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.
We develop prediction equations for the median and standard deviation of the significant duration of earthquake ground motions from shallow crustal earthquakes in active tectonic regions. We consider ...significant duration parameters for 5–75%, 5–95%, and 20–80% of the normalized Arias intensity. The equations were derived from a global database with M 3.0–7.9 events. We find significant noise effects on duration parameters that compel us to exclude some records that had been used previously to develop models for amplitude parameters. Our equations include an M-dependent source duration term that also depends on focal mechanism. At small M, the data suggest approximately M-independent source durations that are close to 1 sec. The increase of source durations with M is slower over the range ∼5 to 7.2–7.4 than for larger magnitudes. We adopt an additive path term with breaks in distance scaling at 10 km and 50 km. We include site terms that increase duration for decreasing VS30 and increasing basin depth. Our aleatory variability model captures decreasing between- and within-event standard deviation terms with increasing M.
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.
AbstractOne-dimensional ground response analyses (GRAs) are often used with an expectation that they provide unbiased estimates of site effects. Under that hypothesis, epistemic uncertainty in site ...response arises mainly from uncertain soil properties. This approach has dominated practice for projects where site-specific site response is estimated for use in probabilistic seismic hazard analyses. We extend the uncertainty framework to also consider modeling errors (i.e., inability of GRAs to model site response for some sites). We quantify this epistemic uncertainty using vertical array data in which the downhole motion is input to GRAs to predict ground motions at the surface. Residuals (i.e., difference between observed and predicted ground motion intensity measures in natural log units) are partitioned into between- and within-site components. After correcting for overprediction bias near the site period, we quantify epistemic uncertainty using between-site standard deviation, which ranges from 0.2 to 0.35 using California data. A Japan data set analyzed in the literature provides similar results. This dispersion is only modestly smaller than the site-to-site variability from ergodic models for active tectonic regions, which limits the apparent benefits of site-specific GRAs. Dispersion results are not appreciably affected by varying damping models, although a model informed by site-specific observations minimizes bias relative to alternative models based on geotechnical laboratory tests and seismological crustal attenuation studies.
NGA-West2 site database Seyhan, Emel; Stewart, Jonathan P; Ancheta, Timothy D ...
Earthquake Spectra,
08/2014, Letnik:
30, Številka:
3
Journal Article, Book Chapter
Recenzirano
The NGA-West2 site database (SDB) contains information on site condition and instrument housing for 4,147 strong-motion stations with recordings in the project flatfile. The stations are from active ...tectonic regions, mainly in California, Japan, Taiwan, China, and the Mediterranean area. The principal site parameter is the time-averaged shear wave velocity in the upper 30 m (VS30), which we characterize using measurements where available (2,013 stations) and proxy-based relationships otherwise. We also provide basin depths from published models for 2,761 sites mostly in California and Japan. We improved the documentation and consistency of site descriptors used as proxies for VS30 estimation (surface geology, ground slope, and geotechnical or geomorphic categories) and analyzed proxy performance relative to VS30 values from measurements. We present protocols for VS30 estimation from proxies that emphasize methods minimizing bias and dispersion relative to data. For each site, we provide the preferred VS30 and its dispersion.
Probabilistic seismic hazard analyses are usually performed with semi-empirical ground motion models (GMMs) following the ergodic assumption whereby average source, path, and site effects from global ...databases apply for a specific site of interest. Site-specific site response is likely to differ from the global average conditional on site parameters used in GMMs (typically VS30 and basin depth). Non-ergodic site response can be evaluated using on-site ground motion recordings and/or one-dimensional wave propagation analyses, and allows site-to-site variability to be removed from the within-event standard deviation. Relative to ergodic, non-ergodic hazard analyses often reduce ground motions at long return periods. We describe procedures for replacing the site term in GMMs with a non-ergodic nonlinear mean over its appropriate range of periods (returning to the ergodic mean outside that range). We also present procedures for computing non-ergodic standard deviation by removing site-to-site variability while considering effects of soil nonlinearity. We illustrate application of these procedures, and their effect on hazard curves and uniform hazard spectra, as implemented in OpenSHA.
NGA-West2 database Ancheta, Timothy D; Darragh, Robert B; Stewart, Jonathan P ...
Earthquake spectra,
08/2014, Letnik:
30, Številka:
3
Journal Article
Recenzirano
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.
AbstractCommonly used simplified one-dimensional nonlinear seismic site response analyses employ constitutive models based on a variation of the hyperbolic model to represent the initial ...stress-strain backbone curve. Desirable features of the backbone curve include provision of (1) an initial shear modulus at zero shear strain, (2) a limiting shear stress at large shear strains, and (3) flexible control of the nonlinear behavior between those boundary conditions. Available hyperbolic models have combinations of two of these features. A new general quadratic/hyperbolic (GQ/H) model is developed from the bivariate quadratic equation to provide all desired features. Nonlinear behavior is controlled by a shear-strain-dependent curve-fitting function. The model’s unload-reload rules and coupling with pore-water pressure generation are also presented. Several total-stress site response analyses are presented to demonstrate the performance of the GQ/H model relative to a commonly used hyperbolic model in which the maximum shear stress cannot be defined. The analyses show the importance of properly representing the maximum shear stress in the constitutive model because it may lead to underestimation or overestimation of the computed site response.
AbstractStrong ground motions from the Mw=6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array at a nuclear power plant. Site conditions consist of about 70 m of ...medium-dense sands overlying clayey bedrock, with groundwater located at 45 m. Ground shaking at the bedrock level had a geometric mean peak acceleration of 0.55g, which reduced to 0.4g at the ground surface, indicating nonlinear site response. One-dimensional ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification levels, provided small-strain damping levels somewhat larger than those from laboratory tests are applied. Nonlinear ground response analyses of strong-motion data using laboratory-based modulus reduction and damping relations valid up to moderate strain levels (<∼0.5%) produce unrealistic strain localization at a velocity contrast. A procedure is presented to more realistically represent the large-strain portion of backbone curves by asymptotically approaching the shear strength at large strains, which removes strain localization for this application and provides reasonable matches of observed and computed ground motions.
We present an ergodic site response model with regional adjustments for use with subduction zone ground-motion models. The model predicts site amplification of peak ground acceleration, peak ground ...velocity, and 5% damped pseudo-spectral accelerations of the orientation-independent horizonal component for oscillator periods from 0.01 to 10 s. The model depends on the time-averaged shear-wave velocity in the upper 30 m (VS30), basin depth, and region and is independent of subduction earthquake type. It has three components: a linear site-amplification term in the form of VS30-scaling, a nonlinear term that depends on VS30 and shaking intensity parameterized by peak ground acceleration at the reference-rock velocity condition of 760 m/s, and a basin sediment-depth term for Japan and Cascadia conditioned on the depth to the 2.5 km/s shear-wave velocity isosurface (Z2.5). A global VS30-scaling model is provided along with regional adjustments for Japan, Taiwan, South America, Alaska, and Cascadia. The nonlinear model is global, with a functional form that has often been used to fit nonlinear responses inferred from simulations, but here we calibrate it empirically. Relative to a prior model for shallow earthquakes in active tectonic regions, our subduction zone global VS30-scaling is comparable at short periods (<1.0 s) but weaker at long periods, while the nonlinear site response is generally less pronounced but extends to lower levels of shaking. Basin depth models are conditioned on the difference of the actual Z2.5 and a VS30-conditioned mean Z2.5. Sites with positive differential depths have increased long-period site responses and decreased short-period responses, with the opposite occurring for negative differential depths.
