An assessment of site amplification factors for the Western United States (original) (raw)
Related papers
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.
GeoHazards, 2021
Site amplification factors in National Building Codes are typically specified as a function of the average shear wave velocity over the first 30 m (Vs30) or site class (A, B, C, D and E) for defined ranges of Vs30 and/or ranges of depth to bedrock. However, a single set of amplification factors may not be representative of site conditions across the country, introducing a bias in seismic hazard and seismic risk analyses. This is exemplified by significant differences in geological settings between East and West coast locations in North America. Western sites are typically characterized by lower impedance contrasts between recent surface deposits and bedrock in comparison to Eastern sites. In North America, site amplification factors have been derived from a combination of field data on ground motions recorded during West Coast earthquakes and numerical models of site responses that are meant to be representative of a wide variety of soil profiles and ground motions. The bias on ampl...
Open File Report, 2000
We estimate site amplification at the location of a proposed bridge near Charleston, South Carolina. Model calculations indicate that amplification at periods of 1 s and longer is likely to be strongly influenced by the effects of a large contrast in shear-wave velocity at a depth of approximately 1 km (3,000 ft). On-site borehole data, regional geological and geophysical information, and data from a geologically similar setting near Memphis, Tennessee allowed us to estimate profiles of shear-wave velocity, shear-wave attenuation, and density from ground level down to metamorphic and igneous rocks that are approximately 3 km (9,500 ft) beneath the site. We modeled amplifications that would be produced at the surface and at the top and bottom of the Cooper Marl. Amplification estimates that are based only on the shallow shear-wave structure, for example in the upper 100 m (300 ft), can severely underestimate long-period amplification at the site. Additional modeling could help determine whether new data should be collected, to resolve remaining uncertainties about likely amplification.
Bulletin of the Seismological Society of America, 1987
Strong ground motion attenuation relations are usually described by smoothly decreasing functions of distance. However, consideration of wave propagation in the crust suggests that attenuation relations should be more complex. Such complexity may be present in strong ground motion data for eastern North American earthquakes, which show amplitudes in the distance range of 60 to 150 km that lie above the trends at smaller and greater distances. Using a wavenumber integration method to compute Green's functions and close-in recordings of several earthquakes as empirical source functions, we have generated synthetic seismograms that are in good agreement with regional and strong-motion recordings of eastern North American earthquakes. From these synthetic seismograms, we have shown that the observed interval of relatively high amplitudes may be attributable to postcritically reflected S waves from the Moho. The presence and location of the interval of relatively high amplitudes is h...
2006
The "Next Generation of Ground Motion Attenuation Models" (NGA) project is a partnered research program conducted by Pacific Earthquake Engineering Research Center-Lifelines Program (PEER-LL), U.S. Geological Survey (USGS), and Southern California Earthquake Center (SCEC). The project has the objective of developing updated ground motion attenuation relationships through a comprehensive and highly interactive research program. Five sets of updated attenuation relationships are developed by teams working independently but interacting throughout the development process. The main technical issues being addressed by the NGA teams include magnitude scaling at close-in distances, directivity effects, polarization of near-field motion (faultstrike-normal component vs. fault-strike-parallel component), nonlinear amplification by shallow soil, and sedimentary basin amplification. The attenuation relationships development is also facilitated by the development of an updated and expanded database of recorded ground motions; conduct of supporting research projects to provide constraints on the selected functional forms of the attenuation relationships; and a program of interactions throughout the development process to provide input and reviews from both the scientific research community and the engineering user community. An overview of the NGA project components, process, and products developed by the project is presented in this paper.
Soil Dynamics and Earthquake Engineering, 2018
The impact of unconsolidated sedimentary basins on ground motion amplification is of particular interest for earthquake engineers and seismologists. The Mississippi embayment (ME) of the New Madrid seismic zone, located in the central United States, is covered with a thick layer of unconsolidated soil deposits. Thus, the estimation of site response in this region is vital to simulate site-specific ground motions and to conduct sitespecific probabilistic seismic hazard analysis. We evaluated site amplification at 11 stations within the ME, employing the horizontal-to-vertical spectral ratio (HVSR) technique. Regarding the results obtained from this study, weak ground motions recorded by stations on the unconsolidated ME sediments are amplified 3-7 times for frequencies less than 5 Hz compared to stations located on bedrock. The fundamental resonant frequencies vary from 0.2 to 0.4 Hz within the ME. We investigated differences between the HVSRs obtained from P-waves, S-waves, coda, and pre-event noise. All fundamental frequencies obtained from different seismic phases are in good agreement with a less than 10% difference. The fundamental frequencies of the P-wave and S-wave are relatively higher due to higher velocity of the P-wave and S-wave compared to other phases since the velocity of seismic waves and fundamental frequencies are proportional. There is a good correlation between the HVSR of the S-wave, coda, and pre-event noise portions for frequencies more than 4 Hz. For the frequencies less than 4 Hz, the HVSR of the S-wave is higher than the HVSR of coda by a factor of 3. Reflections of S-waves from the edges of the unconsolidated sedimentary basin of the ME produce surface waves. The presence of basin-induced surface waves in the coda portion for frequencies less than 3 Hz results in increased amplitude of coda, and as a result, slower decay rate with time implying higher Q values. These basin-induced surface waves have a period of 0.5-4.0 s.
Engineering Geology, 2001
Local soil conditions have a profound in¯uence on the characteristics of ground shaking during an earthquake. Exceptionally deep soil deposits, on the order of 100±1000 m deep, are found in the Upper Mississippi Embayment of the central United States. Shear waves (SH) from earthquakes in the New Madrid seismic zone are expected to be strongly affected by the sharp impedance contrasts at the bedrock/sediment interface, attenuation of seismic waves in the soil column, and the SH-wave velocities of the more poorly consolidated near-surface (#50 m) soils.
Seismic Wave Amplification in Las Vegas: Site Response and Empirical Estimates of Ground Motion
2004
This presentation will summarize a multidisciplinary effort to understand seismic wave amplification in Las Vegas Valley. The project involves weak motion recording and analysis, geotechnical and seismic refraction field studies, geologic and lithologic interpretation and model building. We will provide a brief overview of the project, then focus on specifics of seismic wave amplification including observations and interpretations. We analyzed recordings of nuclear explosions from the Nevada Test Site (NTS) and regional earthquakes to estimate site response in Las Vegas. An empirical transfer function method was used to transform ground motion time-series at one (reference) station to other stations, using frequency dependent site response curves in the band 0.2-5.0 Hz. The method transforms the time-series to the frequency domain by Fast Fourier transform, multiplies the amplitude spectrum by the site response curve and inverse FFT's back to the time domain. The approach is va...
Soil Dynamics and Earthquake Engineering, 2000
Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 M L 6:4: In the epicentral area, when the peak ground velocities are larger than v m Ϸ 15 cm=s; nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion v m Ͼ 30 cm=s; the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses. ᭧