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Distributed acoustic sensing (DAS) is a relatively new technology for recording the propagation of seismic waves, with promising applications in both engineering and geophysics. DAS's ability to simultaneously collect high spatial resolution waveforms over long arrays suggests that it is well-suited for near-surface imaging applications such as 2-D multichannel analysis of surface waves (MASWs), which require, at a minimum, long, linear arrays of single-component receivers. The 2-D MASW method uses a large number of sensor subarrays deployed along a linear alignment to produce 1-D shear-wave velocity (VS) profiles beneath each subarray. The 1-D VS profiles are then combined to form a pseudo-2-D VS image beneath the entire linear alignment that can be used for the purpose of identifying and characterizing lateral variations in subsurface layering. Traditionally, 2-D MASW is conducted using arrays consisting of either 24 or 48 geophones. While additional receivers could easily be incorporated into the testing configuration, it is rare for researchers and practitioners to have access to greater numbers of seismographs and geophones. When a limited number of geophones are available for deployment, there is a need to pre-determine the geophone spacing and subarray length prior to field data acquisition. Studies examining how the choice of subarray geometry impacts the resulting pseudo-2-D VS cross-sections have been largely limited to synthetic data. In response, this study utilizes DAS data to examine the effects of using various subarray lengths by comparing pseudo-2-D VS cross-sections derived from active-source waveforms collected at a well-characterized field site. DAS is particularly useful for 2-D MASW applications because the subarray geometry does not need to be determined prior to field data acquisition. We organize the DAS waveforms into multiple sets of overlapping MASW subarrays of differing lengths, ranging from 11 to 47 m, along the same alignment, allowing for direct comparison of the derived pseudo-2-D VS results at the site. We show that the length of the individual MASW subarrays has a significant effect on the resulting VS cross-sections, including the resolved location of a strong impedance contrasts at our study site, and evaluate the results relative to ground truth from invasive testing. Our results suggest that the choice of subarray length is important and should be carefully chosen to meet project-specific goals. Furthermore, analysts may consider using multiple subarray geometries during the data processing stage, as is made possible by DAS, to properly evaluate the uncertainty of 2-D MASW results. This study demonstrates the potential of using DAS to collect data for 2-D MASW in a manner that is efficient and flexible, and can be easily scaled up for use with very long arrays. [ABSTRACT FROM AUTHOR]

A robust, in situ estimate of shear-wave velocity VS and the small-strain damping ratio DS (or equivalently, the quality factor QS) is crucial for the design of buildings and geotechnical systems subjected to vibrations or earthquake ground shaking. A promising technique for simultaneously obtaining both VS and DS relies on the Multichannel Analysis of Surface Waves (MASW) method. MASW can be used to extract the Rayleigh wave phase velocity and phase attenuation data from active-source seismic traces recorded along linear arrays. Then, these data can be inverted to obtain VS and DS profiles. This paper introduces two novel methodologies for extracting the phase velocity and attenuation data. These new approaches are based on an extension of the beamforming technique which can be combined with a modal filter to isolate different Rayleigh propagation modes. Thus, the techniques return reliable phase velocity and attenuation estimates even in the presence of a multimode wavefield, which is typical of complex stratigraphic conditions. The reliability and effectiveness of the proposed approaches are assessed on a suite of synthetic wavefields and on experimental data collected at the Garner Valley Downhole Array and Mirandola sites. The results reveal that, under proper modelling of wavefield conditions, accurate estimates of Rayleigh wave phase velocity and attenuation can be extracted from active-source MASW wavefields over a broad frequency range. Eventually, the estimation of soil mechanical parameters also requires a robust inversion procedure to map the experimental Rayleigh wave parameters into soil models describing VS and DS with depth. The simultaneous inversion of phase velocity and attenuation data is discussed in detail in the companion paper. [ABSTRACT FROM AUTHOR]

This paper deals with in situ characterization of the small-strain shear-wave velocity VS and damping ratio DS from an advanced interpretation of Multi-channel Analysis of Surface Waves (MASW) surveys. A new approach based on extracting Rayleigh wave data using the CFDBFa method has been discussed in the companion paper. This paper focuses on mapping the experimental Rayleigh wave phase velocity and attenuation into profiles of VS and DS versus depth, which is achieved through a joint inversion procedure. The joint inversion of phase velocity and attenuation data utilizes a newly developed Monte Carlo global search algorithm, which implements a smart sampling procedure. This scheme exploits the scaling properties of the solution of the Rayleigh eigenvalue problem to modify the trial earth models and improve the matching with the experimental data. Thus, a reliable result can be achieved with a limited number of trial ground models. The proposed algorithm is applied to the inversion of synthetic data and of experimental data collected at the Garner Valley Downhole Array site, as described in the companion paper. In general, inverted soil models exhibit well-defined VS profiles, whereas DS profiles are affected by larger uncertainties. Greater uncertainty in the inverted DS profiles is a direct result of higher variability in the experimental attenuation data, the limited wavelength range at which reliable values of attenuation parameters can be retrieved, and the sensitivity of attenuation data to both DS and VS. Nonetheless, the resulting inverted earth models agree well with alternative in situ estimates and geological data. The results stress the feasibility of retrieving both stiffness and attenuation parameters from active-source MASW testing and the effectiveness of extracting in situ damping ratio estimates from surface wave data. [ABSTRACT FROM AUTHOR]

