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PREdiction of NOn-LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strongmotion recordings obtained at well-known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log10 scale) for a peak ground acceleration of 5 m=s2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.

Soil nonlinear behavior is often triggered in soft sedimentary deposits subjected to strong ground shaking and has led to catastrophic damage to civil infrastructure in many past earthquakes. Nonlinear behavior in soils is associated with large shear strains, increased material damping ratio, and reduced stiffness. However, most investigations of the high-frequency spectral decay parameter κ⁠, which captures attenuation, have focused on low-intensity ground motions inducing only small shear strains. Because studies of the applicability of the κ model when larger deformations are induced are limited, this article investigates the behavior of κ (both κr per record and site-specific κ0 estimates) beyond the linear-elastic regime. About 20 stations from the Kiban–Kyoshin network database, with time-average shear-wave velocities in the upper 30 m between 213 and 626  m/s⁠, are used in this study. We find that the classification scheme used to identify ground motions that trigger soil nonlinear behavior biases estimates of κ0 in the linear and nonlinear regimes. A hybrid method to overcome such bias is proposed considering proxies for in situ deformation (via the shear-strain index) and ground shaking intensity (via peak ground acceleration). Our findings show that soil nonlinearity affects κr and κ0 estimates, but this influence is station dependent. Most κ0 at our sites had a 5%–20% increase at the onset of soil nonlinear behavior. Velocity gradients and impedance contrasts influence the degree of soil nonlinearity and its effects on κr and κ0⁠. Moreover, we observe that other complexities in the wave propagation phenomenon (e.g., scattering and amplifications in the high-frequency range) impose challenges to the application of the κ0 model, including the estimation of negative values of κr⁠. [ABSTRACT FROM AUTHOR]

Earthquake ground motions are strongly affected by the upper tens of meters of the Earth's crust and consequently local site effects need to be included in any ground-motion prediction. In ground-motion prediction equations (GMPEs), it is increasingly common to account for possible nonlinear behavior of near-surface materials (soil). These nonlinear site terms adjust observations made on soft soil sites to the ground motion expected on bedrock and hence allow these abundant soil records to be used within the regression analysis for the derivation of empirical GMPEs. These nonlinear site terms also allow rapid predictions of the expected ground motions on soil rather than requiring a site-response analysis to be conducted. In this study, we compare the signature on observed peak ground acceleration as a function of a strain proxy of nonlinear soil behavior within four large strong-motion databases to the predicted signature from four recent GMPEs, three of which explicitly include nonlinear site terms. We find that observed nonlinearity in the databases, interpreted in terms of strain-stress relationships and reduction of shear modulus, is limited, but even this limited effect is underestimated by the nonlinear site terms of the considered GMPEs, which suggests that predictions from these GMPEs could be biased for soft soil sites but also on bedrock. Some of this mismatch could be explained by the use of the average shear-wave velocity in the top 30 m (VS30) to characterize sites as well as errors in these values. [ABSTRACT FROM AUTHOR]

We analyze the nonlinear and near-surface geological effects of two Network for Earthquake Engineering Simulation at University of California, Santa Barbara (NEES@UCSB) instrumented sites: the Garner Valley Downhole Array (GVDA) and the Wildlife Liquefaction Array (WLA). The seismic interferometry by deconvolution method is applied to earthquake data recorded by the multisensor vertical array between January 2005 and September 2013. Along the cross section, local shear-wave velocity is extracted by estimating travel time between sensors. The S-wave velocity profiles are constructed and compared with classical in situ geophysical surveys. We show that velocity values change according to the amplitude of the ground motion, and we find anisotropy between east-west and north-south directions at the GVDA site. The ratio between average peak particle velocity v* and local S-wave velocity Vs* between two boreholes is tested as a deformation proxy. Using average peak particle acceleration a*, the a* versus v*/Vs* curve is used to represent the stress-strain curve for observing the site's nonlinear responses under different levels of excitation. Nonlinearity is observed from quite low shear-strain levels (~1×10-5) and a classic hyperbolic model is derived. proves to be a good deformation proxy. Finally, the shear modulus degradation curves are constructed for each depth and test site, and they are similar to previous laboratory measurements or in situ geophysical surveys. A simple comparison regarding nonlinear behavior between GVDA and WLA is performed. [ABSTRACT FROM AUTHOR]

