Reduction of Bias and Uncertainty in Regional Seismic Site Amplification Factors for Seismic Hazard and Risk Analysis (original) (raw)
Related papers
Probabilistic Seismic Hazard Estimates Incorporating Site Effects--An Example from Indiana, U.S.A
Environmental and Engineering Geoscience, 2010
The U.S. Geological Survey (USGS) has published probabilistic earthquake hazard maps for the United States based on current knowledge of past earthquake activity and geological constraints on earthquake potential. These maps for the central and eastern United States assume standard site conditions with Swave velocities of 760 m/s in the top 30 m. For urban and infrastructure planning and long-term budgeting, the public is interested in similar probabilistic seismic hazard maps that take into account near-surface geological materials. We have implemented a probabilistic method for incorporating site effects into the USGS seismic hazard analysis that takes into account the first-order effects of the surface geologic conditions. The thicknesses of sediments, which play a large role in amplification, were derived from a P-wave refraction database with over 13,000 profiles, and a preliminary geology-based velocity model was constructed from available information on S-wave velocities. An interesting feature of the preliminary hazard maps incorporating site effects is the approximate factor of two increases in the 1-Hz spectral acceleration with 2 percent probability of exceedance in 50 years for parts of the greater Indianapolis metropolitan region and surrounding parts of central Indiana. This effect is primarily due to the relatively thick sequence of sediments infilling ancient bedrock topography that has been deposited since the Pleistocene Epoch. As expected, the Late Pleistocene and Holocene depositional systems of the Wabash and Ohio Rivers produce additional amplification in the southwestern part of Indiana. Ground motions decrease, as would be expected, toward the bedrock units in south-central Indiana, where motions are significantly lower than the values on the USGS maps.
Factors influencing soil surface seismic hazard curves
Soil Dynamics and Earthquake Engineering, 2016
Performance-based seismic design of important structures requires design ground motions from probabilistic seismic hazard analysis (PSHA) that incorporate the effects of local site conditions. Seismic hazard curves incorporating site-specific soil conditions can be generated through the convolution of rock hazard curves with statistical models for site-specific ground motion amplification factors (AF). The AF relationships are developed from a series of site response analyses. The goal of this study is to evaluate how the AF relationships and the resulting surface hazard curves are influenced by different approaches in the site response analysis, specifically the time series (TS) vs. random vibration theory (RVT) approaches, and by different levels of shear wave velocity variability introduced in the site response analysis. The results show that the median AF relationships derived from TS and RVT analyses are similar, except at periods near the site period, where RVT analysis may predict larger AF. Including the effect of shear wave velocity variability reduces the median AF and increases the standard deviation associated with the AF relationship (σ lnAF ). Generally, the soil hazard curve derived by the AF relationship with the largest σ lnAF generates the largest ground motions, and this effect is most significant at small annual frequencies of exceedance. The effect of σ lnAF on soil hazard curves is larger than the effect of different median AF relationships. The value of σ lnAF is influenced significantly by the variability in the shear wave velocity and therefore proper specification of this variability is critical when developing soil hazard curves.
Impediments to Predicting Site Response: Seismic Property Estimation and Modeling Simplifications
Bulletin of the Seismological Society of America, 2009
We compare estimates of the empirical transfer function (ETF) to the plane SH-wave theoretical transfer function (TTF) within a laterally constant medium for invasive and noninvasive estimates of the seismic shear-wave slownesses at 13 Kiban-Kyoshin network stations throughout Japan. The difference between the ETF and either of the TTFs is substantially larger than the difference between the two TTFs computed from different estimates of the seismic properties. We show that the plane SH-wave TTF through a laterally homogeneous medium at vertical incidence inadequately models observed amplifications at most sites for both slowness estimates, obtained via downhole measurements and the spectral analysis of surface waves. Strategies to improve the predictions can be separated into two broad categories: improving the measurement of soil properties and improving the theory that maps the 1D soil profile onto spectral amplification. Using an example site where the 1D plane SH-wave formulation poorly predicts the ETF, we find a more satisfactory fit to the ETF by modeling the full wavefield and incorporating spatially correlated variability of the seismic properties. We conclude that our ability to model the observed site response transfer function is limited largely by the assumptions of the theoretical formulation rather than the uncertainty of the soil property estimates.