There is a growing need to characterize the engineering material properties of the shallow subsurface in three dimensions for advanced engineering analyses. However, imaging the near-surface in three dimensions at spatial resolutions required for such purposes remains in its infancy and requires further study before it can be adopted into practice. To enable and accelerate research in this area, we present a large subsurface imaging data set acquired using a dense network of three-component (3C) nodal stations acquired in 2019 at the Garner Valley Downhole Array (GVDA) site. Acquisition of this data set involved the deployment of 196 stations positioned on a 14 × 14 grid with a 5 m spacing. The array was used to acquire active-source data generated by a vibroseis truck and an instrumented sledgehammer, and passive-wavefield data containing ambient noise. The active-source acquisition included 66 vibroseis and 209 instrumented sledgehammer source locations. Multiple source impacts were recorded at each source location to enable stacking of the recorded signals. The active-source recordings are provided in terms of both raw, uncorrected units of counts and corrected engineering units of meters per second. For each source impact, the force output from the vibroseis or instrumented sledgehammer was recorded and is provided in both raw counts and engineering units of kilonewtons. The passive-wavefield data include 28 h of ambient noise recorded over two nighttime deployments. The data set is shown to be useful for active-source and passive-wavefield three-dimensional imaging and other subsurface characterization techniques, which include horizontal-to-vertical spectral ratios (HVSRs), multichannel analysis of surface waves (MASW), and microtremor array measurements (MAM). [ABSTRACT FROM AUTHOR]

Seismic cone penetration testing (SCPT) is a powerful geotechnical site characterization tool, allowing for simultaneous collection of routine cone penetration testing data and rapid downhole-type measurements of shear-wave velocity ([V.sub.S]). However, the uncertainties associated with developing [V.sub.S] profiles from SCPT measurements are rarely considered or communicated to the end-user. One important source of [V.sub.S] uncertainty is related to how the shear wave travel times are interpreted from the recorded waveforms, while another critical source of uncertainty is related to the analysis method used to transform the travel times to velocities. In this study, four common ways of obtaining travel times were considered: (i) first arrival picks, (ii) peaks and troughs picks, (iii) crossover picks, and (iv) the peak response of the cross-correlation function. Using these different travel times, a number of [V.sub.S] profiles were developed using four different velocity analysis methods: (i) pseudo-interval, (ii) true-interval, (iii) corrected vertical travel time slope-based, and (iv) raytracing. Through consideration of multiple wave arrival time and velocity analysis methods, a robust and meaningful quantification of the intramethod, depth-dependent epistemic uncertainty in [V.sub.S] obtained from several example SCPT datasets has been developed. [V.sub.S] uncertainty is further examined through consideration of the intermethod variability and bias between SCPT and direct-push crosshole testing. Key words: seismic cone penetration testing, downhole seismic testing, shear-wave velocity, epistemic uncertainty, invasive seismic testing, true-interval method, pseudo-interval method. L'essai de penetration au cone sismique (SCPT) est un puissant outil de caracterisation geotechnique du site, permettant la collecte simultanee de donnees d'essai de routine de penetration au cone et de mesures rapides de la vitesse des ondes de cisaillement ([V.sub.S]) en fond de trou. Cependant, les incertitudes associees a l'elaboration des profils [V.sub.S] a partir des mesures SCPT sont rarement prises en compte ou communiquees a l'utilisateur final. Une source importante d'incertitude [V.sub.S] est liee a la facon dont les temps de propagation des ondes de cisaillement sont interpretes a partir des formes d'onde enregistrees, tandis qu'une autre source critique d'incertitude est liee a la methode d'analyse utilisee pour transformer les temps de propagation en vitesses. Dans cette etude, nous avons examine quatre facons courantes d'obtenir les temps de deplacement : (i) les premiers choix d'arrivee, (ii) les choix de cretes et tremies, (iii) les choix de croisement, et (iv) la reponse de crete de la fonction de correlation croisee. En utilisant ces differents temps de parcours, un certain nombre de profils [V.sub.S] ont ete developpes en utilisant quatre methodes d'analyse de vitesse differentes: (i) pseudo-intervalle, (ii) intervalle vrai, (iii) temps de parcours vertical corrige-base sur la pente, et (iv) raytracing. Grace a la prise en compte de multiples methodes d'analyse du temps d'arrivee des vagues et de la vitesse, une quantification robuste et significative de l'incertitude epistemique intra-methode, dependante de la profondeur, dans [V.sub.S] a ete developpee a partir de plusieurs ensembles de donnees SCPT d'exemple. L'incertitude [V.sub.S] est examinee plus en detail par l'examen de la variabilite et biais intermethodes entre le SCPT et les essais de trous transversaux a poussee directe. [Traduit par la Redaction] Mots-cles : essais de penetration des cones sismiques, essais sismiques en fond de puits, vitesse des ondes de cisaillement, incertitude epistemique, essais sismiques invasifs, methode de l'intervalle vrai, methode du pseudo-intervalle., Introduction Measuring the small-strain constrained compression modulus ([M.sub.o]) and shear modulus ([G.sub.o]) of near-surface soil and rock is an important part of many civil engineering projects. Small-strain moduli values anchor [...]

