ALBERT

Description: We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)-West2 project. A third model is based on a near-surface velocity profile developed from the NGA-West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation ( 0 ) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013 , 2016, unpublished manuscript, see Data and Resources ) and Yenier and Atkinson (2015) , the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty.

Description: This short note contains two contributions related to deriving depth-dependent velocity and density models for use in computing generic crustal amplifications. The first contribution is a method for interpolating two velocity profiles to obtain a third profile with a time-averaged velocity to depth Z that is equal to a specified value (e.g., for shear-wave velocity V S , for Z =30 m, in which the subscript S has been added to indicate that the average is for shear-wave velocities). The second contribution is a procedure for obtaining densities from V S . The first contribution is used to extend and revise the Boore and Joyner (1997) generic rock V S model, for which , to a model with the more common . This new model is then used with the densities from the second contribution to compute crustal amplifications for a generic site with .

Description: The stochastic method of ground-motion simulation specifies the amplitude spectrum as a function of magnitude ( M ) and distance ( R ). The manner in which the amplitude spectrum varies with M and R depends on physical-based parameters that are often constrained by recorded motions for a particular region (e.g., stress parameter, geometrical spreading, quality factor, and crustal amplifications), which we refer to as the seismological model. The remaining ingredient for the stochastic method is the ground-motion duration. Although the duration obviously affects the character of the ground motion in the time domain, it also significantly affects the response of a single-degree-of-freedom oscillator. Recently published updates to the stochastic method include a new generalized double-corner-frequency source model, a new finite-fault correction, a new parameterization of duration, and a new duration model for active crustal regions. In this article, we augment these updates with a new crustal amplification model and a new duration model for stable continental regions. Random-vibration theory (RVT) provides a computationally efficient method to compute the peak oscillator response directly from the ground-motion amplitude spectrum and duration. Because the correction factor used to account for the nonstationarity of the ground motion depends on the ground-motion amplitude spectrum and duration, we also present new RVT correction factors for both active and stable regions. Online Material: Files of coefficients for evaluating distance ( D rms ), time-domain–to–random-vibration ratios, and SMSIM parameter files.

Description: We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)-West2 project. A third model is based on a near-surface velocity profile developed from the NGA-West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation ( 0 ) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013 , 2016, unpublished manuscript, see Data and Resources ) and Yenier and Atkinson (2015) , the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty.

Description: Adjustment factors are provided for converting ground-motion intensity measures between central and eastern North America (CENA) sites with different reference-rock site conditions ( V S 30 =760, 2000, and 3000 m/s) for moment magnitudes ranging from 2 to 8, rupture distances ranging from 2 to 1200 km, Fourier amplitude spectra (FAS) for frequencies ranging from 0.01 to 100 Hz, response spectra for periods ranging from 0.01 to 10.0 s, peak ground acceleration, and peak ground velocity. The adjustment factors are given for a wide range of the site diminution parameters ( 0 ) for sites with V S 30 =760 m/s and for a 0 of 0.006 s for two harder rock sites. Fourteen CENA velocity profiles with V S 30 values within a factor of 1.1 of 760 m/s were used to derive average FAS amplification factors as a function of frequency, which were then used in simulations of peak ground-motion parameters and response spectra to derive the adjustment factors. The amplification function differs from that used in western North America (e.g., Campbell and Boore, 2016 ) in having a peak near 9 Hz, due to the resonance of motions in the relatively thin low-velocity material over hard rock that characterizes many CENA sites with V S 30 near 760 m/s. We call these B/C sites, because this velocity marks the boundary between National Earthquake Hazards Reduction Program site classes B and C ( Building Seismic Safety Council, 2004 ). The adjustments for short-period motions are sensitive to the value of 0 , but there are very few if any determinations of 0 for CENA B/C sites. For this reason, we determined 0 from multiple recordings at Pinyon Flat Observatory (PFO), California, which has a velocity-depth profile similar to those of CENA B/C sites. The PFO and other results from the literature suggest that appropriate values of 0 for CENA B/C sites are expected to lie between 0.01 and 0.03 s. Electronic Supplement: Zip files with parameters used by Stochastic-Method SIMulation (SMSIM) and ratios of the ground-motion intensity measures between hard-rock sites and National Earthquake Hazards Reduction Program (NEHRP) B/C sites.

Description: Various measures using the two horizontal components of recorded ground motions have been used in a number of studies that derive ground-motion prediction equations and construct maps of shaking intensity. We update relations between a number of these measures, including those in Boore et al. (2006) and Boore (2010) , using the large and carefully constructed global database of ground motions from crustal earthquakes in active tectonic regions developed as part of the Pacific Earthquake Engineering Research Center–Next Generation Attenuation-West2 project. The ratios from the expanded datasets generally agree to within a few percent of the previously published ratios. We also provide some ratios that were not considered before, some of which will be useful in applications such as constructing ShakeMaps. Finally, we compare two important ratios with those from a large central and eastern North American database and from many records from subduction earthquakes in Japan and Taiwan. In general, the ratios from these regions are within several percent of those from crustal earthquakes in active tectonic regions. Electronic Supplement: Figures of ground-motion intensity measure (GMIM) ratios, and csv files with average ratios and the coefficients of fits to the ratios.