Attenuation and Velocity Structure for Site Response Analyses via Downhole Seismogram Inversion

We investigate the propagation of uncertainty in site-response analyses from the soil model parameters to the ground surface motion at three downhole array sites in the Los Angeles (LA) Basin. For this purpose, we develop realistic stochastic models of elastic and nonlinear dynamic soil properties using extensive site-specific and generic geotechnical data on the variability of soil properties. We also generate synthetic ground motions using a finite source dynamic rupture model over a wide range of magnitudes and distances and use this statistically significant number of ground motions in the analysis. For each of the three sites, we evaluate the effects of soil parameter uncertainty as a function of the seismic input intensity and frequency content. We show that the frequency range, where the ground-motion variability due to soil parameter uncertainty is maximized, is a function of both the site and the seismogram characteristics. We compare our results with previously published studies and show that different soil models, statistical descriptions of soil parameters, or groundmotion scenarios may yield substantial differences in the estimated site-response scatter. We conclude that the effects of nonlinear soil property uncertainties on the ground-motion variability strongly depend on the seismic motion intensity, and this dependency is more pronounced for soft soil profiles. By contrast, the effects of velocity profile uncertainties are less intensity dependent and more sensitive to the velocity impedance in the near surface that governs the maximum site amplification.

Section: Site Conditions and Ground-motion Syntheticsmentioning

confidence: 99%

Site- and Motion-Dependent Parametric Uncertainty of Site-Response Analyses in Earthquake Simulations

Li¹,

Assimaki²

2010

“…For this purpose a seismic waveform inversion algorithm developed by Assimaki et al (2006) is initially implemented for the evaluation of high-resolution, low-strain shear-wave velocity (V s ), attenuation (Q s ), and density profiles at 37 geotechnical array sites of the Japanese strongmotion network KiK-Net (Aoi et al, 2000); based on the inversion results, V s -Q s correlations are derived for the near-surface (0-30 m) and underlying (30-100 m) soil layers and are interpreted to account for the effects of scattering and intrinsic (material) attenuation. Successively, the sites are classified through the weighted average inverted velocity of the upper 30 m (V s30 ) according to the National Earthquake Hazards Reduction Program (NEHRP) site classification system.…”

Section: Introductionmentioning

confidence: 99%

Site Amplification and Attenuation via Downhole Array Seismogram Inversion: A Comparative Study of the 2003 Miyagi-Oki Aftershock Sequence

Assimaki¹,

Wei²,

Steidl³

et al. 2008

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of nearsurface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V s ), attenuation (Q s ), and density (ρ) profiles at the sites of interest. Based on the inversion results, V s versus Q s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surfaceto-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average V s of the upper 30 m (V s30 ) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B-C boundary site conditions (V s30 760 m=sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations.

“…Averaging of the optimal solution for multiple events has been shown to minimize the error propagation of the measured process and the error translation of the forward idealized model limitations, leading to a robust estimate of the bestfit solution to the inverse problem. For more information, the reader is referred to Assimaki et al (2006) and Assimaki and Steidl (2007).…”

Section: CMmentioning

confidence: 99%

“…The criteria are developed on the basis of a comprehensive nonlinear site-response modeling uncertainty analysis at three downhole array sites in the Los Angeles basin, which includes both detailed soil profile descriptions and statistical adequacy of ground-motion time histories. To achieve high resolution of the near-surface stratigraphy, seismogram inversion is employed to weak-motion recordings (Assimaki et al, 2006). Also, the scarcity of ground-motion recordings is addressed by generating a suite of synthetic ground motions for rupture scenarios of weak, medium, and large magnitude events (M 3:5-7:5).…”

Section: Introductionmentioning

confidence: 99%

“…A detailed description of the near-surface stratigraphy is achieved by employing the seismogram inversion algorithm developed by Assimaki et al (2006) to weak ground motions recorded at the three arrays (Table 2). This optimization technique comprises a genetic algorithm in the wavelet domain coupled to a nonlinear least-squares fit in the frequency domain, and it has been shown to improve the computational efficiency of the former while avoiding the pitfalls of using local linearization techniques such as the latter (Houck et al, 1996).…”

Section: Introductionmentioning

confidence: 99%

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Quantifying Nonlinearity Susceptibility via Site-Response Modeling Uncertainty at Three Sites in the Los Angeles Basin

Assimaki¹,

Li²,

Steidl³

et al. 2008

The effects of near-surface soil stratigraphy on the amplitude and frequency content of ground motion are accounted for in most modern U.S. seismic design codes for building structures as a function of the soil conditions prevailing in the area of interest. Nonetheless, currently employed site-classification criteria do not adequately describe the nonlinearity susceptibility of soil formations, which prohibits the development of standardized procedures for the computationally efficient integration of nonlinear ground response analyses in broadband ground-motion simulations. In turn, the lack of a unified methodology for nonlinear site-response analyses affects both the prediction accuracy of site-specific ground-motion intensity measures and the evaluation of site-amplification factors when broadband simulations are used for the development of hybrid attenuation relations. In this article, we introduce a set of criteria for quantification of the nonlinearity susceptibility of soil profiles based on the site conditions and incident ground-motion characteristics, and we implement them to identify the least complex ground response prediction methodology required for the simulation of nonlinear site effects at three sites in the Los Angeles basin. The criteria are developed on the basis of a comprehensive nonlinear site-response modeling uncertainty analysis, which includes both detailed soil profile descriptions and statistical adequacy of ground-motion time histories. Approximate and incremental nonlinear models are implemented, and the limited site-response observations are initially compared to the ensemble site-response estimates. A suite of synthetic ground motions for rupture scenarios of weak, medium, and large magnitude events (M 3:5-7:5) is next generated, parametric studies are conducted for each fixed magnitude scenario by varying the source-to-site distance, and the variability introduced in ground-motion predictions is quantified for each nonlinear site-response methodology. A frequency index is developed to describe the frequency content of incident ground motion relative to the resonant frequencies of the soil profile, and this index is used in conjunction with the rock-outcrop acceleration peak amplitude (PGA RO ) to identify the site conditions and ground-motion characteristics where incremental nonlinear analyses should be employed in lieu of approximate methodologies. We show that the proposed intensity-frequency representation of ground motion may be implemented to describe the nonlinearity susceptibility of soil formations in broadband simulations by accounting both for the magnitude-distance-orientation characteristics of seismic motion and the profile stiffness characteristics. The synthetic ground-motion predictions are next used for the development of site-amplification factors for the alternative site-response methodologies, and the results are compared to published site factors of attenuation relations. For the site conditions investigated, currently established amplification fac...