Jamison H. Steidl

Site Response in Southern California for Probabilistic Seismic Hazard Analysis

This study determines site-response factors that can be applied as cor- rections to a rock-attenuation relationship for use in probabilistic seismic-hazard analysis. The site-response factors are amplitude and site-class dependent. These amplification factors are determined by averaging ratios between observed and pre- dicted ground motions for peak ground acceleration (PGA) and for 5% damped re- sponse spectral acceleration at 0.3, 1.0, and 3.0 sec oscillator periods. The observa- tions come from the strong-motion database of the Southern California Earthquake Center (SCEC), and the predictions are based on the Sadigh (1993) rock-attenuation relation. When separated and averaged according to surface geology, significantly different site-response factors are found for Quaternary and Mesozoic units, but a subclassification of Quaternary is generally not justified by the data. The low input- motion amplification factors are consistent with those obtained from independent aftershock studies at the PGA and 0.3-second period. An observed trend of decreasing Quaternary site amplification with higher input motion is consistent with nonlinear soil behavior; however, the trend exists for Mesozoic sites as well, implying that this may be an artifact of the Sadigh relationship. There is a correlation between larger site-response factors and lower average shear-wave velocity in the upper 30 m for low predicted PGA input motions, with an increase in the correlation with increasing period. The 0.3-sec site response factors for Quaternary data in southern California determined in this study are consistent with 0.3-sec NEHRP site-response correction factors; however, at 1.0-sec period some inconsistencies remain. A trend is also seen with respect to sediment basin depth, where deeper sites have higher average site- response factors. These results constitute a customized attenuation relationship for southern California. The implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in Field and Petersen (2000).

Borehole Response Studies at the Garner Valley Downhole Array, Southern California

The Garner Valley Downhole Array (GVDA) consists of a set of seven downhole strong-motion instruments ranging from 0- to 500-m depth. One of the objectives of this experiment is to estimate site response and study wave propagation as the energy travels from the bedrock underneath the site up through the soil column. The GVDA velocity structure is studied by computing synthetic accelerograms for a small event located at an epicentral distance of 10 km. These synthetics simulate well the data recorded at the borehole stations. In addition, theoretical transfer functions are calculated using the obtained velocity model and compare well with the empirical transfer functions from 54 recorded events. It is also observed that the downgoing wave effect is predominant in the first 87 m and is strongly reduced at depth. Using the velocity structure at GVDA and the transfer function results, it has also been possible to develop a simple method to compute the incident wave field, which is needed in nonlinear site response for instance. Recently there have been many comparative studies between horizontal-to-vertical (H/V) spectral ratios and traditional spectral ratios. Although many of these studies show that H/V spectral ratios can reproduce the shape of the site response curve, most show differences in the amplitude level. In the case of Garner Valley, where we have both surface and multiple borehole instruments, we find that this discrepancy in amplitude of the site response estimates is because the vertical component has significant site response associated with it due to S -to- P conversions that begin in the weathered granite boundary at 87-m depth. Manuscript received 27 August 2001.

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

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 near-surface 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. Surface-to-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 Vs 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.

Site-Response Estimation for the 2003 Miyagi-Oki Earthquake Sequence Considering Nonlinear Site Response

The M w 7.0 Miyagi-Oki earthquake, which occurred on 26 May 2003, was well recorded by the KiK-net and K-net networks. A large number of stations recorded very high peak ground accelerations above 0.5g and large peak ground velocities above 0.5 m/sec. These high ground-motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high-frequency ground-motion content associated with this intraslab earthquake. The objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration ! 0.3g. Linear site response is first estimated from observed weak motion (aftershock) records. In this case, we use a spectral inversion method, without reference stations, to separate the source, path, and site-response effects. The resulting weak motion analysis for the source, path, and site response agree with other previous studies. The mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency-dependent radiation pattern. The comparison of the site am- plification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the Mw 7.0 event. The results also suggest a correlation between lownear-surface materialvelocity and thedegreeofnonlinearity.

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

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 factors compare well with synthetic simulations for class C and D site conditions, while long-period amplification factors are overestimated by a factor of 1.5 at the class E site, where site-specific nonlinear analyses should be employed for levels of PGA_(RO)>0.2g.

