Jonathan P. Stewart

NGA-West2 Equations for Predicting PGA, PGV, and 5% Damped PSA for Shallow Crustal Earthquakes

We provide ground motion prediction equations for computing medians and standard deviations of average horizontal component intensity measures (IMs) for shallow crustal earthquakes in active tectonic regions. The equations were derived from a global database with M 3.0–7.9 events. We derived equations for the primary M- and distance-dependence of the IMs after fixing the VS30-based nonlinear site term from a parallel NGA-West2 study. We then evaluated additional effects using mixed effects residuals analysis, which revealed no trends with source depth over the M range of interest, indistinct Class 1 and 2 event IMs, and basin depth effects that increase and decrease long-period IMs for depths larger and smaller, respectively, than means from regional VS30-depth relations. Our aleatory variability model captures decreasing between-event variability with M, as well as within-event variability that increases or decreases with M depending on period, increases with distance, and decreases for soft sites.

NGA-West2 Database

The NGA-West2 project database expands on its predecessor to include worldwide ground motion data recorded from shallow crustal earthquakes in active tectonic regimes post-2000 and a set of small-to-moderate-magnitude earthquakes in California between 1998 and 2011. The database includes 21,336 (mostly) three-component records from 599 events. The parameter space covered by the database is M 3.0 to M 7.9, closest distance of 0.05 to 1,533 km, and site time-averaged shear-wave velocity in the top 30 m of VS30 = 94 m/s to 2,100 m/s (although data becomes sparse for distances >400 km and VS30 > 1,200 m/s or <150 m/s). The database includes uniformly processed time series and response spectral ordinates for 111 periods ranging from 0.01 s to 20 s at 11 damping ratios. Ground motions and metadata for source, path, and site conditions were subject to quality checks by ground motion prediction equation developers and topical working groups.

Ground motion evaluation procedures for performance-based design

The objective of performance-based earthquake engineering (PBEE) is the analysis of performance objectives with a specified annual probability of exceedance. Increasingly undesirable performance is caused by increasing levels of strong ground motion having decreasing annual probabilities of exceedance. Accordingly, the evaluation of ground motion intensity measures is a vital component of PBEE. This paper provides a brief synthesis of ground motion analysis procedures within a performance-based design framework, and is a summary of a recent report to which the reader is referred for details. The principal topics addressed are probabilistic characterizations of source, path, and site effects, with the discussion of these effects focusing principally on applications in active regions such as California.

Nonlinear Site Amplification as Function of 30 m Shear Wave Velocity

We develop empirical relationships to predict nonlinear (i.e., amplitude-dependant) amplification factors for 5% damped response spectral acceleration as a continuous function of average shear wave velocity in the upper 30 m, Vs-30. We evaluate amplification factors as residuals between spectral accelerations from recordings and modified rock attenuation relationships for active regions. Amplification at low- and mid-periods is shown to increase with decreasing Vs-30 and to exhibit nonlinearity that is dependent on Vs-30. The degree of nonlinearity is large for NEHRP Category E (Vs-30<180 m/s) but decreases rapidly with Vs-30, and is small for Vs-30>∼300 m/s. The results can be used as Vs-30-based site factors with attenuation relationships. The results also provide an independent check of site factors published in the NEHRP Provisions, and apparent bias in some of the existing NEHRP factors is identified. Moreover, the results provide evidence that data dispersion is dependent on Vs-30.

NGA-West2 Research Project

The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.

Prediction Equations for Significant Duration of Earthquake Ground Motions Considering Site and Near-Source Effects

For engineering systems having a potential for degradation under cyclic loading (e.g., landslides, soil profiles subject to liquefaction, some structural systems), the characterization of seismic demand should include the amplitude and duration of strong shaking within the system. This article is concerned with significant-duration parameters, which are defined as the time interval across which a specified amount of energy is dissipated (as measured by the integral of the square of the ground acceleration or velocity). We develop ground-motion prediction equations for significant-duration parameters as a function of magnitude, closest site-source distance, site parameters that reflect shallow geologic conditions as well as deep basin structure, and near-source parameters. The relations are developed using a modern database and a random-effects regression procedure. We find significant duration to increase with magnitude and site-source distance (effects that had been identified previously), but also to decrease with increasing shear-wave velocity of near-surface sediments and to increase with increasing basin depth. Parameters that principally measure the duration of body waves were also found to decrease in near-fault areas subject to forward rupture directivity, although such effects were not apparent for other duration parameters that tend to reflect the combined duration of body and surface waves.

