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Peninsula Emergency Preparedness Coalition
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Gig Harbor, WA - USA

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Earthquake of May 2, 1983, Coalinga, California, USA. Affected Area: 205,000 km2; Damage: 31 million. The most serious damage occurred in the eight-block downtown commercial district, but residents were also heavily damaged. More then 800 single-family houses were destroyed or incurred major damage. The majority of the 94 injuries occurred in residential sections of the city.
Understanding Intensity
Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location. Intensity levels are expressed numerically according to observation of damage done to structures, changes in the earth's surface and subjective reports by people who experienced the shaking.

The Mercalli Scale is based on observable earthquake damage. From a scientific standpoint, the Richter scale is based on seismic records while the Mercalli scale is based on observable data which can be subjective. Thus, the Richter scale is considered scientifically more objective and therefore more accurate. For example a level I-V on the Mercalli scale would represent a small amount of observable damage. At this level doors would rattle, dishes break and weak or poor plaster would crack. As the level rises toward the larger numbers, the amount of damage increases considerably. The top number, 12, represents total damage.

Following an earthquake, intensity maps are sometimes drawn. On these maps are isoseismals, or contour lines, that separate one level of shaking intensity from another. MMI shaking intensity VI is usually considered the threshold of shaking that produces damage to poorly constructed man-made structures. Major damage occurs at intensity levels VIII and above.

Move on to the "Locating Epicenters" page