, especially in populated areas. Each year, there are around 20 earthquakes with magnitudes between 7.0 and 7.9. These major quakes are responsible for serious damage. Great earthquakes only occur about once every five to ten years. However, when they do, they cause catastrophic damage near their epicenter.
Modified Mercalli Intensity Scale
The Modified Mercalli Intensity Scale is used in the United States to determine a quake's intensity. Unlike magnitude, which is based on mathematics, this scale is based on observed effects. The scale was originally formulated around 1900 by Giuseppe Mercalli, and revised in 1931 by Harry Wood and Frank Neumann. The scale has twelve increasing levels of intensity, designated by Roman numerals. Unlike magnitude, which is based on amplitude, is scientifically measured and results in one overall value, a quake's intensity will vary greatly by location and is dependent on the observer's subjective evaluation.