We describe a technique to derive first-order site condition maps directly from topographic data. For calibration, we use global 30 arc sec topographic data and V S 30 measurements (here V S 30 , refers to the average shear-velocity down to 30 m) aggregated from several studies in the U.S., as well as in Taiwan, Italy, and Australia. V S 30 values are correlated against topographic slope to develop two sets of parameters for deriving V S 30 : one for active tectonic regions where topographic relief is high, and one for stable shields where topography is more subdued. By taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear-velocity observations, we can recover, to first order, many of the spatially varying features of sitecondition maps developed for California. Our site-condition map for the low-relief Mississippi Embayment also predicts the bulk of the V S 30 observations in that region despite rather low slope ranges. We find that maps derived from the slope of the topography is often well correlated with other independently-derived, regional-scale site-condition maps, but the latter maps vary in quality and continuity, and subsequently, also in their ability to match observed V S 30 measurements contained therein. Alternatively, the slope-based method provides a simple approach to uniform site condition mapping.

Topographic Slope as a Proxy for Seismic Site Conditions and Amplification

We describe a technique to derive first-order site-condition maps di- rectly from topographic data. For calibration, we use global 30 arc sec topographic data and VS 30 measurements (here VS 30 refers to the average shear-velocity down to 30 m) aggregated from several studies in the United States, as well as in Taiwan, Italy, and Australia. VS 30 values are correlated against topographic slope to develop two sets of parameters for deriving VS 30 : one for active tectonic regions where to- pographic relief is high, and one for stable shields where topography ismoresubdued. By taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear-velocity observations, we can recover, to first order, many of the spatially varying features of site-condition maps developed for California. Our site-condition map for the low-relief Mississippi Embayment also predicts the bulk of the VS 30 observations in that region despite rather low slope ranges. We find that maps derived from the slope of the topography are often well cor- related with other independently derived, regional-scale site-condition maps, but the latter maps vary in quality and continuity, and subsequently, also in their ability to match observed V S 30 measurements contained therein. Alternatively, the slope-based method provides a simple approach to uniform site-condition mapping. After validating this approach in regions with numerous V S 30 observations, we subsequently estimate and map site conditions for the entire continental United States using the respective slope correlations.

Review Article Topographic Slope as a Proxy for Seismic Site Conditions and Amplification

Earthquake losses due to ground failure

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.

/pdf/prediction-equations-for-significant-duration-of-earthquake-3vsq7piajx.pdf

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

The average velocity to 30 m (V s (30)) is a widely used parameter for classifying sites to predict their potential to amplify seismic shaking. In many cases, however, models of shallow shear-wave velocities, from which V s(30) can be com- puted, do not extend to 30 m. If the data for these cases are to be used, some method of extrapolating the velocities must be devised. Four methods for doing this are described here and are illustrated using data from 135 boreholes in California for which the velocity model extends to at least 30 m. Methods using correlations be- tween shallow velocity and V s(30) result in significantly less bias for shallow models than the simplest method of assuming that the lowermost velocity extends to 30 m. In addition, for all methods the percent of sites misclassified is generally less than 10% and falls to negligible values for velocity models extending to at least 25 m. Although the methods using correlations do a better job on average of estimating V s(30), the simplest method will generally result in a lower value of V s(30) and thus yield a more conservative estimate of ground motion (which generally increases as V s(30) decreases).

Estimating V̄s(30) (or NEHRP Site Classes) from Shallow Velocity Models (Depths < 30 m)

Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude‐dependent site amplification factors are specified: Fa for short periods and Fv for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯s). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A ‐ E, replace site classes in previous codes denoted by S1 ‐ S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / ver...

New Site Coefficients and Site Classification System Used in Recent Building Seismic Code Provisions

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New Site Coefficients and Site Classification System Used in Recent Building Seismic Code Provisions