Establishing Appropriate Setback Widths for Active Faults GLENN BORCHARDT Soil Tectonics, P.O. Box 5335, Berkeley, CA 94705-0335 Key Terms: Active Tectonics, Site Investigations, Geologic Hazards, Seismic Risk, Soils ABSTRACT Tectonically undisturbed materials adjacent to active fault traces are defined herein as ‘‘freeboard’’ soils. Such materials have surface fault rupture potentials that are an inverse function of soil age and the activity of the adjacent fault. Surface rupture probability is low for old soils adjacent to extremely active shear zones but relatively high for young soils adjacent to moderately active shear zones. For a soil surface to be ruptured where it has never ruptured before (the ‘‘freeboard soil’’), the width of the shear zone must increase. As shown in this paper, this becomes increasing unlikely as a shear zone matures (.30 events), at which time a setback should not be required. Young freeboard soils require setbacks, which only can be determined from shear zone widths measured through older soils along a strike. A minor instance in which setbacks would be required involves the grading of flower structures, which may give a false impression of the expected width of the shear zone. Once primary faults are identified, any secondary faults can be avoided only after the mature width of the shear zone has been determined. No setback is then necessary. When this is not possible, structural mitigation to withstand minor offsets is preferable to an arbitrary setback, which gives little more than a false sense of security. INTRODUCTION For 35 years, consultants and reviewers for development projects within California’s Alquist- Priolo Earthquake Fault Zones have usually required setbacks from Holocene active faults. These setbacks are designed to minimize the potential for surface fault rupture (SFR) to damage structures for human occupancy. Setbacks generally were measured from the nearest active trace, with some jurisdictions routinely requiring a setback of 50 ft (15 m) even when there was no readily apparent geomorphic or subsurface evidence for faulting. Unfortunately, the traditional 50-ft (15-m) setback from active faults was never adequately justified. It was only a suggestion that ‘‘Unless proven otherwise, the area within 50 feet of an active fault is presumed to be underlain by active branches of the fault’’ (Bryant and Hart, 2007, p. 2). Some federal and state jurisdictions even have mandated setbacks of 200 ft (61 m) in an equally unsubstantiated belief that an increase in setback would provide an increase in safety (e.g., California Integrated Waste Management Board, 2002, pp. 13, 59). Utah has minimum setbacks ranging from 15 to 50 ft (5–15 m) geared to ‘‘criticality’’ of the proposed structure (Batatian, 2002) based on the calculation methodology of Batatian and Nelson (1999). This paper attempts to provide a preliminary framework for determining what a reasonable setback should be. PREDICTING FUTURE OFFSET OF ‘‘FREEBOARD’’ SOILS I first define the term ‘‘freeboard’’ soil as the ground surface adjacent to an active fault that has no evidence of SFR (defined here as shearing, folding, or warping associated with the current tectonic regime). Because age information is critical for establishing the potential for SFR, I use the term ‘‘soil’’ here in a broad sense, encompassing all near-surface materials. For example, in the absence of pedogenesis, the age of tectonically undeformed bedrock is used as the age of the ‘‘freeboard soil.’’ The challenge is to determine whether or not a freeboard soil is suitable for construction, consistent with health and safety involving the potential for SFR. The principal factors are 1) the age of the freeboard soil and 2) the number of ground-rupturing earthquakes undergone by the adjacent shear zone since soil formation began. Age of the Soil In general, we expect the probability of SFR through a freeboard soil to be an inverse function of its age. Thus, in general, a 100,000-year-old freeboard soil might be deemed a hundred times less likely to rupture than a 1,000-year-old freeboard soil. Environmental & Engineering Geoscience, Vol. XVI, No. 1, February 2010, pp. 47–53 47