Full waveform inversion (FWI) and distributed acoustic sensing (DAS) are powerful tools with potential to improve how seismic site characterization is performed. FWI is able to provide true 2D or 3D images of the subsurface by inverting stress wave recordings collected over a wide variety of scales. DAS can be used to efficiently collect high-resolution stress wave recordings from long and complex fiber optic arrays and is well-suited for large-scale site characterization projects. Due to the relative novelty of combining FWI and DAS, there is presently little published literature regarding the application of FWI to DAS data for near-surface (depths <30 m) site characterization. We perform 2D FWI on DAS data collected at a well-characterized site using four different, site-specific 1D and 2D starting models. We discuss the unique benefits and challenges associated with inverting DAS data compared to traditional geophone data. We examine the impacts of using the various starting models on the final 2D subsurface images. We demonstrate that while the inversions performed using all four starting models are able to fit the major features of the DAS waveforms with similar misfit values, the final subsurface images can be quite different from one another at depths greater than about 10 m. As such, the best representation(s) of the subsurface are evaluated based on: (1) their agreement with borehole lithology logs that were not used in the development of the starting models, and (2) consistency at shallow depths between the final inverted images derived from multiple starting models. Our results demonstrate that FWI applied to DAS data has significant potential as a tool for near-surface site characterization while also emphasizing the significant impact that starting model selection can have on FWI results. [ABSTRACT FROM AUTHOR]

In this paper, efforts to characterize and compare the full-scale in situ three-dimensional (3D) dynamic response of a low-plasticity silt deposit to multidirectional loading from two different sources, a vibroseis shaker named T-Rex and controlled blasting, are presented. Horizontal vibroseis shaking at a frequency, f , of 10 Hz, produced dynamic responses in the silt that ranged from linear-elastic to nonlinear-inelastic, inducing maximum equivalent direct simple shear (DSS) shear strains, γDSS,max , up to 0.15% and residual excess pore pressure ratios, ru,r , of 14.1%. Blast-induced shear waves with predominant frequencies ranging from 9.6 to 14.6 Hz excited nonlinear-elastic and nonlinear-inelastic responses in the silt deposit, with γDSS,max of 1.14% and maximum ru,r of 61%. Importantly, these responses were observed to be minimally influenced by high frequency compression waves. Multidirectional loading, and excess pore pressure, ue , migration and impedance were identified as the predominant factors for achieving the large ru,r in the silt deposit from these two in situ testing techniques. The cyclic threshold shear strain, γtp , to trigger ru,r observed from the T-Rex shaking equaled 0.007% to 0.011% and varied with the initial soil stiffness. The two testing techniques demonstrated that the in-situ shear modulus, G , reduced to 90% of the maximum shear modulus, Gmax , at γDSS,max≈γtp , whereas by γDSS,max≈1% , G further reduced to 10 to 30% of Gmax corresponding to ru,r of ∼60%. Changes in soil fabric were quantified using small-strain shear-wave velocity measurements performed before and/or upon initiation and after each stage of dynamic testing, and were linked to the observed increase and decrease in γtp for the shallower and deeper 3D elements, respectively, following T-Rex shaking. The side-by-side comparison of the dynamic responses and soil properties derived from these two distinctly different field-testing techniques validate the use of controlled blasting for quantifying in situ dynamic soil properties and responses. [ABSTRACT FROM AUTHOR]

Surface wave testing is a powerful tool for noninvasive seismic site characterization. It includes a wide variety of active-source and passive-wavefield methods [e.g., spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), microtremor array measurements (MAM)] that can be applied to many different field conditions. The dispersion data collected with all these methods can be inverted to produce (one-dimensional) 1D shear wave velocity (VS) profiles of the subsurface. A critical part of this inversion procedure is the need to develop several trial model parameterizations with different numbers of layers as a means for investigating epistemic uncertainty and nonuniqueness when attempting to fit the experimental dispersion data. This is especially important when a priori information either is not available or does not extend to great enough depths to constrain the inversions. These trial parameterizations are used to vary the number of potential layers present in the subsurface models and to set the acceptable search ranges for depths to boundaries and stiffness in each layer. The inversion results are highly affected by the parameterizations. When performing high-quality inversions, it is important to use different parameterization options to fully explore potential conditions at the site and characterize the epistemic uncertainty of the inversion process. While this practice allows for a robust analysis of the experimental dispersion data and its uncertainty, it also generates many potential subsurface models for the site, which can represent a challenge for engineers when deciding which VS profiles best represent the true subsurface layering for use in subsequent analysis and design (e.g., seismic site response). This paper presents an approach, called the DeltaVs method, for systematically evaluating the depths of layer boundaries across all acceptable models determined from several different inversion layering parameterizations as a means to identify distinct clusters of boundaries. Using these clusters, the number and location of meaningful layer boundaries can be determined for many sites, thereby allowing for the elimination of some parameterization options containing more, or fewer, boundaries and a reduction in epistemic uncertainty. The distributions of layer boundaries within these clusters can then be used to determine statistics for the depths of layer boundaries that are consistent with the epistemic uncertainty of the models produced by the inversion procedure. The DeltaVs method is demonstrated on 12 synthetic ground models as well as one field data set. [ABSTRACT FROM AUTHOR]

