Implications for Earthquake Hazards

Earthquake probabilities are one key component in estimating the seismic hazard in a region, but not the only one. Most earthquake damage is caused by strong, sustained ground shaking. The strength and duration of shaking at a particular location depends on the earthquake's size, its distance from the location, soil conditions at the location, and details about the rupture itself and the propagation of the seismic waves from it.

WG02 has identified 35 potential earthquake rupture sources on the seven faults characterized in this study. For each potential source, a "scenario" map of the expected shaking intensity was constructed, using existing knowledge about the expected propagation and site effects in the SFBR. Figure 4 shows the expected shaking intensity distribution related to a M>=6.7 event on the southern Hayward fault, similar to a repeat of the 1868 earthquake.  This particular event has a likelihood of occurrence of 0.11 [0.02 - 0.32] over the next 30 years.

The full suite of 35 potential earthquake sources (and their probabilities) have been combined with the likelihood of background earthquakes to produce regional shaking hazard maps (Figure 5). These shaking hazard maps quantify the expected shaking in terms of modified Mercalli intensity (MMI), a scale that is related to damage. These maps represent average expectations and do not attempt to characterize the variations in the ground shaking and damage expected in any individual earthquake. The hazard map shown in Figure 5 depicts the MMI shaking level at a given site with a 50% chance of being exceeded in 30 years. This type of information is used as the input into the seismic design criteria in building codes.

Both the scenario shaking intensity maps and regional shaking hazard maps show that future earthquakes, regardless of where they occur in the San Francisco Bay region, are capable of producing damaging ground motions over broad areas and at substantial distances from the causative fault. Furthermore, the hazard maps show that sites located on rock have even odds in 30 years of experiencing up to MMI VII shaking, which is likely to cause structural damage in some structures not built to current earthquake design standards. In contrast, most sites on soft soils surrounding San Francisco Bay and the Sacramento River Delta generally have even odds in 30 years of experiencing MMI VIII or stronger shaking, which may cause significant damage in engineered structures.

Scenario ShakeMap

Figure 4. Scenario ShakeMap illustrating the strength and regional extent of shaking that can be expected from a future M 6.7 earthquake on the southern Hayward fault.

Shaking hazard of the SFBR, expressed
                      as the modified Mercalli Intensity.

Figure 5. Shaking hazard of the SFBR, expressed as the modified Mercalli Intensity (MMI) having even odds of being exceeded in 30 years. Shaking hazard is high (MMI VII) throughout the region, and especially pronounced on the soft-soil areas surrounding the bays and the Sacramento River Delta.