Bulletin of the Seismological Society of America, 2011
Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811-1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismichazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5g, values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m=s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.
1999
Site-specific amplification factors, Fa and Fv used in current US building codes are dependent on amplitude of the input motion. The Northridge earthquake of January 17, 1994 provided a large set of in-situ recordings of input “base” motions up to about 0.5g. Extensive sets of borehole geotechnical data, collected at many of these sites since the earthquake, provide an improved basis to reexamine the dependency of site specific amplification factors on input base shaking level. This paper summarizes recent results for this dependency as implied by empirical amplification factors inferred with respect to nearby “rock” stations underlain by granite. Preliminary results suggest that the site factors are in good agreement with those suggested for the code provisions at the 0.1 g base acceleration level. At successively higher input ground-motion levels the preliminary empirical estimates do not show a well-defined tendency to decrease with increasing amplitude. The detailed geotechnical...
2012
The time-averaged shear wave velocity in the upper 30 m of a site (V s30) is the most common site parameter used in ground motion prediction equations for the evaluation of seismic site response. It is often the case that V s30 is not available at sites with earthquake recordings; for example in the NGA-East site database only 45 of 1149 sites have measured values of V s30. Accordingly, estimates of V s30 are often made on the basis of available proxies that are widely available such as ground slope, geomorphic terrain categories, and surface geology. We compile a database of 1930 measured and inferred V s30 values in Central and Eastern North America (CENA) to test slope and geomorphology-based proxy methods. The results indicate that these existing proxy methods are biased for sites with V s30 greater than 400 m/s. Based on a careful review of geological conditions in the CENA, we propose nineteen geologic classes based on setting (i.e., glaciated or non-glaciated), age, and depositional environmental that can form the basis for geology-based proxy estimates of V s30 as well as for simplified stratigraphic columns.
The 2018 update of the US National Seismic Hazard Model: Overview of model and implications
Earthquake Spectra, 2019
During 2017–2018, the National Seismic Hazard Model for the conterminous United States was updated as follows: (1) an updated seismicity catalog was incorporated, which includes new earthquakes that occurred from 2013 to 2017; (2) in the central and eastern United States (CEUS), new ground motion models were updated that incorporate updated median estimates, modified assessments of the associated epistemic uncertainties and aleatory variabilities, and new soil amplification factors; (3) in the western United States (WUS), amplified shaking estimates of long-period ground motions at sites overlying deep sedimentary basins in the Los Angeles, San Francisco, Seattle, and Salt Lake City areas were incorporated; and (4) in the conterminous United States, seismic hazard is calculated for 22 periods (from 0.01 to 10 s) and 8 uniform VS30 maps (ranging from 1500 to 150 m/s). We also include a description of updated computer codes and modeling details. Results show increased ground shaking i...
Site-specific seismic hazard analysis
Probabilistic seismic hazard analysis has traditionally been calculated using rock conditions and modifying the rock hazard results using deterministic site-specific amplification factors.
An assessment of site amplification factors for the Western United States
The Next Generation Attenuation (NGA) ground motion relationships enable an assessment of the site class coefficients presented in the 2003 NEHRP Provisions and ASCE-7-05 for the Western United States. A site amplification study is performed using the three NGA relationships used by the USGS to update their seismic hazard maps for the Western United States. The average NGA site amplification factors show a clear dependency on period for average shear-wave velocity smaller than 270 m/s; can vary significantly within a site class (D or E) for a given bedrock spectral intensity in the mid-and long-period ranges; and are substantially greater than the current NEHRP/ASCE-7 site class coefficients in some cases.