How Big is Big - Measuring Earthquake Size

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 the shaking hazard maps in this report are based on fault length, they, in effect, bypass magnitude. (See Appendix A.)

Fault segments generate "characteristic" earthquakes. Some faults are weak and tend to generate earthquakes with moment magnitudes of 5 and 6. However, at least ten fault segments in the Bay Area are relatively strong and can store up enough energy to generate earthquakes of magnitude 7 or so. These stronger faults will generate these large earthquakes, not magnitude 5 and 6 events. The concept of "characteristic" earthquakes means that we can anticipate, with reasonable certainty, the actual damaging earthquakes that will occur on these fault segments. These anticipated events are the scenario earthquakes depicted in the color maps.

The probability of one of these scenario earthquakes occurring varies from fault segment to fault segment. The two Hayward fault segments and the peninsula segment of the San Andreas are felt to have, roughly, a probability of one in four of occurring in the next 30 years (Ref. 3). Other fault segments are less well understood; equivalent probabilities are being developed.

Note: No probability data is provided in Ref. 3 for the entire Hayward fault rupturing at once. However, experts at the U.S. Geological Survey working on a map to be included in the Uniform Building Code are now assuming that one in every four Hayward earthquakes will involve both segments of the Hayward rupturing at once, yielding a probability of 5-6% in 30 years (personal communication, Art Frankel).

The following table shows the 11 scenario earthquakes on 10 fault segments in the Bay Area for which intensity maps have been generated. (Both segments of the Hayward fault rupturing at once provides the eleventh scenario.) Approximate magnitudes calculated for the scenario earthquakes can aid in relating these scenarios to past earthquakes. All of these earthquakes result in areas of modified Mercalli intensities of V to X. To view the modified Mercalli intensity scale,click here.

Moment Magnitude Based on Fault Length for Scenario Earthquakes