Amplification Factors for Spectral Acceleration in Tectonically Active Regions

Empirical relationships are developed to predict amplification factors for 5% damped response spectral acceleration (period range T = 0.01-5 sec) as a function of site category. We evaluate amplification factors by normalizing response spectral accelerations computed from recordings by reference spectral accelerations derived from a modified attenuation relationship for active regions. Strong motion sites are categorized according to several schemes, which are based on surface geology (age-only, age + depositional environment, and age + material texture), shallow (30 m) shear-wave velocity, and geotechnical data. Criteria for selection of the optimal classification scheme are that the amplification models for categories within the scheme (1) minimize the global dispersion of prediction residuals and (2) are significantly distinct across a broad period range. The results of the regressions indicate that the greatest levels of distinction in amplification levels across categories occur for the shallow shear-wave velocity scheme, but that the dispersion of residuals for this scheme are relatively high for soil sites (although not for rock sites). Conversely, schemes based on detailed surface geology generally have the smallest dispersion for soil sites, but amplification levels across Quaternary categories are only significantly distinct at small periods. These findings suggest that detailed surface geology provides an effective means of soil site categorization at small periods, whereas shallow shear-wave velocity provides an effective means for rock site categorization at small periods. At longer periods, none of the schemes are optimized relative to both the dispersion and distinction criteria. The principal application of the amplification factors is as a modifier of attenuation relations (much like a site term). The present results significantly reduce the bias associated with the site terms in a widely used attenuation relationship for rock and soft soil site conditions. Manuscript received 25 February 2002.

System identification for evaluating soil–structure interaction effects in buildings from strong motion recordings

Parametric system identification is used to evaluate seismic soil–structure interaction effects in buildings. The input–output strong motion data pairs needed for evaluations of flexible- and fixed-base fundamental mode parameters are derived. Recordings of lateral free-field, foundation, and roof motions, as well as foundation rocking, are found to be necessary for direct evaluations of modal parameters for both cases of base fixity. For the common situation of missing free-field or base rocking motions, procedures are developed for estimating the modal parameters that cannot be directly evaluated. The accuracy of these estimation procedures for fundamental mode vibration period and damping is verified for eleven sites with complete instrumentation of the structure, foundation, and free-field.

Broadband simulations for Mw 7.8 southern San Andreas earthquakes: Ground motion sensitivity to rupture speed

Using the high-performance computing resources of the Southern California Earthquake Center, we simulate broadband (0–10 Hz) ground motions for three M_w 7.8 rupture scenarios of the southern San Andreas fault. The scenarios incorporate a kinematic rupture description with the average rupture speed along the large slip portions of the fault set at 0.96, 0.89, and 0.84 times the local shear wave velocity. Consistent with previous simulations, a southern hypocenter efficiently channels energy into the Los Angeles region along the string of basins south of the San Gabriel Mountains. However, we find the basin ground motion levels are quite sensitive to the prescribed rupture speed, with peak ground velocities at some sites varying by over a factor of two for variations in average rupture speed of about 15%. These results have important implications for estimating seismic hazards in Southern California and emphasize the need for improved understanding of earthquake rupture processes.

Semi-Empirical Nonlinear Site Amplification from NGA-West2 Data and Simulations

We analyze NGA-West2 data and simulations to develop a site amplification model that captures ground motion scaling with VS30 and soil nonlinear effects. We parameterize nonlinearity as the gradient of site amplification with respect to peak acceleration for reference (firm) sites. Both data analyses and simulations indicate nonlinearity for sites with VS30 < 500 m/s and spectral periods T < ∼3 s. Following approximate removal of nonlinear effects from the data, we evaluate VS30-scaling of ground motions, which is most pronounced for T ≥ ∼0.2 s and saturates for hard rock sites. Regional trends in VS30-scaling and nonlinearity were not found to be sufficiently robust to justify inclusion in our model. We apply the site amplification model to derive site factors now approved for building code applications. Principal causes of changes relative to previous values are reduction of the reference velocity (at which amplification is unity) to 760 m/s and reduced nonlinearity.