GROUND-SURFACE VIBRATIONS FROM ENSEMBLE SEISMIC ANALYSES

Abstract

We investigate ground-surface vibrations within tens of meters from surface or near-surface excitations. Our interest is not for a specific site, but generically by subdivisions of National Earthquake Hazards Reduction Program site categories associated with stiff and dense soil. To this end we consider four soil categories specified by VS 30 values 210, 270, 330, and 425 m/s. For each category we use an ensemble of propagation analyses to examine energy level and distribution of vibration responses over a surface wave field. The ensembles have as a basis a statistical layering model with correlation properties dependent on depth and layer thickness. Randomly assigned properties follow a Latin hypercube sampling. These include VS 30 , shear wave speed as a function of depth, shear-wave quality factor, water table, and the compression-to-shear wave speed ratio above the water table. The dependent variables density, compression wave speed below the water table, and compression-to-shear quality factor ratio, are based on relationships from the seismic literature. We present an example result to show effects of varying ensemble group over a 1-222 Hz band. The effects are observed by the energy in the frequency response of an axial strain quantity at 1-m depth out to 50 m range from a horizontal point force. Differences and standard deviations in energy level field reveal the relative responses of the ensembles.