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An H/V Geostatistical Approach for Developing Pseudo-3D Vs Models to Account for Spatial Variability in Site Response

Many recent studies have shown that we are generally unable to accurately replicate recorded ground motions at most borehole array sites using a single shear wave velocity (Vs) profile and one-dimensional (1D) ground response analyses (GRAs), presumably due to the presence of spatially variable subsurface conditions. While 3D GRAs are possible and believed to be more accurate, there is rarely a 3D subsurface model available for these analyses. The lack of affordable and reliable 3D site characterization methods, particularly regarding Vs measurements needed for GRAs, has pushed researchers to adopt stochastic methods, such as Vs randomization and spatially correlated random fields. However, these stochastically generated models require the assumption of generic, or guessed, input parameters, introducing significant uncertainties into the site response predictions.

To address these challenges, this study proposes a new approach that can be used to build pseudo-3D Vs models by integrating simple, cost- and time-effective horizontal-to-vertical spectral ratio measurements (H/V) with geostatistical modeling tools. Importantly, it requires only a single measured Vs profile and a number of H/V noise measurements. Using Gaussian geostatistical regression, irregularly sampled estimates of fundamental site frequency from H/V measurements (f_0,H/V) are used to generate a uniform grid of f_0,H/V across the site with accompanying Vs profiles that have been scaled to match each f_0,H/V value, thereby producing a pseudo-3D Vs model. While the pseudo-3D Vs models can be used to incorporate spatial variability into 1D, 2D, or 3D GRAs, their implementation in 1D GRAs at the Treasure Island and Delaney Park Downhole Array sites is assessed, where the proposed approach is shown to produce superior fits to the small-strain recorded site response relative to other approaches used to account for spatial variability in 1D GRAs. We also investigate the lateral area likely influencing site response and show that it could extend to significant distances (as much as 1 km) from the borehole arrays.

Presenting Author: Mohamad M. Hallal

Student Presenter: Yes

Day: 4/19/2021

Time: 5:00 PM - 6:15 PM Pacific

Additional Authors