Visual Glossary

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.
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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. |