Site-specific subsurface conditions are known to significantly influence the frequency content and amplitude of seismic ground shaking. When attempting to model these effects, known as seismic site effects, most studies have focused on the importance of vertically characterizing subsurface geomaterials. However, laterally characterizing the geomaterials' variability has generally either been overlooked or oversimplified. As such, little is actually known about the spatial area that influences seismic site response. In this article, we discuss insights that have been gained regarding the spatial area that influences site response from performing two-dimensional (2D) ground response analyses (GRAs) for the Treasure Island Downhole Array (TIDA) site. In these numerical analyses, we use a site-specific, large-scale, three-dimensional (3D) shear wave velocity (Vs) model that has been developed to a depth of 150 m over the entirety of Treasure Island (an area of approximately 1.6×1.0 km2). We investigate the lateral extent influencing site response and its azimuthal variability using: (1) four cross sections with increasing lateral extent along the same azimuth; and (2) 12 cross sections with different azimuths that uniformly sample the full 3D Vs model. By comparing 2D GRA predictions to recorded earthquake ground motions at TIDA, we systematically highlight the large spatial area that influences site response, which extends to distances as much as 1 km from the TIDA sensors. Although the TIDA site is often assumed to be archetype of one-dimensional (1D) site conditions, our findings support the idea that there may not be many truly 1D sites due to the large spatial area that influences site response. While some sites can be reasonably modeled using 1D GRAs, moving the engineering profession forward requires focusing our efforts on developing site-specific subsurface models over larger areas to better understand and accurately model seismic site response. [ABSTRACT FROM AUTHOR]

Non-invasive subsurface imaging using full waveform inversion (FWI) has the potential to fundamentally change near-surface (<30 m) site characterization by enabling the recovery of high-resolution (metre-scale) 2-D/3-D maps of subsurface elastic material properties. Yet, FWI results are quite sensitive to their starting model due to their dependence on local-search optimization techniques and inversion non-uniqueness. Starting model dependence is particularly problematic for near-surface FWI due to the complexity of the recorded seismic wavefield (e.g. dominant surface waves intermixed with body waves) and the potential for significant spatial variability over short distances. In response, convolutional neural networks (CNNs) are investigated as a potential tool for developing starting models for near-surface 2-D elastic FWI. Specifically, 100 000 subsurface models were generated to be representative of a classic near-surface geophysics problem; namely, imaging a two-layer, undulating, soil-over-bedrock interface. A CNN has been developed from these synthetic models that is capable of transforming an experimental wavefield acquired using a seismic source located at the centre of a linear array of 24 closely spaced surface sensors directly into a robust starting model for FWI. The CNN approach was able to produce 2-D starting models with seismic image misfits that were significantly less than the misfits from other common starting model approaches, and in many cases even less than the misfits obtained by FWI with inferior starting models. The ability of the CNN to generalize outside its two-layered training set was assessed using a more complex, three-layered, soil-over-bedrock formation. While the predictive ability of the CNN was slightly reduced for this more complex case, it was still able to achieve seismic image and waveform misfits that were comparable to other commonly used starting models, despite not being trained on any three-layered models. As such, CNNs show great potential as tools for rapidly developing robust, site-specific starting models for near-surface elastic FWI. [ABSTRACT FROM AUTHOR]

Quantitative dynamic strain measurements of the ground would be useful for engineering scale problems such as monitoring for natural hazards, soil-structure interaction studies, and non-invasive site investigation using full waveform inversion (FWI). Distributed acoustic sensing (DAS), a promising technology for these purposes, needs to be better understood in terms of its directional sensitivity, spatial position, and amplitude for application to engineering-scale problems. This study investigates whether the physical measurements made using DAS are consistent with the theoretical transfer function, reception patterns, and experimental measurements of ground strain made by geophones. Results show that DAS and geophone measurements are consistent in both phase and amplitude for broadband (10 s of Hz), high amplitude (10 s of microstrain), and complex wavefields originating from different positions around the array when: (1) the DAS channels and geophone locations are properly aligned, (2) the DAS cable provides good deformation coupling to the internal optical fiber, (3) the cable is coupled to the ground through direct burial and compaction, and (4) laser frequency drift is mitigated in the DAS measurements. The transfer function of DAS arrays is presented considering the gauge length, pulse shape, and cable design. The theoretical relationship between DAS-measured and pointwise strain for vertical and horizontal active sources is introduced using 3D elastic finite-difference simulations. The implications of using DAS strain measurements are discussed including directionality and magnitude differences between the actual and DAS-measured strain fields. Estimating measurement quality based on the wavelength-to-gauge length ratio for field data is demonstrated. A method for spatially aligning the DAS channels with the geophone locations at tolerances less than the spatial resolution of a DAS system is proposed. [ABSTRACT FROM AUTHOR]

A significant amount of seismic site response research over the past decade has focused on our abilities to replicate recorded ground motions at borehole array sites, where both the input (rock) and output (surface) ground motions are known. When viewed in aggregate, these studies have found that approximately 50% of borehole array sites are poorly modeled using one-dimensional (1D) ground response analyses (GRAs) based on a single shear wave velocity (Vs) profile, with individual studies reporting values between approximately 30%–80%. While there is no doubt that some sites are indeed too variable to be modeled using 1D GRAs, it is possible that simple 1D analyses could still be effectively used at many sites if spatial variability is accounted for via a rational, site-specific approach. In this study, we investigate five alternative approaches that can be used to account for spatial variability in 1D GRAs: (1) Vs randomization, (2) shear wave travel time randomization, (3) utilization of Vs suites derived from surface wave testing covering a large area, (4) incorporation of a pseudo-3D Vs model derived from a horizontal-to-vertical spectral ratio geostatistical approach, and (5) damping modifications. These approaches are investigated at two US borehole array sites (the Treasure Island and Delaney Park Downhole Arrays) so that the GRA results can be compared with recorded small-strain ground motions. Spatial variability is accounted for by generating approximately 250 Vs profiles for each approach, except for damping modifications, wherein only a single Vs profile is used, but with increased damping to account for wave scattering originating from spatial discontinuities. Through qualitative and quantitative comparisons, we assess the relative and absolute effectiveness of each approach, highlight their limitations, and propose potential improvements that can help overcome these limitations in practice. [ABSTRACT FROM AUTHOR]

