Luis Fabian Bonilla

Hysteretic and Dilatant Behavior of Cohesionless Soils and Their Effects on Nonlinear Site Response: Field Data Observations and Modeling

In this study we present evidence that nonlinearity can be directly observed in acceleration time histories such as those recorded at the Wildlife Refuge and Kushiro Port downhole arrays from the 1987 Superstition Hills, California, and the 1993 Kushiro-Oki, Japan, earthquakes, respectively. These accelerograms and others compiled in this study present a characteristic waveform composed of intermittent high-frequency peaks riding on a low-frequency carrier. In addition, soil amplification of the surface records is strongly observed compared to their downhole counterpart; this is contrary to the expected amplification reduction produced by the nonlinear soil behavior. Laboratory studies show that the physical mechanism that produces such phenomena is the dilatant nature of cohesionless soils, which introduces the partial recovery of the shear strength under cyclic loads. This recovery translates into the ability to produce large deformations followed by large and spiky shear stresses. The spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure. These results are significant in strong-motion seismology because these spikes produce large if not the largest acceleration. They are site related, not source related. Using the in situ observations from the Kushiro Port downhole array, we have modeled the 1993 Kushiro-Oki earthquake. The synthetic accelerograms show the development of intermittent behavior—high frequency peaks—as observed in the recorded acceleration time histories. Shear modulus degradation due to pore pressure produces large strains in the soil with large amplification in the low-frequency band of the ground motion. We also modeled data from the 1987 Superstition Hills earthquake recorded at the Wildlife Refuge station. The results show the importance of better soil characterization when pore pressure may develop and the effects of dilatancy in the understanding of nonlinear site response.

Nonstationary Stochastic Simulation of Strong Ground Motion Time Histories Including Natural Variability: Application to the K-Net Japanese Database

Physical models that can be used to obtain realistic accelerograms usually require a thorough knowledge of the source, path, and site effects. In addition, the computational resources needed might be expensive. Thus, empirical models still represent a good alternative for simulating strong ground motion. In this work, we modify and improve the model developed by Sabetta and Pugliese (1996). This new method models the time-domain accelerogram based on the assumption that the phase is random and that the time envelope can be represented by the so-called average instantaneous power. This is, in turn, described as a lognormal distribution for P and S waves combined with an algebro-exponential function representing the envelope of coda waves. In addition, the frequency content of the signal is nonstationary and follows a modified ω -square model. The method depends on four common indicators in earthquake engineering: peak ground acceleration, strong-motion duration, Arias intensity, and central frequency. These indicators are empirically connected to a given database by means of ground-motion prediction equations. In this study we calibrate the model using Japanese data recorded by the K-net array, which has high-quality digital accelerograms and station-site conditions characterized by geotechnical measurements. In addition, this technique permits the inclusion of the uncertainty of the model parameters to take into account the ground-motion natural variability in the stochastic generation of the time histories. The main goal of this work is to provide the earthquake engineering community with a flexible tool to generate realistic accelerograms for dynamic studies.

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 Classification Using Horizontal-to-vertical Response Spectral Ratios and its Impact when Deriving Empirical Ground-motion Prediction Equations

We classify sites based on their predominant period computed using average horizontal-to-vertical (H/V) response spectral ratios and examine the impact of this classification scheme on empirical ground-motion models. One advantage of this classification is that deep geological profiles and high shear-wave velocities are mapped to the resonance frequency of the site. We apply this classification scheme to the database of Fukushima et al. [2003], for which stations were originally classified as simply rock or soil. The calculation of average H/V response spectral ratios permits the majority of sites in the database to be unambiguously classified. Soft soil conditions are clearly apparent using this technique. Ground-motion prediction equations are then computed using this alternative classification scheme. The aleatoric variability of these equations (measured by their standard deviations) is slightly lower than those derived using only soil and rock classes. However, perhaps more importantly, predicted response spectra are radically different to those predicted using the soil/rock classification. In addition, since the H/V response spectral ratios were used to classify stations the predicted spectra for different sites show clear separation. Thus, site classification using the predominant period appears to be partially mapped into the site coefficients of the ground-motion model.

Eurocode 8 design response spectra evaluation using the K-net Japanese database.

