ABAG Earthquake Quiz - Answer 4

Answer #4 Fiction!

Magnitude is a measure of overall earthquake size (which is related to the length of the fault that ruptures). On the other hand, intensity is a measure of the effect of the earthquake at a specific location. Thus, an earthquake has one moment magnitude, but a range of intensities. The intensity of ground shaking at a site varies for any particular earthquake based on:
  1. the size (magnitude) of the earthquake;
  2. the distance from the site to the fault source for the earthquake;
  3. the directivity (or focusing of earthquake energy along the fault axis rather than perpendicular to the fault); and
  4. 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, Earthquake Program Manager, Association of Bay Area Governments.

To learn more about intensity, click here.
To learn more about magnitude, click here.

ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area. This information is based on the ABAG reports, "The San Francisco Bay Area - On Shaky Ground."

jbp 9/16/03