Thousands of earthquakes, many too small to feel, occur each year across the United States. Earthquakes occur most often west of the Rocky Mountains, yet there are 39 states that have a medium to high potential for quakes.
Although most people think of California as the "earthquake state," Alaska actually experiences the greatest number of major quakes.
However, the New Madrid Fault, which lies under America's heartland – Illinois, Indiana, Kentucky, Missouri, Tennessee, Arkansas, and Mississippi – is the site of the three largest earthquakes known to have occurred in North America. In 1811 and 1812, the region near New Madrid, Missouri, experienced earthquakes with magnitudes estimated between 7.2 and 8.3 (on the Richter Scale), as well as 203 damaging after shocks. Soil actually liquefied!
The places that pose the greatest danger during earthquakes are outside buildings, at the exits and exterior walls. In 1933, many of the 120 deaths in the Long Beach (California) Earthquake occurred when people ran from shaking buildings.
The Richter Scale
Developed in 1935 by Charles F. Richter of the California
Institute of Technology, this scale is actually a measurement of
an earthquake's magnitude.
Magnitude scales, like the Richter and the Moment Scales, measure the size of the earthquake at its source by the amplitude of the largest seismic wave recorded. Intensity scales, like the Modified Mercelli and the Rossi-Forel Scales, are more subjective, since they measure the amount of damage an earthquake causes.
The Richter Scale is based on a logarithmic scale or base 10. That means for each one point increase on the magnitude scale, there is 10 times the amount of energy released. The amount of energy released goes up each magnitude faster than the ground velocity by a factor of 32.
For example, a magnitude 6.0 earthquake, as measured on the Richter Scale, has 32 times more energy than an earthquake with a 5.0 magnitude, and almost 1,000 times more energy than an earthquake with a 4.0 magnitude.
United States Geological Survey Earthquake Magnitude Classes |
|||
Class |
Magnitude |
Est. No. Each Year (globally) |
Effects |
| Great | > 8.0 | one every 5-10 years | great earthquake; can totally destroy communities near the epicenter |
| Major | 7.0-7.9 | 20 | major earthquake; serious damage |
| Strong | 6.0-6.9 | 100 | may cause a lot of damage in very populated areas |
| Moderate | 5.5-5.9 | 500 | slight damage to buildings and other structures |
| Minor to Moderate | 2.5-5.4 | 30,000 | often felt, but only causes minor damage |
| Very Minor | < 2.5 | 900,000 | usually not felt, but can be recorded by a seismograph |
[Some content on this page is
courtesy of the Department of Geological Engineering and
Sciences, Michigan
Technological University in Houghton, Michigan, and the United States Geological Survey.]
Information gathered from Weather.com.
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