SWinvert is a workflow developed at The University of Texas at Austin for the inversion of surface wave dispersion data. SWinvert encourages analysts to investigate inversion uncertainty and non-uniqueness in shear wave velocity (Vs) by providing a systematic procedure and specific actionable recommendations for surface wave inversion. In particular, the workflow encourages the use of multiple layering parametrizations to address the inversion's non-uniqueness, multiple global searches for each parametrization to address the inverse problem's non-linearity and quantification of Vs uncertainty in the resulting profiles. While the workflow uses the Dinver module of the popular open-source Geopsy software as its inversion engine, the principles presented are of relevance to analysts using other inversion programs. To illustrate the effectiveness of the SWinvert workflow and to develop a set of benchmarks for use in future surface wave inversion studies, synthetic experimental dispersion data for 12 subsurface models of varying complexity are inverted. While the effects of inversion uncertainty and non-uniqueness are shown to be minimal for simple subsurface models characterized by broad-band dispersion data, these effects cannot be ignored in the Vs profiles derived for more complex models with band-limited dispersion data. To encourage adoption of the SWinvert workflow, an open-source Python package (SWprepost), for pre- and post-processing of surface wave inversion data, and an application on the DesignSafe-Cyberinfrastructure (SWbatch), for performing batch-style surface wave inversions with Dinver using high-performance computing, have been developed and released in conjunction with this work. The SWinvert workflow is shown to provide a methodical procedure and a powerful set of tools for performing rigorous surface wave inversions and quantifying the uncertainty in the resulting Vs profiles. [ABSTRACT FROM AUTHOR]

The horizontal-to-vertical spectral ratio (HVSR) of ambient noise is commonly used to infer a site's resonance frequency (⁠|${f_{0,site}}$|⁠). HVSR calculations are performed most commonly using the Fourier amplitude spectrum obtained from a single merged horizontal component (e.g. the geometric mean component) from a three-component sensor. However, the use of a single merged horizontal component implicitly relies on the assumptions of azimuthally isotropic seismic noise and 1-D surface and subsurface conditions. These assumptions may not be justified at many sites, leading to azimuthal variability in HVSR measurements that cannot be accounted for using a single merged component. This paper proposes a new statistical method to account for azimuthal variability in the peak frequency of HVSR curves (⁠|${f_{0,HVSR}}$|⁠). The method uses rotated horizontal components at evenly distributed azimuthal intervals to investigate and quantify azimuthal variability. To ensure unbiased statistics for |${f_{0,HVSR}}$| are obtained, a frequency-domain window-rejection algorithm is applied at each azimuth to automatically remove contaminated time windows in which the |${f_{0,HVSR}}$| values are statistical outliers relative to those obtained from the majority of windows at that azimuth. Then, a weighting scheme is used to account for different numbers of accepted time windows at each azimuth. The new method is applied to a data set of 114 HVSR measurements with significant azimuthal variability in |${f_{0,HVSR}}$|⁠ , and is shown to reliably account for this variability. The methodology is also extended to the estimation of a complete lognormal-median HVSR curve that accounts for azimuthal variability. To encourage the adoption of this statistical approach to accounting for azimuthal variability in single-station HVSR measurements, the methods presented in this paper have been incorporated into hvsrpy , an open-source Python package for HVSR processing. [ABSTRACT FROM AUTHOR]

The horizontal-to-vertical spectral ratio (HVSR) of ambient noise measurement is commonly used to estimate a site's resonance frequency (⁠|${f_0}$|⁠). For sites with a strong impedance contrast, the HVSR peak frequency (⁠|${f_{0,\mathrm{ HVSR}}}$|⁠) has been shown to be a good estimate of |${f_0}$|⁠. However, the random nature of ambient noise (both in time and space), in conjunction with variable environmental conditions and sensor coupling issues, can lead to uncertainty in |${f_{0,\mathrm{ HVSR}}}$| estimates. Hence, it is important to report |${f_{0,\mathrm{ HVSR}}}$| in a statistical manner (e.g. as a mean or median value with standard deviation). In this paper, we first discuss widely accepted procedures to process HVSR data and estimate the variance in |${f_{0,\mathrm{ HVSR}}}$|⁠. Then, we propose modifications to improve these procedures in two specific ways. First, we propose using a lognormal distribution to describe |${f_{0,\mathrm{ HVSR}}}$| rather than the more commonly used normal distribution. The use of a lognormal distribution for |${f_{0,\mathrm{ HVSR}}}$| has several advantages, including consistency with earthquake ground motion processing and allowing for a seamless transition between HVSR statistics in terms of both frequency and its reciprocal, period. Second, we introduce a new frequency-domain window-rejection algorithm to decrease variance and enhance data quality. Finally, we use examples of 114 high-variance HVSR measurements and 77 low-variance HVSR measurements collected at two case study sites to demonstrate the effectiveness of the new rejection algorithm and the proposed statistical approach. To encourage their adoption, and promote standardization, the rejection algorithm and lognormal statistics presented in this paper have been incorporated into hvsrpy , an open-source Python package for HVSR processing. [ABSTRACT FROM AUTHOR]

