What Is the Role of Fault Segment Boundaries in Bay Area Earthquake Hazards?
	Based on ABAG "On Shaky Ground" Reports

The entire length of major faults do not always rupture at one time to create a single major earthquake. Often, only a portion of a fault ruptures in an earthquake. These pieces of major faults that rupture on their own are called "fault segments." Geologists and paleoseismologists work to identify the likely portions of faults to rupture. These fault segments generate "characteristic" earthquakes which serve as the sources of earthquakes used in creating ABAG's shaking hazard maps.

This process of identifying fault segments is not an exact science; professional judgment is involved, in part because little evidence of past earthquake ruptures may be present. Scientists at USGS and other earthquake researchers are reexamining the segments for use in future earthquake scenarios.

For example, ABAG used the definition of the Peninsula segment of the San Andreas fault identified in the 1990 report by the Working Group on California Earthquake Probabilities (1990) (Ref. 17) in our 1995 On SHAKY Ground report. The 1996 report by the Working Group on Northern California Earthquake Potential (Ref. 16) changes the definition of some fault segments, including the peninsula San Andreas, slightly. The length of this fault segment was changed from 52 to 88 km; this longer fault segment (now called the Peninsula-Golden Gate San Andreas by ABAG) extends north to outside the Golden Gate, rather than stopping on the peninsula. This longer fault segment will result in an earthquake which is much more serious than the shorter fault segment for both the City of San Francisco and the entire Bay Area. The segment length was again revised in the 2003 Working Group report and became approximately 3 km shorter.

1990 Peninsula San Andreas Earthquake Scenario

Extent

The Peninsula San Andreas fault segment was defined in the 1990 report by the Working Group on California Earthquake Probabilities (Ref. 17) as extending from the northern end of the fault source of the 1989 Loma Prieta earthquake to the northern end of Lower Crystal Springs Reservoir in San Mateo County. This fault segment is approximately 52 km long and is capable of generating a magnitude 7.l earthquake. It is included in the 1995 ABAG report, On SHAKY Ground (Ref. 1).

Estimate Housing Impacts

Approximately 45,700 total uninhabitable housing units, including approximately 19,200 units in San Francisco. See the 1996 ABAG report, Shaken Awake! (Ref. 3).

Estimated Transportation Impacts

Approximately 428 road closures, including about 84 in San Francisco. See the 1997 ABAG report, Riding Out Future Quakes (Ref. 8).

1996 Peninsula-Golden Gate San Andreas Earthquake Scenario

Extent

The Peninsula-Golden Gate San Andreas fault segment was used in the 1996 report by the Working Group on California Earthquake Potential (Ref. 16) as extending from the northern end of the fault source of the 1989 Loma Prieta earthquake beyond Lower Crystal Springs Reservoir to off the Golden Gate. This fault segment is approximately 88 km long and is capable of generating a magnitude 7.3 earthquake.

Estimated Housing Impacts

Approximately 107,200 total uninhabitable housing units, including approximately 65,100 units in San Francisco. Thus, in San Francisco alone, ABAG estimates this scenario will have over triple the impact of the smaller Peninsula-San Andreas scenario and almost double the impact of the entire Hayward scenario of approximately 38,700 uninhabitable units.

Estimated Transportation Impacts

Using this scenario, ABAG forecast approximately 836 road closures, including about 356 in San Francisco. Thus, in this city, this scenario will have over four times the impact of the smaller Peninsula San Andreas scenario and more than the 228 closures from the Hayward scenario.

2003 Peninsula-Golden Gate San Andreas Earthquake Scenario

Extent

The Peninsula-Golden Gate San Andreas fault segment used in the 2003 report by the Working Group on California Earthquake Probabilities extending from the northern end of the fault source of the 1989 Loma Prieta earthquake beyond Lower Crystal Springs Reservoir to off the Golden Gate, but stops approximately 3 km further south than previously. This fault segment is approximately 85 km long and is capable of generating a magnitude 7.2 earthquake.

Estimated Housing Impacts

Using this scenario, but combining it with slightly improved housing data, ABAG now forecasts approximately 108,500 total uninhabitable housing units, including approximately 65,300 units in San Francisco. Thus, in San Francisco alone, ABAG estimates this scenario will have over triple the impact of the smaller Peninsula-San Andreas scenario.

Estimated Transportation Impacts

Using this scenario, but combining it with more detailed information on the status of retrofit of local (not Caltrans) bridges in the region, ABAG now forecasts approximately 866 road closures, including about 335 in San Francisco. Thus, in this city, this scenario will still have four times the impact of the smaller Peninsula San Andreas scenario and more than the Hayward scenario.

jbp 10/15/03