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.

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