Effect of Sediment Column on Weak-Motion Site Response for a Deep Basin Fill

Unique challenges arise when projecting dynamic site response in a deep, steep sided, irregularly shaped, sediment-filled basin. The influence of shallow sediments on site response was investigated for a 1-km-deep alluvial column in the Las Vegas Basin, subjected to weak ground motions. A one-dimensional equivalent-linear model was applied. Response analyses for deep deposits are complicated because dynamic material properties at depth are uncertain. To compensate, the model half space was placed well above the physical bottom of the sediment column. The depth to half space was selected by matching characteristics of projected surface response to measured data or expectations. Weak ground-motion datasets resulting from underground nuclear tests were considered. The bestmatch half space depth, 375 m, greatly exceeded the depth of the threshold shear wave velocity for engineering bedrock. The analysis captured site response over the period range 0.3-1.3 s. When the parameterization was tested for a weak-ground-motion earthquake dataset, projections were poorer but still instructive.