Site effects may be assessed using a standard soil classification parameter, Vs30 (the harmonic average shear-wave velocity in the first 30 m); however, this index does not account for the complexity of the velocity profile, especially its variability at depth. In the present study, in addition to VS30, we propose consideration of the gradient of the profile from 0 to 30 m depth, denoted S30. A lower gradient value means low velocity increases with depth; a higher gradient indicates a rapid velocity increase with depth in the shallow layers. In addition, we consider the fundamental resonance frequency of the soil (ƒ0), which has been shown to be a relevant parameter for site-effect assessment and which is obtained from the empirical site response. Using the Japanese KiK-net database, we analyze the variability of the Vs profiles and the empirical borehole site responses of selected sites through the US30, the velocity gradient fi30, and ƒ 0. We select 289 sites for which the ID linear numerical modeling is close to the empirical site response and a Vs at the downhole station is greater than 1000 m/s. For a given VS30 class, S30, and ƒ 0 can be used to distinguish between two types of sites: deep sedimentary sites and sites with high velocity contrast at shallow depths. We find that, even if the gradient is calculated using shallow information, its use improves the site amplification characterization, compared to using only VS30, by reducing the intersite site-response variability. As expected, however, this improvement is limited for deep sedimentary sites. On the other hand, / 0 is able to reduce the intersite response variability for deep sedimentary sites even though it is limited to specific Vs30 classes. Thus, the combined use of Vs30, S30, and /o improves the assessment of linear site amplification. [ABSTRACT FROM AUTHOR]

This article investigates the effects of the nonlinear behavior of soils on site response, through various earthquake recordings from the KiK-net database in Japan. This network is composed of more than 688 surface-borehole instruments, from which a characterization of the shear- and compressive-wave velocity profiles down to the borehole depth is available. We selected events with a peak ground acceleration (PGA) at the downhole station of < 10 cm/s² in order to characterize the linear soil behavior by computing the surface to downhole spectral ratios at each site. Modifications of site-response curves computed with strong events (PGA > 50 cm/s²) compared to the linear characterization are supposed to be caused by nonlinear soil behavior. To describe the effects of soil nonlinear behavior on site response per event, we propose the percentage of modification (either amplification or deamplification) of the site-response curve compared to the linear evaluation (PNLev, percentage of nonlinearity) and the associated shift frequency (Shev)). These parameters are used to estimate the probability that nonlinear site response is significantly different that the linear counterpart. We find that, regardless the site, this probability is important even for low input-motion PGA (values equal to or larger than 30 cm/s² at downhole sensor). This indicates that nonlinear soil behavior must be taken into account in site-response evaluation for moderate to strong motion. In addition, for 54 sites of the KiK-net database that have recorded at least two strong events (PGA at the downhole station > 5 0 cm/s²), we define four additional parameters that characterize the effects of soil nonlinear behavior on site responses for each site: (1) a PGA threshold (PGAth), defined as the PGA value for which PNLev, is higher than 10%; (2) a site-specific PNL for a PGA of 50 cm/s² (PNLs jte); (3) a site-specific shift of the predominant frequency for a PGA of 50 cm/s² (Shsite); and (4) a frequency from which we observe deamplification between nonlinear and linear site responses (fNL). We observe that nonlinear soil behavior can increase the amplification at frequencies below fNL. We find that fNL lies in between the fundamental and the predominant resonance frequencies of the site response and that sites having Vs contrast close to the surface trigger nonlinear behavior at a lower input-motion PGA threshold. These results suggest that nonlinear behavior occurs mostly in the superficial soil layers. Furthermore, by investigating the nonlinear behavior of soils on earthquake horizontal-to-vertical spectral ratios at the surface, we find that they can give satisfactory results (equivalent to the analysis of borehole site responses) for the evaluation of the fNL frequency and shift frequency (Shsite), which indicates that part of the results obtained in this study can be extended to other databases without downhole sensors. [ABSTRACT FROM AUTHOR]

