How Strong Is STRONG - Measuring Earthquake Strength
Based on ABAG "On Shaky Ground" Reports
Intensity is a measure of the effect of the earthquake at a specific location.

An earthquake has one moment magnitude, but a range of intensities. The most commonly used intensity scale is the modified Mercalli intensity scale (MMI scale). The intensity of ground shaking at a site varies for any particular earthquake based on several factors:


the size (magnitude) of the earthquake (which is related to the length of the fault that ruptures);
the distance from the site to the fault source for the earthquake;
the directivity (focusing of earthquake energy along the fault axis rather than perpendicular to the fault); and
the type of geologic material underlying the site, with stronger shaking occurring on softer soils

Just as a light bulb above my desk is 100 watts regardless of where I'm sitting, and the intensity of the light varies with where I am in my office, an earthquake has a single moment magnitude and a variety of intensities distributed throughout the region.
Jeanne Perkins

Click HERE for more information on the Modified Mercalli Intensity Scale.

ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.
Source - 2003 "On Shaky Ground" documentation prepared by ABAG.


jbp 10/15/03