How Strong is Strong - Measuring Shaking

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.