We propose a site-classification scheme based on the predominant period of the site, as determined from the average horizontal-to-vertical (H/V) spectral ratios of ground motion. Our scheme extends Zhao et al. (2006) classifications by adding two classes, the most important of which is defined by flat H/V ratios with amplitudes less than 2. The proposed classification is investigated by using 5%-damped response spectra from Italian earthquake records. We select a dataset of 602 three-component analog and digital recordings from 120 earthquakes recorded at 214 seismic stations within a hypocentral distance of 200 km. Selected events are in the momentmagnitude range 4.0 ⩽ Mw⩽ 6.8 and focal depths from a few kilometers to 46 km. We computed HIV ratios for these data and used them to classify each site into one of six classes. We then investigate the impact of this classification scheme on empirical ground-motion prediction equations (GMPE5) by comparing its performance with that of the conventional rock/soil classification. Although the adopted approach results in only a small reduction of the overall standard deviation, the use of H/V spectral ratios in site classification does capture the signature of sites with flat frequency-response, as well as deep and shallow-soil profiles, characterized by longand short-period resonance, respectively; in addition, the classification scheme is relatively quick and inexpensive, which is an advantage over schemes based on measurements of shearwave velocity. [ABSTRACT FROM AUTHOR]

Current ground-motion prediction equations invariably assume that site conditions at strong-motion stations, often characterized by the average shear-wave velocity to a depth of 30 m (VS30), are known to a uniform accuracy. This is, however, rarely the case. In this article, we present a regression procedure based on generalized least-squares and maximum-likelihood approaches that take into account the varying uncertainties on VS30. Assuming that VS30S for various groups of stations are known to different accuracies, application of this procedure to a large set of records from the Japanese KiK-net shows that the regression coefficients are largely insensitive to the assumption of nonuniform uncertainties. However, this procedure allows the computation of a site-specific standard deviation (σ) that should be used for sites where VS30 is known to different accuracies (e.g., a site only specified by class or a site with a measured VS profile). For sites with a measured VS profile, this leads to lower site-specific σ than for a site that is poorly characterized because this technique explicitly models the separation between the epistemic uncertainty in VS30 and the aleatory variability in predicted ground motion. [ABSTRACT FROM AUTHOR]

This article proposes a new framework for the inclusion of site effects in empirical ground-motion prediction equations (GMPEs) by characterizing stations through their one-quarter wavelength velocities and assessed confidence limits. The approach is demonstrated for 14 stations of the French accelerometric network (Réseau Accélérométrique Permanent). This method can make use of all the available information about a given site, for example, the surface geology, the soil profile, standard penetration test measurements, near-surface velocity estimated from the topographic slope, depth to bedrock, and crustal structure. These data help to constrain the velocity profile down to a few kilometers. Based on a statistical study of 858 real profiles from three different regions (Japan, western North America, and France) physically realistic profiles are generated that comply with the information available for each site. In order to evaluate the confidence limits for the shear-wave velocity profiles and derived site amplifications for each station, a stochastic method is adopted: several thousand profiles are randomly generated based on parameters derived in the statistical study and the constraints available for each station. Then, the one-quarter wavelength assumption is used to estimate the amplification for each station. It is found that a good knowledge of near-surface attenuation (i.e., κ or Q) is mandatory for obtaining precise amplification estimates at high frequencies. Nevertheless, the proposed scheme highlights the important differences in the uncertainties of the site amplifications, depending on the information available for a given station. We suggest that these results could, therefore, be used when developing GMPEs by weighting records from each station depending on the variability in the computed one-quarter wavelength velocities. This approach relies on the assumption that local site effects are only onedimensional, which is far from true, especially in sedimentary basins. However, most GMPEs only model one-dimensional site effects, so this is not an issue specific to this study. Finally, a way to improve this technique is to use earthquakes or noise recorded at the stations to further constrain the shear-wave velocity profiles and to consequently derive more accurate one-quarter wavelength velocities. [ABSTRACT FROM AUTHOR]