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Recommended setback distances from active normal faults
Proceedings of the Symposium on Engineering Geology and Soils Engineering (1987)
  • James P. McCalpin
Abstract
The geometry of near-surface ground breakage was analyzed from 40 trenches across Quaternary normal faults to help define reasonable setback distances. From each of the trench logs (28 on the Wasatch Fault, 11 on other Great Basin faults) eight parameters characteristic of surface rupture style were measured. Parameters included: 1) position of the main fault in relation to scarp morphology. 2) dip of the main fault. 3) number of faults on the upthrown block, 4) width of the upthrown block fault zone. 5) number of faults in the downthrown block. 6) width of the downthrown deformation zone, 7) ratio of antithetic to main fault displacement, and 8) degree and type of tilt or drag. A typical rupture event on the Wasatch Fault resulted in a 2-3 m displacement along a main fault lying near the scarp mid-point (48 +/- 11% of distance from base to crest) with a dip of 78 +/- 10 degrees , often fronted by a gouge zone 0.3-0.6 m wide. A single minor fault typically occurs on the upthrown block within a meter of the main fault, in contrast to an average of four faults on the downthrown block over a width of 12-14 m. Tilt or drag occurs in 50% of the cases, averaging 90 over a distance of 5 m from main fault. The typical rupture and ensuing scarp degradation scenario define a zone of hazardous geologic processes to be avoided in siting structures. On smoothly sloping, previously unfaulted terrain. setbacks from the inferred fault trace of 40 feet on the upthrown side and 50 feet on the downthrown side are recommended as minimums. For pre-existing fault scarp areas, setbacks of 20 feet from the 30' slope break at the scarp crest, and 30 feet from the 30% slope break at the scarp base are recommended. In many areas, the standard building setback from lot boundaries (20-50 feet) would be an appropriate setback from the non-buildable area (>30% slopes) of the fault scarp face. Data further suggest that building within graben should only be allowed after a detailed site investigation, wherein building recommendations are based on extensive trenching studies.
Keywords
  • active fault,
  • surface rupture,
  • seismic hazard,
  • Wasatch fault,
  • trench
Publication Date
1987
Citation Information
James P. McCalpin. "Recommended setback distances from active normal faults" Proceedings of the Symposium on Engineering Geology and Soils Engineering Vol. 23 (1987)
Available at: http://works.bepress.com/james_mccalpin/7/