NEHRP SITE CLASS AND LIQUEFACTION SUSCEPTIBILITY MAPS DERIVED FROM STATEMAP GEOLOGIC MAPPING, IDAHO FALLS-REXBURG AREA, EASTERN SNAKE RIVER PLAIN, IDAHO

The Idaho Falls-Rexburg area is a rapidly developing region with significant seismic shaking hazards. New NEHRP site class and liquefaction susceptibility maps were derived for this area using 1:24,000-scale STATEMAP geologic maps produced in 2005-2009. The new maps should improve the accuracy of regional simulations of earthquake losses and provide guidance to planners performing analysis of local hazards. Digital maps and ArcGIS data are available at www.idahogeology.org.

Major geologic units in the study area are late Tertiary rhyolitic tuffs, Quaternary basaltic lavas, late Pleistocene loess, late Pleistocene glacial outwash, Holocene Snake River and sidestream alluvium, and Holocene sand dunes. NEHRP site classes are based upon shear wave velocity and standard penetration test data from ID Transportation Dept bridge plans, ID National Lab earthquake design studies, and seismic stability studies for Ririe dam. Data were unavailable for rhyolitic tuffs and classification is based upon published measurements of similar units. Bedrock overlain by >3 m of loess was identified with water well logs and classified on the basis of loess.

Liquefaction susceptibility (low, moderate, high) is based upon deposit age, grain size, and environment of deposition. Numerical weightings for each factor are used so that susceptibility ratings can be changed easily in the GIS environment. Since liquefaction susceptibility is critically sensitive to saturation, two independent methods are used to outline areas with shallow water tables. County soil maps identify aquic soils with redoximorphic features characteristic of transient saturation within 1.5 m of the surface. Contours showing average depth-to-water were created using hundreds of water well logs and kriging techniques. The two methods agree well where contours indicate <6 m to water. Water tables have large seasonal fluctuations with highest levels in April-June. Multi-year fluctuations are influenced by droughts and wet cycles, as well as local pumping and irrigation practices. Liquefaction hazards are highest in Holocene alluvium near the junction of the Henrys Fork and South Fork of the Snake River between Rigby and Rexburg.

Funding provided by the Idaho Bureau of Homeland Security under TO-001-FY2009 and TO-003-FY2009.