The Eurocode 8 (EC8) currently proposes two standard shapes for the design response spectra. Type 1 spectra are enriched in long period and are suggested for high seismicity regions. Conversely, Type 2 spectra are proposed for low to moderate seismicity areas (like France), and exhibit both a larger amplification at short period, and a much smaller long period contents, with respect to Type 1 spectra. These propositions, however, were constrained using few events mostly recorded on analogical instruments. In the present study, we use the Japanese high quality digital K-net array in order to evaluate the proposed ECS response spectra. Furthermore, all K-net stations have geotechnical characterisation. We first constructed a database of shallow events, depth less than 25 km, to avoid subduction related records. The database spans six years of seismicity from 1996 until 2003. Thus, 591 events were selected with moment magnitude between 4 and 7.3, recorded at 691 stations, giving a total of 6812 two horizontal components accelerograms. Using these records, we computed spectral ground-motion prediction equations and we used them to review the shape of the proposed EC8 spectra. In particular, we studied the plateau-PGA ratio level, the period interval where this plateau is constant, and site amplification effects. The results show surprisingly that the Type 2 rock better envelope the Japanese data. Another interesting observation is that the K-net data corresponding to all soil classes are rich in short periods around 0.1 s. This characteristic has not been observed in other worldwide databases. Normalised empirical predictions show a widening of the plateau as the soil conditions degrade. This suggests that the Type 2 EC8 spectra do not cover enough the long periods for EC3-soil classes C, D and E. Finally, the computed ground-motion prediction equations show that the peak ground acceleration (PGA) is nearly invariant to the soil conditions. Soil effects are mainly seen in the shape and plateau level.

Influence of the Vs profiles beyond 30 m depth on linear site effects: assessment from the KiK-net data

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 VS profile from 0 to 30 m depth, denoted B30. 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 (f0), 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 VS30, the velocity gradient B30, and f0. We select 289 sites for which the 1D 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, B30, and f0 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, f0 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, B30, and f0 improves the assessment of linear site amplification.

Accounting for site characterization uncertainties when developing ground-motion prediction equations

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 sitespecific 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.

Estimation of Site‐Specific Kappa (κ0)‐Consistent Damping Values at KiK‐Net Sites to Assess the Discrepancy between Laboratory‐Based Damping Models and Observed Attenuation (of Seismic Waves) in the FieldEstimation of Site‐Specific Kappa (κ0)‐Consistent Damping Values at KiK‐Net Sites

In this article, we compare field estimates of near-surface attenuation, as captured by site-specific o-values (i.e., o0) with laboratory-based estimates of minimum shear-strain damping (Ÿmin). We propose models for Ÿmin based on o0 measured at selected stations of the KiK-net database, which are found to be generally larger than lowstrain damping values obtained from laboratory testing. The latter can only quantify intrinsic material damping, whereas other attenuation mechanisms such as scattering of the wavefield contribute to field-based estimates. In addition, we evaluate the difference in damping at the surface and at borehole stations to determine the contribution of shallow layers to attenuation as captured by o0-values at the surface. Thus, values of o0 are computed at the surface and at the downhole instrument depth. The difference between both values, correlates well with the averaged shear-wave velocity over the top 30 m of the profile, VS30, and with the depth to bedrock. Estimates of o0 for hard-rock and stiff sites in Japan are also examined and compared with other regional o0-values proposed for high VS30 materials in New Zealand, Greece, and Switzerland. Two values of o0, which are lower than the corresponding estimates for the aforementioned regions, are deemed potential descriptors of hard-rock conditions in Japan. The ability of the proposed o0-consistent damping models to predict ground motions using the vertical array data from the KiK-net sites has yet to be tested.

Eurocode 8-compatible synthetic time-series as input to dynamic analysis

Nonlinear dynamic analysis of existing or planned structures often requires the use of accelerograms that match a target design spectrum. Here, our main concern is to generate a set of motions with a good level of fit to the Eurocode 8 design spectra for France. Synthetic time series are generated by means of a non-stationary stochastic method. To calibrate the input parameters in the stochastic approach, we select a reference set of accelerograms for a Eurocode 8 type B site category from the PEER Ground-Motion Database, which are then adjusted to the target spectrum through wavelet addition. Then, we compute nonlinear seismic responses of a soil column, including pore pressure effects, and brittle and ductile structures to the stochastic time-series, the natural accelerograms and time-series generated using stationary stochastic approaches. The results of these calculations reveal considerable variability in response despite the similarities in terms of spectral acceleration.