Improved Implementation of Travel Time Randomization for Modeling Vs Uncertainty

Accounting for the inevitable uncertainty in shear wave velocity (Vs) profiles has been recognized to be critical to seismic ground response analysis (GRA) and probabilistic seismic hazard analysis (PSHA). Vs stochastic randomization models are among the most widely used approaches to account for this uncertainty. However, the suitability of these models, as currently implemented, has been undermined by their inability to match observations at vertical seismometer arrays.

This motivated modifications, such as randomizing the cumulative shear wave travel time (tts) instead of directly Vs. While some preliminary assessments suggest that tts randomization allows for better management of Vs uncertainty, the framework has only been implemented in a few case studies. The aim of this study is to fill this gap by scrutinizing the different steps of tts randomization.

By using real experimental data and Monte Carlo simulations, we argue that some of the steps of this framework pose some challenges. For each challenge, we devise recommendations and demonstrate that they provide a clear improvement on the current framework. The findings of this study will enhance our understanding of the tts randomization framework and will enable researchers and engineers to adopt it robustly and confidently at engineering scales.

Collaborators: Dr. Brady Cox (USU), Dr. Adrian Rodriguez-Marek (VA Tech), Dr. Sebastiano Foti (PoliTO), Dr. Ellen Rathje (UT Austin)

In-Situ Vs and Damping Inversions at the Treasure Island and Delaney Park Downhole Arrays

This study extends a recently proposed heterogeneous data assimilation technique to estimate compression and shear wave velocity (Vp and Vs, respectively) and damping at Treasure Island and Delaney Park Downhole Arrays. The adopted method is based on the joint inversion of earthquake acceleration time series and experimental surface wave dispersion data.

We first use synthetic data at these two sites to refine the proposed approach and then apply the refined algorithm to real data sets available at the Treasure Island and Delaney Park Downhole Arrays. The joint inversion results show that the estimated Vs and Vp profiles are in very good agreement with measured profiles at these two sites.

We also compare the results from this study with other methods that are used to incorporate spatial variability and wave scattering effects in 1D ground response analysis (GRA). The comparisons show that the joint inversion-based Vs and damping profiles estimated in this study can effectively integrate the effects of spatial variability and wave scattering into 1D GRAs, especially at the Delaney Park Downhole Array, which is poorly modeled using traditional 1D GRA.

Collaborators: Dr. Elnaz Seylabi (UNR), Dr. Brady Cox (USU)

Estimating Unbiased Statistics for f0 Using Spatially Distributed HVSR Measurements

A site’s fundamental resonance frequency (f0), or its reciprocal, the fundamental period (T0), is a critical parameter in seismic studies due to its proven ability to aid in predicting local site effects. The horizontal-to-vertical spectral ratio (HVSR) method is a popular nonintrusive technique that can be used to estimate f0 in a time-efficient and cost-effective manner.

Although it is becoming more common to perform several HVSR measurements, the measurements often are irregularly spaced due to access and/or budget restrictions. This has the potential of introducing significant bias when attempting to estimate a single, representative f0 across an area of interest. To address this problem, we propose the use of Voronoi tessellations to obtain an unbiased, statistical representation of f0 or T0 from spatially distributed HVSR measurements.

Three example applications are presented to illustrate potential uses. The first application demonstrates the effectiveness of the adopted approach in correcting for bias introduced by irregular spatial sampling. The second application illustrates how better-informed seismic site classifications can be made using a statistical representation of T0. The third application compares the relative degree of spatial variability in f0 at two downhole array sites to assess the applicability of performing one-dimensional ground response analyses.

Collaborators: Tianjian Cheng, Dr. Brady Cox (USU), Joseph Vantassel (UT Austin)