The SCEC Phase III Strong-Motion Database

As part of the Southern California Earthquake Center (SCEC) Phase III effort to include site effects in hazard models for southern California, a regional database of strong-motion observations was developed. The observations consist of the peak ground acceleration (PGA) and 5% damped response spectral acceleration (SA) at 0.3-, 1.0-, and 3.0-sec periods from 28 earthquakes and 281 stations. A total of 449 pairs of horizontal PGA and SA observations that were taken from the SCEC Strong-Motion Database (SMDB) are presented here. The phase III database includes earthquakes with moment magnitudes larger than 5.0 and stations in southern California with locations between 32° and 36° north latitude. Observations from buildings with more than two stories and dam abutments or crests are excluded from the database. Observations with distances of 150 km or greater were also excluded. The agencies that provided the data to SMDB are the U.S. Geological Survey, California Strong Motion Instrumentation Program, University of Southern California, and the Los Angeles Department of Water and Power. The database also contains site classification information for each station. A first general classification is based on the 1:750,000 California map of Quaternary, Tertiary, and Mesozoic geologic units by Jennings (1977) as modified by Park and Elrick (1998). A second more detailed classification is based on Quaternary mapping in the Los Angeles region by Tinsley and Fumal (1985) as modified by Park and Elrick (1998). A third classification is based on the correlation of surface geology with shear-wave velocity in the upper 30 m (Wills et al., 2000). An arbitrary “depth-to-basement” parameter is assigned to stations that are located within the boundary of the 3D velocity model used by Olsen (2000), which is based on the SCEC 3D velocity model (version 1). This parameter is defined as the depth to the 2.5km/sec velocity isosurface. Special parameters associated with particular attenuation relations, such as different distance measures, a hanging wall flag, and fault-type flag are also assigned to each observation. These data are all presented within the tables and figures of this article, and also have been made available via a downloadable file on the Internet ( http://smdb.crustal.ucsb.edu/~phase3 ).

Finite-fault site-specific acceleration time histories that include nonlinear soil response

To estimate the broadband strong ground motion one might expect at a given site we develop a method that includes heterogeneous slip on a finite-fault, full wave propagation with high frequencies, and site-specific material properties with nonlinear soil response. The faulting is simulated as a stochastic process with the spatial variation of the key parameters determined by probability distribution functions. The wave propagation from source to site is accounted for by using small earthquake recordings as empirical Green’s functions (EGF). This accounts for the regional effects of scattering, attenuation and structure while providing the basis for a broadband (0.5–10 Hz) time history. Because we are interested in sites where the ground motion is expected to be severe, we have included nonlinear wave propagation through the soil. The material properties of the soil column have been determined from laboratory tests, borehole logs and confirmed through seismological modeling of weak motion. We have computed 240 three-component acceleration time histories to represent the range of ground motion one might expect from a M 6.8 earthquake for a site that is located 10 km above the hanging wall of blind thrust (7.1 km closest distance). Based on the suite of time histories we computed a S.D. of 0.45 (natural log units) for the acceleration response spectra in the passband 0.5–10 Hz. The total S.D. (modeling plus parameterization) is 0.6 in natural log units. The mean acceleration response spectrum is near the median 10% in 50-year probabilistic seismic hazard analysis (PSHA) spectrum for the site; the 84% spectrum of the simulations is closer to the 5% in 50 years median spectrum.

In Situ Assessment of the G–γ Curve for Characterizing the Nonlinear Response of Soil: Application to the Garner Valley Downhole Array and the Wildlife Liquefaction Array

Abstract 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 between two boreholes is tested as a deformation proxy. Using average peak particle acceleration a * , the a * versus 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.

Nonlinear site response from the 2003 and 2005 Miyagi-Oki earthquakes

We estimate nonlinear site response by comparing site response estimates from the 16 August 2005 Mj=7.2 and 26 May 2003 Mj=7.0 Miyagi-Oki earthquakes with site response estimates from aftershocks of the 2003 event. Site response is solved by a spectral inversion technique to separate source, path, and site components. The constraint motion in the inversion is a regional attenuation model derived from fitting the spectra of data recorded at borehole KiK-net stations in the region and a theoretical source spectrum for each event determined using the same borehole stations. Site response is calculated at the surface of the KiK-net and K-NET stations. In general, the average aftershock site response is larger than for the two mainshocks, especially at a higher frequency. When comparing site response with input ground motion level, the predominant frequency and the site response values tend to decrease as the level of input ground motion increases.

The Cosmos Virtual Data Center

The COSMOS VDC is a comprehensive, unrestricted, on-line, interactive strong ground-motion search engine for engineers, seismologists, and other earthquake professionals which implements a variety of search interfaces and allows users to preview data and configure design spectra overlays on response spectra. Users may download data files from data providers’ servers transparently through this web portal: http://db.cosmos-eq.org.