How Big Is BIG - Measuring Earthquake Size
Based on ABAG "On Shaky Ground" Reports
Magnitude is a measure of overall earthquake size. Larger magnitude earthquakes generally cause a larger area of ground to shake hard, and to shake longer. This relationship is generally well understood. Thus, one principal factor in determining shaking hazard is the magnitude of the earthquake.

Seismologists now have several measures of earthquake magnitude in addition to the familiar Richter (or "local") magnitude. The Richter magnitude has difficulty differentiating the size of large and great (7-1/2+) magnitude earthquakes. To overcome this difficulty, modern seismologists use moment magnitude because it best reflects the energy released by the earthquake. The moment magnitude is proportional to the area of the fault surface that has slipped. Thus, it is directly related to the fault length. Because the models used to generate ABAG's shaking hazard maps are based on fault length, they, in effect, bypass magnitude. (See 1995 "On Shaky Ground" report Appendix A for more technical documentation.)

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