We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Five trenches across a Holocene fault scarp yield the first radiocarbon-measured earthquake recurrence intervals for a crustal fault in western Washington. The scarp, the first to be revealed by laser imagery, marks the Toe Jam Hill fault, a north-dipping backthrust to the Seattle fault. Folded and faulted strata, liquefaction features, and forest soil A horizons buried by hanging-wall-collapse colluvium record three, or possibly four, earthquakes between 2500 and 1000 yr ago. The most recent earthquake is probably the 1050-1020 cal. (calibrated) yr B.P. (A.D. 900-930) earthquake that raised marine terraces and triggered a tsunami in Puget Sound.

Geologic and geodetic studies in California indicate that about 1 cm yr−1 of right‐lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is ∼N60° ± 20°W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction ∼N90° ± 30°W at a rate of ∼0.5 mm yr−1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 ± 1 mm yr−1 in a direction N55° ± 10°W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr−1 oriented ∼N60° to 70°W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific‐North American transform motion and much of the lateral component of relative motion between the Juan de Fuca and North American plates.

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