A borehole array data–based approach for conducting 1D site response analyses I: Damping and V_S randomization

Abstract

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.

Applicability of 1D site response analysis to shallow sedimentary basins: A critical evaluation through physics‐based 3D ground motion simulations

“…However, it is unlikely that wave scattering and soil nonlinearity alone would explain the large amplification near the site periods, as this is also observed in relatively well-characterized soil profiles with small lateral velocity changes and under the excitation of weak motions (i.e., essentially linear behaviour). [45][46][47] We speculate that this phenomenon may have strong correlation with the limitation in the simplified 1D modelling method for inclined seismic waves that have led to complex wave interferences of trapped waves in the 1D soil column vibrating near the site periods for an excessively long time. 38 For demonstration purposes, we show in Figure 3 the 1D approximation results of two different 2D plane wavefield cases in a homogeneous half-space with wave velocity v, and a reference point A that records the 1D in-column input motions at depth H. In both Figure 3A and B, the actual 2D wavefield is shown on the left-hand side, with the upgoing incident and downgoing reflected waves denoted by the red and green pulses, respectively; the 1D soil column employed in 1D SRA with the in-column input motion is shown on the right-hand side, with the colour coding of the wave pulses corresponding to the resultant waves incurred by the respective red and green pulses from the actual 2D wavefield on the left.…”

“…In view of the large amplification near the site periods, in existing site response studies, researchers have often tried to optimize the soil damping profile to achieve a best match scenario with the recorded data, 9,13,19,26,46,47 typically resulting in increased damping levels compared to the small-strain damping profile determined based on laboratory measurements, even for small tremors. It should be noted that although this optimization process may be useful in improving the 1D predictions empirically on a site-to-site basis, it does not necessarily provide persuasive physical basis on the soil profile change and may have undesirably extended the range where the 1D approach is considered applicable.…”

“…Similar observations have also been made in past site response studies based on instrumented downhole arrays. 47,65 It is well recognized that, for vertically propagating waves passing through horizontally layered soil profiles, destructive wave interferences between the upgoing incident wave and the downgoing reflected wave from the surface can occur at certain frequencies at a given depth, known as the downgoing wave effect, 69 leading to local minimums in the spectral acceleration at these frequencies. For site amplification more prone to the 1D wave propagation behaviour, this downgoing wave effect is expected to be more pronounced.…”

A borehole array data–based approach for conducting 1D site response analyses II: Accounting for modeling errors

“…Such errors are referred to as “intrinsic errors.” While parametric errors are commonly addressed by using multiple alternative input parameters, for example, baseline, upper, and lower V S profiles (Electric Power Research Institute (EPRI), 2013), intrinsic and modeling errors are generally overlooked. This and the companion paper (Pretell et al, 2023) develop and propose an approach for conducting 1D SRAs that removes the intrinsic errors and reduce the discrepancies between observations and 1D SRA-based predictions given by the modeling errors.…”