This article presents ground~motion and site~effect observations in the Wellington region of New Zealand from the 14 November 2016 Mw 7.8 Kaikōura earthquake. Despite being ~60 km from the northern extent of the causative earthquake rupture, amplification of long~period ground motions due to site and basin~edge effects resulted in appreciable ground motions and subsequent damage to the built environment in this major urban area and capital city of New Zealand. The largest long~period ground motions were observed in the Thorndon and Te Aro basins in central Wellington, where similar site amplification effects were also observed during the 2013 Cook Strait earthquake sequence. Comparisons of pseudoacceleration response spectra with current estimates of fundamental site period across central Wellington indicate that this long~period amplification, relative to nearby rock stations, cannot be explained by 1D site effects alone (i.e., layered impedance), and thus it is inferred that there is a significant contribution from basin~edge~generated surface waves. In contrast, in the Lower Hutt-Petone area, north of central Wellington, ground motions from the GeoNet strong~motion station array across a deep alluvial valley clearly demonstrate the influence of 1D site effects. The 5%-95% significant duration of ground motions in central Wellington was on the order of 30 s, consistent with empirical models for this earthquake magnitude and source~to~site distance. The observations from the 2016 Kaikōura earthquake and comparison with the ground~motion characteristics of recent earthquakes has highlighted the need to improve characterization of the regional basin structures, particularly in regard to quantifying the contribution of basin resonance and basin~edge~generated surface waves. [ABSTRACT FROM AUTHOR]

An experimental study aimed at investigating potential topographic amplification of seismic waves was conducted on a 50-m-tall and 185-m-wide soft-rock ridge located at Los Alamos National Laboratory near Los Alamos, New Mexico. Ten portable broadband seismograph stations were placed in arrays across the ridge and left to record ambient vibration data for ~9 hours. Clear evidence of topographic amplification was observed by comparing spectral ratios calculated from ambient noise recordings at the toe, slope, and crest of the instrumented ridge. The inferred resonance frequency of the ridge obtained from the experimental recordings was found to agree well with several simple estimates of the theoretical resonance frequency based on its geometry and stiffness. Results support the feasibility of quantifying the frequency range of topographic amplification solely using ambient vibrations, rather than strong or weak ground motions. Additionally, comparisons have been made between a number of widely used experimental methods for quantifying topographic effects, such as the standard spectral ratio, median reference method, and horizontal-to-vertical spectral ratio. Differences in the amplification and frequency range of topographic effects indicated by these methods highlight the importance of choosing a reference condition that is appropriate for the site-specific conditions and goals associated with an experimental topographic amplification study. [ABSTRACT FROM AUTHOR]

A number of strategies exist to account for the epistemic uncertainty and aleatory variability in shear-wave velocity (Vs) profiles used in site response analyses. Epistemic uncertainty may be accounted for by using median and bounding-type profiles (e.g., ±20%), while aleatory variability may be accounted for by using Vs randomization procedures. A robust, quantitative method to help judge how well these statistically derived Vs profiles represent actual subsurface stiffness or layering conditions is currently not available. This paper presents a surface-wave dispersion approach for evaluating statistical models meant to account for Vs uncertainty in site response. Specifically, surfacewave dispersion data from two geologically disparate sites were used to generate 1,000 Vs profiles and layered earth models whose theoretical dispersion curves fit within the uncertainty bounds of the experimental data collected at each site. Additionally, theoretical dispersion curves generated from statistical Vs profiles such as the median, bounding-type, and randomly generated Vs profiles for each site were compared with the experimentally measured dispersion data. It was found that the theoretical dispersion curve from the median Vs profile provided a satisfactory fit to the experimental data, but the theoretical dispersion curves from bounding-type Vs profiles did not. Furthermore, randomly generated Vs profiles resulted in some theoretical dispersion curves that fit the experimental data and many that did not. The authors recommend comparing theoretical dispersion curves for candidate Vs profiles and layered earth models with the measured dispersion data at a site prior to using the candidate profiles to account for uncertainty in site response analyses as a quantitative way to judge if the Vs profiles represent realistic site conditions. [ABSTRACT FROM AUTHOR]

Uncertainty in site response analyses can be attributed to a number of parameters, including analysis methods, input ground motions, nonlinear dynamic soil properties, and shear-wave velocity profiles. In this paper, several approaches commonly used to account for shear-wave velocity (Vs) uncertainty in site response are investigated. Specifically, the Vs profiles considered are categorized into three groups: (1) Vs profiles determined directly from surface-wave inversion, (2) simple statistical Vs profiles derived indirectly from the surface-wave Vs profiles (including bounding-type, median, and other percentile Vs profiles), and (3) statistically based, randomly generated Vs profiles. A companion paper discusses the development of these Vs profiles for two international blind-study sites. In this paper, the effects of using each approach to account for Vs uncertainty in site response are investigated by linking the dispersion misfit values for each Vs profile to variability in equivalent linear site response estimates. Clear trends exist between variability in site response estimates and dispersion misfit values at both sites. Thus, the experimental dispersion data can be used to help select suites of Vs profiles, generated either directly from inversion or through a randomization model, that account for uncertainty in a meaningful way without including unrealistic statistical profiles that result in too much site response variability. [ABSTRACT FROM AUTHOR]

