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USGS Geology in the Parks

Visual Glossary

Loma Prieta earthquake
Aftermath of the 1989 Loma Prieta earthquake. Ground view of collapsed building and burned area, Marina District, San Francisco. Photo by C.E. Meyer, U.S.G.S.

Measuring Earthquakes:
Magnitude

There are two basic ways to measure the strength of an earthquake; magnitude and intensity. Intensity measures of the effect of an earthquake on buildings and reactions of people. Intensity levels range from not felt (I) to total destruction (XII). It is a useful measure in built-up urban areas, but not so useful in remote areas without any buildings to damage or people to react!

Magnitude is the most commonly used measure of an earthquake's size. It describes the total amount of energy released during an earthquake, allowing geologists to compare earthquakes occurring in different parts of the world.

In the 1930's, C.F. Richter devised a way measure the magnitude of an earthquake using an instrument called a seismograph to measure the speed of ground motion during an earthquake. Geologists discovered that the energy released in an earthquake goes up with magnitude faster than the ground speed by a factor of 32.

If you do the calculations, you'll see that a magnitude 7 earthquake has 32 times more energy than a magnitude 6 and almost 1,000 times more energy than a magnitude 5 earthquake! This doesn't mean there will be 1,000 times more shaking at your house. Large earthquakes last longer and spread their energy out over a much larger area.

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