Seismic Hazard Mapping at 1:24,000 Scale, Wasatch Front, Utah-- A Digital Pilot Project

(Summarized from McCalpin, J.P., 1999, Seismic hazards mapping at 1:24,000 scale, Wasatch Front, Utah—a digital pilot project: unpublished report submitted to the Utah Geological Survey, Salt Lake City, UT by GEO-HAZ Consulting, Inc., Estes Park, CO, Dec. 31, 1999, 112 p. plus digital appendices).

          This study produced three types of seismic hazard maps for the Logan, Smithfield, Wellsville, and Newton 7.5' quadrangles in north-central Utah. First a 1:24,000-scale Map of Amplified Ground Motions was created for each quadrangle. The basic procedure to produce this map is the adaptation of the 1997 NEHRP "Recommended Provisions" procedure for identifying Site Classes (a site-specific procedure) to areal mapping at 1:24,000 scale (see the PDF file Expanded Flowchart of GIS Protocol

. NEHRP Site Classes were mapped using SPT blow counts as the geotechnical parameter. Due to the scarcity of geotechnical boreholes in the area, water well logs were used as "surrogate" boreholes and pseudo-SPT values were assigned water wells to assist in contouring SPT average of the upper 30 m. Ground motion amplification factors are then cited from the 1997 NEHRP Recommended Provisions, for short- and long-period spectral accelerations, for the probabilistic ground motion with a 2% Probability of Exceedance in 50 years (return period ca. 2500 years). For Site Class E (soft clays) the long period amplification factor is 2.8, but the short period amplification factor is only 0.9. Site Class E soils are inferred in one area between major canyon-mouth fans, and at the distal edge of such fans.

          Second, a Map of Liquefaction Potential was created for the Smithfield quadrangle. Because geotechnical boreholes were scarce I devised a geologically-based rating system for Liquefaction susceptibility. This rating system categorized 5 variables at each water well and yielded a weighted rating value that was correlated with critical accelerations computed by the simplified method of Seed et al (1985). The only areas with high liquefaction potential are the natural levees (Qals) and floodplain (Qal1) of the Bear River. Three areas of Low-Moderate potential exist off the distal edges of Provo deltas and subdeltas where sand is abundant in the subsurface. The remainder of the quadrangle is either Low potential (where water table depth is <50 ft) or Very Low potential (water table depth >50 ft).

          Third, seismic slope stability was analyzed by the 3-pronged approach developed by the Oregon Department of Geology and Mineral Industries. In this approach lateral spread displacements (Group A) are calculated for all soil slopes less than 6%, and Newmark displacements (Group B) for all soil slopes greater than 6%. Rock slope stability (Group C) is analyzed by the empirical flow chart procedure of Keefer (1993). For lateral spreads, high displacements (>0.9 m) are predicted only for the natural levees of the Bear River and for the western edge of the youngest Provo subdelta in downtown Logan. Over 90% of the valley floor falls into the Low or Very Low displacement categories. High Newmark displacements (>30 cm) are predicted on many soil slopes that lie at the base of much steeper bedrock slopes. In most of these cases the soil slope angles have been exaggerated by the DEM smoothing process, and thus Newmark displacements are overestimated. Rock slopes of the Bear River Range generally fall into the Low hazard category, except for rockfall source zones (Moderate hazard) and pre-existing mapped landslides (High hazard). However, a large part of the outcrop area of the Salt Lake Group in the Smithfield quadrangle is classified as Very High hazard, because slopes steeper than 25 degrees are developed on the "poorly indurated" Salt Lake Group. A composite map for each quadrangle shows all three hazard ratings.

          These seismic hazard maps, and the accompanying text that describes the analysis algorithms in detail, have been published by the Utah Geological Survey in their Miscellaneous Publications series. To view the maps, go to http://www.ugs.state.ut.us/maps/geohazmap/index.htm, and look for Hazard Maps—Cache Valley

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