Surface wave methods provide a cost effective means of developing shear wave velocity (Vs) profiles for applications such as dynamic site characterization and seismic site response analyses. However, the inverse problem involved in obtaining a realistic layered earth model from surface wave dispersion data is inherently ill-posed, non-linear and mix-determined, without a unique solution. When available, a priori information such as geotechnical boreholes or geologic well logs should be used to aid in constraining site-specific inversion parameters. Unfortunately, a priori information is often unavailable, particularly at significant depths, and a 'blind analysis' must be performed. In these situations, the analyst must decide on an appropriate number of layers and ranges for their corresponding inversion parameters (i.e. trial number of layers and ranges in their respective thicknesses, shear wave velocities, compression wave velocities and mass densities). Selection of these parameters has been shown to significantly impact the results of an inversion. This paper presents a method for conducting multiple inversions utilizing systematically varied inversion layering parametrizations in order to identify and encompass the most reasonable layered earth models for a site. Each parametrization is defined by a unique layering ratio, which represents a multiplier that systemically increases the potential thickness of each layer in the inversion parametrization based on the potential thickness of the layer directly above it. The layering ratio method is demonstrated at two sites associated with the InterPacific Project, wherein it is shown to significantly aid in selecting reasonable Vs profiles that are close representations of the subsurface. While the goal of the layering ratio inversion methodology is not necessarily to find the 'optimal' or 'best' Vs profile for a site, it may be successful at doing so for certain sites/datasets. However, the primary reason for using the layering ratio method is to find Vs profiles that realistically represent the uncertainty in Vs resulting from surface wave inversion, and to avoid selection of Vs profiles that are unrealistic and adversely influenced by the choice of inversion parametrization. [ABSTRACT FROM AUTHOR]

This paper presents an overview of the soil profile characteristics at strong motion station (SMS) locations in the Christchurch Central Business District (CBD) based on recently completed geotechnical site investigations. Given the variability of Christchurch soils, detailed investigations were needed in close vicinity to each SMS. In this regard, CPT, SPT and borehole data, and shear wave velocity (Vs) profiles from surface wave dispersion data in close vicinity to the SMSs have been used to develop detailed representative soil profiles at each site and to determine site classes according to the New Zealand standard NZS1170.5. A disparity between the NZS1170.5 site classes based on Vs and SPT N60 investigation techniques is highlighted, and additional studies are needed to harmonize site classification based on these techniques. The short period mode of vibration of soft deposits above gravels, which are found throughout Christchurch, are compared to the long period mode of vibration of the entire soil profile to bedrock. These two distinct modes of vibration require further investigation to determine their impact on the site response. According to current American and European approaches to seismic site classification, all SMSs were classified as problematic soil sites due to the presence of liquefiable strata, soils which are not directly accounted for by the NZS1170.5 approach. [ABSTRACT FROM AUTHOR]

The Christchurch-specific empirical correlation between shear wave velocity (Vs) and cone penetration test (CPT) data developed by McGann et al. [1-3] for the non-gravel soils of the Christchurch and Springston Formations is evaluated through comparison to Vs profiles obtained using surface wave analysis techniques at twelve Christchurch strong motion stations. These comparisons highlight the similarities and differences between the Vs profiles obtained from each approach, and allow for an assessment of the relative strengths and weaknesses of each. It is shown that, with known differences, the results of the surface wave analysis and CPT correlation compare well in terms of their independently obtained Vs magnitudes. The sources of the differences between the results of each method are identified and discussed. [ABSTRACT FROM AUTHOR]

The 2011 M W = 9.0 Tohoku-oki earthquake generated a large number of unique soil liquefaction case histories, including cases with strong ground motion recordings on liquefiable or potentially liquefiable soils. We have compiled a list of 22 strong motion stations (SMS) where surface evidence of liquefaction was observed and 16 SMS underlain by geologically recent sediments or fills where surface evidence of liquefaction was not observed. Pre-earthquake standard penetration test data and borehole shear wave velocity (V s) profiles are available for some stations, but critical information, such as grain size distribution and fines plasticity, are often lacking. In the heavily damaged city of Urayasu, we performed post-earthquake cone penetration testing at seven SMS and V s profiles, using surface wave methods at 28 additional locations to supplement existing geotechnical data. We describe the liquefaction effects in Urayasu, the available site characterization data, and our initial data interpretations. [ABSTRACT FROM AUTHOR]

Relationships between seismic site classification and surficial geology for the city of Fairbanks, Alaska, for both frozen (winter) and unfrozen (summer) surface conditions are investigated in this article. The work is based on 59 shear wave velocity () profiles collected using the spectral analysis of surface waves (SASW) method. Forty-three profiles were collected during the summer when the ground surface was fully thawed, and 16 profiles were collected by reoccupying some of the test locations during fully frozen winter ground surface conditions. Relationships between mean over the top 30 m of the subsurface () and surficial geologic units in Fairbanks were developed, permitting estimates of seismic site classification throughout the city for both frozen and unfrozen conditions. Much of Fairbanks was determined to have been founded on NEHRP Site Class D material when unfrozen. When frozen, the of the near-surface material increased significantly (by 700% on average). However, this drastic increase in stiffness was typically limited to the top 1 to 2 m of the subsurface and only resulted in an increase in site class from NEHRP D to NEHRP C at three of the 16 sites retested during the winter. [ABSTRACT FROM AUTHOR]

