How Did We Choose Which Faults to Use as Earthquake Sources?
	Based on ABAG "On Shaky Ground" Reports

Fault segments generate "characteristic" earthquakes. Some faults are weak and tend to generate earthquakes with moment magnitudes of 5 and 6. However, several 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 ABAG ground shaking hazard maps.

Various researchers have produced lists of faults capable of generating major earthquakes affecting the San Francisco Bay Area. The most recent list, and the one most widely accepted at the present time, was prepared by the Working Group on Northern California Earthquake Probabilities and released in 2003. This report provides information on a number of faults or fault segments which might impact the Bay Area.

This Working Group did not compile information on some additional faults in the Bay Area that are felt to be unlikely to generate a large (greater than magnitude 6) earthquake in the next 30 years or so. These faults are responsible for background seismicity and are often poorly understood. An earthquake can occur on one or more of these other faults. While they could produce a large earthquake, they are less likely to generate a significant earthquake than the major faults included in their report and in the following table. These faults include the Maacama in northern Sonoma County, the Monte Vista on the western side of the Santa Clara Valley, and the West Napa fault in southern Napa County. ABAG has mapped shaking intensity for these three faults because the earthquakes they may generate are useful for emergency planning.

Note that the Santa Cruz Mts.-San Andreas is similar, but not identical, to the fault causing the Loma Prieta earthquake, which occurred deeper and at an angle to the principal trace of the San Andreas fault. On the other hand, the Southern Calaveras, the Southern San Gregorio, and the northern North Coast-San Andreas faults are outside of the Bay Area. The Bay Area impacts of earthquakes on these fault segments are dwarfed by their Bay Area segments so they are not included.

	On the basis of research conducted since the 1989 Loma Prieta earthquake, the U.S. Geological Survey (USGS) Working Group on Northern California Earthquake Probabilities published a report in 2003 concluding that there is a 62% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2032. Thus, a major quake is about twice as likely to happen as not to happen in the next 30 years. This overall regional probability is broken down by fault system on the adjacent map, with probabilities for individual fault sytems shown in the smaller boxes. As is shown, many earthquake faults realistically generate these large earthquakes and the faults are located throughout the Bay Area. Probabilities for individual fault scenarios are availble from USGS as part of a larger report on earthquake probabilities .
Graphic Source: U.S. Geological Survey
The Califonria Geological Survey and USGS have also produced a probabilistic shaking hazard map of California that shows the probability of a variety of shaking accelerations being exceeded over the next 50 years throughout California.

All of these scenario earthquakes result in areas of modified Mercalli intensities of V to X. To view the modified Mercalli intensity scale,click here.

To view a map of active faults in the Bay Area, click here. (Adobe Acrobat PDF)

jbp 10/15/03