One-dimensional site response analyses (1D SRAs) with shear-wave velocity ( VS) randomization are commonly performed to estimate median site-specific amplification factors (AFs) under the implicit assumption that this approach yields a realistic response. In this work, an investigation is conducted to determine the appropriate amount of VS randomization (σlnVs) needed to capture a median response that accounts for 2D VS spatial variability effects. Results from 2D SRAs and 1D SRAs with VS randomization show that the median 2D seismic responses are generally higher than 1D responses at the site’s fundamental frequency, and that higher VS variability has a mild impact on the median 2D seismic response amplitude at the fundamental frequency, whereas it significantly reduces the median 1D response. Findings indicate that the 84th percentile AFs based on 1D SRAs conducted with VS randomization using σlnVs = 0.25, approximate well with the more realistic median 2D SRA-based AFs around the fundamental frequency, while the 70th to 60th percentiles might be more appropriate at higher frequencies. The benefit of using percentiles of the 1D SRA-based AFs higher than the median is shown for different site conditions and supported by comparisons against empirical data from four downhole sites.

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One-dimensional site response analysis (1D SRA) remains the state of practice to estimate site-specific seismic response, despite the ample evidence of discrepancies between observations and 1D SRA-based predictions. These discrepancies are due to errors in the input parameters, intrinsic limitations in the predicting capabilities of 1D SRAs even for sites relatively compliant with the 1D SRA assumptions, and the inability of 1D SRAs to model three-dimensional (3D) wave propagation phenomena. This article aims at reducing 1D SRA mispredictions using small-strain damping profiles factored by a damping multiplier ( Dmul) and randomized shear-wave velocity ( VS) profiles. An approach for conducting 1D SRAs for site-specific site response assessment is developed to reduce the 1D SRA errors in magnitude and variability. First, sites from a database of 534 downhole sites are classified as 1D- or 3D-like, depending on the substructure conditions inferred from observed transfer functions. Second, data from the 1D-like sites are compared against predictions from 1D SRAs conducted using various trials of Dmul and VS standard deviations [Formula: see text] for VS randomization. Third, Dmul and [Formula: see text] are selected based on their combined ability to reduce the root mean square error (RMSE) in SRA predictions. Results indicate that 1D SRAs conducted with Dmul = 3 and [Formula: see text] lead to an overall minimum RMSE and thus provide more accurate site response estimates. The use of these parameters in forward SRA predictions is discussed in a companion paper.

The seismic performance of Balboa Boulevard during the 1994 M W 6.7 Northridge earthquake was examined through nonlinear deformation analyses (NDAs) using advanced tools to (1) investigate the failure mechanism leading to ground deformations at this site; (2) evaluate the accuracy of the adopted analysis methods, engineering procedures, and state-of-the-art tools to reasonably estimate horizontal ground displacements; and (3) identify key factors and parameters contributing to earthquake-induced ground deformations at this site. One-dimensional (1D) liquefaction vulnerability indexes (LVIs) and permanent displacements using Newmark sliding block analyses were also estimated and compared against ground deformations observed after the earthquake. The geotechnical characterization of Balboa Boulevard was assessed based on field and laboratory data obtained from two investigation campaigns. Transitional probability geostatistics were used to develop stratigraphic models that capture the heterogeneity and the spatial variability patterns of sand-like and clay-like soils present at this site. The stratigraphic models were implemented in the finite difference software FLAC and the behavior of sand-like and clay-like soils simulated using the PM4Sand and PM4Silt constitutive models, respectively. Sensitivity analyses were performed to address uncertainties associated with the spatial variability of soils, input ground motions, the proportion of sand-like and clay-like soils within the soil deposit, and the strength properties of these materials. Results from NDAs suggest that a compounded effect of both liquefaction of sand-like soils and cyclic softening of clay-like soils led to the excessive ground deformations at Balboa Boulevard. This study sheds light on the importance of using appropriate engineering procedures and numerical modeling protocols in the prediction of deformation patterns, the selection of key input parameters, as well as the applicability of LVIs in complex sites.