The 12 January 2010 Haiti earthquake (Mw 7.0) caused extensive damage to the Port-au-Prince region, including severe liquefaction failures along the Gulf of Gonâve coastline, along rivers north of Port-au-Prince draining into the Gulf, and a liquefaction-induced structural/bearing capacity failure of a three-story concrete hotel along the southern coast of the Gulf. During two reconnaissance missions, the authors documented ground conditions and performance at eight sites that liquefied and two sites that did not liquefy. Geotechnical characterization included surface mapping, dynamic cone penetration tests, hand auger borings, and laboratory index tests. The authors estimated median peak ground accelerations (PGAs) of approximately 0.17g to 0.48g at these sites using the Next Generation Attenuation (NGA) relations summarized by Power et. al. (2008). These case histories are documented here so that they can be used to augment databases of level-ground/near level-ground liquefaction, lateral spreading, liquefaction flow failure, and liquefaction-induced bearing capacity failure. [ABSTRACT FROM AUTHOR]

The 2010 Haiti earthquake represents one of the most devastating earthquakes in history. Damage to structures was widespread across the city of Port-au-Prince, but its intensity varied considerably from neighborhood to neighborhood. This paper integrates damage statistics with geologic data, shear wave velocity measurements, and topographic information to investigate the influence of these conditions on the damage patterns in the city. The results indicate that the most heavily damaged areas in downtown Port-au-Prince are underlain by Holocene alluvium with shear wave velocities that average about 350 m/s over the top 30 m. The remainder of Port-au-Prince is underlain mostly by older geologic units with higher shear wave velocities. Damage was also concentrated on hillsides around Port-au-Prince. These pockets of damage appear to have been caused by a combination of factors, including topographic amplification, soil amplification, and failure of weakly cemented, steep hillsides. [ABSTRACT FROM AUTHOR]

Presented herein are the results of geotechnical investigations and subsequent laboratory and data analyses of the Port-au-Prince seaport following the Mw7.0 2010 Haiti earthquake. The earthquake caused catastrophic ground failures in calcareous-sand artificial fills at the seaport, including liquefaction, lateral spreads, differential settlements, and collapse of the pile-supported wharf and pier. The site characterization entailed geotechnical borings, hand-auger borings, standard penetration tests, and dynamic cone penetration tests. The laboratory tests included grain size and carbonate content tests. The observations and results presented herein add valuable field performance data for calcareous sands, which are relatively lacking in liquefaction case history databases, and the overall response of the artificial fills are consistent with predictions made using semi-empirical relations developed primarily from field data of silica sands. [ABSTRACT FROM AUTHOR]

A seismic site classification microzonation for the city of Port-au-Prince is presented herein. The microzonation is based on 35 shear wave velocity (V S) profiles collected throughout the city and a new geologic map of the region. The V S profiles were obtained using the multichannel analysis of surface waves (MASW) method, while the geologic map was developed from a combination of field mapping and geomorphic interpretation of a digital elevation model (DEM). Relationships between mean shear wave velocity over the upper 30 m of the subsurface (V S30) and surficial geologic unit have been developed, permitting code-based seismic site classification throughout the city. A site classification map for the National Earthquake Hazards Reduction Program/International Building Code (NEHRP/IBC) classification scheme is provided herein. Much of the city is founded on deposits that classify as either NEHRP Site Class C or D, based on V S30. Areas of the city requiring additional subsurface information for accurate site classification are noted. [ABSTRACT FROM AUTHOR]

To assess the level and nature of ground shaking in Hawaii for the purposes of earthquake hazard mitigation and seismic design, empirical ground-motion prediction models are desired. To develop such empirical relationships, knowledge of the subsurface site conditions beneath strong-motion stations is critical. Thus, as a first step to develop ground-motion prediction models for Hawaii, spectral-analysis-of-surface-waves (SASW) profiling was performed at the 22 free-field U.S. Geological Survey (USGS) strong-motion sites on the Big Island to obtain shear-wave velocity (VS) data. Nineteen of these stations recorded the 2006 Kiholo Bay moment magnitude (M) 6.7 earthquake, and 17 stations recorded the triggered M 6.0 Mahukona earthquake. VS profiling was performed to reach depths of more than 100 ft. Most of the USGS stations are situated on sites underlain by basalt, based on surficial geologic maps. However, the sites have varying degrees of weathering and soil development. The remaining strong-motion stations are located on alluvium or volcanic ash. VS30 (average VS in the top 30 m) values for the stations on basalt ranged from 906 to 1908 ft/s [National Earthquake Hazards Reduction Program (NEHRP) site classes C and D], because most sites were covered with soil of variable thickness. Based on these data, an NEHRP site-class map was developed for the Big Island. These new VS data will be a significant input into an update of the USGS statewide hazard maps and to the operation of ShakeMap on the island of Hawaii. [ABSTRACT FROM AUTHOR]

We report about a preliminary study to evaluate the use of semi-automated imaging analysis of remotely-sensed DEM and field geophysical measurements to develop a seismic-zonation map of Port-au-Prince, Haiti. For in situ data, Vs30 values are derived from the MASW technique deployed in and around the city. For satellite imagery, we use an ASTER GDEM of Hispaniola. We apply both pixel- and object-based imaging methods on the ASTER GDEM to explore local topography (absolute elevation values) and classify terrain types such as mountains, alluvial fans and basins/near-shore regions. We assign NEHRP seismic site class ranges based on available Vs30 values. A comparison of results from imagery-based methods to results from traditional geologic-based approaches reveals good overall correspondence. We conclude that image analysis of RS data provides reliable first-order site characterization results in the absence of local data and can be useful to refine detailed site maps with sparse local data. [ABSTRACT FROM AUTHOR]