EVALUATING LIQUEFACTION SUSCEPTIBILITY OF QUATERNARY DEPOSITS IN SEATTLE, WASHINGTON

Recent advances in geologic mapping in Seattle, Washington have significantly increased the total area of loose to normally consolidated deposits that may be susceptible to liquefaction during an earthquake event (Troost and others, 2005). Using current subsurface data, seismic information, and modeling approaches, this study presents a modern liquefaction hazard assessment for the City of Seattle. The empirically-based assessment results in a 1:12,000-scale map and digital geodatabase delineating relative liquefaction potential on a scale of very low to very high.

Modeling methods for liquefaction assessment include the most current approaches described by Kramer (2008) and the National Academies of Sciences, Engineering, and Medicine (2016). The evaluation considers several tens of thousands of points of subsurface data obtained from GeoMapNW’s digital database. Input parameters modelled include groundwater levels, geologic unit types, unit composition, SPT blow counts, and earthquake ground motions.

The highest liquefaction potential in Seattle is in loose to normally consolidated material where ground water is shallow, such as Holocene-age alluvium and nonengineered fill. Increased spatial extent of potentially liquefiable deposits are found in upland valleys, such as Rainier Valley, and shorelines, such as those of Green Lake and Lake Union. Material that has been glacially overridden is generally resistant to liquefaction due to soil density.

Seattle is subject to high magnitude earthquakes from several sources such as the Cascadia subduction zone and the Seattle fault zone . Earthquake ground motion records and surficial evidence of liquefaction from the recent 2001 Nisqually Earthquake allow for sensitivity analyses and initial calibration of the model. The final liquefaction susceptibility map is most accurate using the performance-based approach described by Kramer (2008), which considers all earthquake sources weighted by relative probability, resulting in a complete picture of liquefaction hazard for the City of Seattle.

The improved liquefaction susceptibility map benefits City agencies, engineers, and residents. As Seattle continues to rapidly urbanize, it is imperative to use the best available science for the preparation and mitigation of geologic hazards.