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  • Articles  (57)
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  • 1
    Publication Date: 2014-02-04
    Description: We develop an empirical model of the decay of Fourier amplitudes for earthquakes of M  3–6 recorded on rock sites in eastern North America and discuss its implications for source parameters. Attenuation at distances from 10 to 500 km may be adequately described using a bilinear model with a geometric spreading of 1/ R 1.3 to a transition distance of 50 km, with a geometric spreading of 1/ R 0.5 at greater distances. For low frequencies and distances less than 50 km, the effective geometric spreading given by the model is perturbed using a frequency- and hypocentral depth-dependent factor defined in such a way as to increase amplitudes at lower frequencies near the epicenter but leave the 1 km source amplitudes unchanged. The associated anelastic attenuation is determined for each event, with an average value being given by a regional quality factor of Q =525 f 0.45 . This model provides a match, on average, between the known seismic moment of events and the inferred low-frequency spectral amplitudes at R =1 km (obtained by correcting for the attenuation model). The inferred Brune stress parameters from the high-frequency source terms are about 600 bars (60 MPa), on average, for events of M 〉4.5.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2021-01-05
    Description: We propose a site-classification scheme based on the predominant period of the site, as determined from the average horizontal-to-vertical (H/V) spectral ratios of ground motion. Our scheme extends Zhao et al.(2006) classifications by adding two classes, the most important of which is defined by flat H/V ratios with amplitudes less than 2. The proposed classification is investigated by using 5%-damped response spectra from Italian earthquake records. We select a dataset of 602 three-component analog and digital recordings from 120 earthquakes recorded at 214 seismic stations within a hypocentral distance of 200 km. Selected events are in the moment-magnituderange 4.0 ≤ Mw ≤ 6.8 and focal depths from a few kilometers to 46 km. We computed H/V ratios for these data and used them to classify each site into one of six classes. We then investigate the impact of this classification scheme on empirical ground-motion prediction equations (GMPEs) by comparing its performance with that of the conventional rock/soil classification. Although the adopted approach results in only a small reduction of the overall standard deviation, the use of H/V spectral ratios in site classification does capture the signature of sites with flat frequency-response, as well as deep and shallow-soil profiles, characterized by long- and short-period resonance, respectively; in addition, the classification scheme is relatively quick and inexpensive, which is an advantage over schemes based on measurements of shear wave velocity.
    Description: Published
    Description: 12-36
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: open
    Keywords: site effects, site classification, predictive equations ; 04. Solid Earth::04.02. Exploration geophysics::04.02.06. Seismic methods
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 3
    Publication Date: 2019-11-04
    Description: JCR Journal
    Description: open
    Keywords: GSHAP ; Seismic hazard ; 04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 4
    Publication Date: 2013-06-08
    Description: The mainshock and moderate-magnitude aftershocks of the 6 April 2009 M  6.3 L’Aquila seismic sequence, about 90 km northeast of Rome, provided the first earthquake ground-motion recordings in the urban area of Rome. Before those recordings were obtained, the assessments of the seismic hazard in Rome were based on intensity observations and theoretical considerations. The L’Aquila recordings offer an unprecedented opportunity to calibrate the city response to central Apennine earthquakes—earthquakes that have been responsible for the largest damage to Rome in historical times. Using the data recorded in Rome in April 2009, we show that (1) published theoretical predictions of a 1 s resonance in the Tiber valley are confirmed by observations showing a significant amplitude increase in response spectra at that period, (2) the empirical soil-transfer functions inferred from spectral ratios are satisfactorily fit through 1D models using the available geological, geophysical, and laboratory data, but local variability can be large for individual events, (3) response spectra for the motions recorded in Rome from the L’Aquila earthquakes are significantly amplified in the radial component at periods near 1 s, even at a firm site on volcanic rocks, and (4) short-period response spectra are smaller than expected when compared to ground-motion predictions from equations based on a global dataset, whereas the observed response spectra are higher than expected for periods near 1 s. Online Material: Velocity models used in computing theoretical site response.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2016-04-07
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
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  • 6
    Publication Date: 2016-01-27
    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 .
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2014-10-04
    Description: The stochastic method of simulating ground motions requires the specification of the shape and scaling with magnitude of the source spectrum. The spectral models commonly used are either single-corner-frequency or double-corner-frequency models, but the latter have no flexibility to vary the high-frequency spectral levels for a specified seismic moment. Two generalized double-corner-frequency 2 source spectral models are introduced, one in which two spectra are multiplied together and another where they are added. Both models have a low-frequency dependence controlled by the seismic moment and a high-frequency spectral level controlled by the seismic moment and a stress parameter. A wide range of spectral shapes can be obtained from these generalized spectral models, which makes them suitable for inversions of data to obtain spectral models that can be used in ground-motion simulations in situations in which adequate data are not available for purely empirical determinations of ground motions, such as in stable continental regions. As an example of the use of the generalized source spectral models, data from up to 40 stations from seven events, plus response spectra at two distances and two magnitudes from recent ground-motion prediction equations, were inverted to obtain the parameters controlling the spectral shapes, as well as a finite-fault factor that is used in point-source, stochastic-method simulations of ground motion. The fits to the data are comparable to or even better than those from finite-fault simulations, even for sites close to large earthquakes.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2014-10-04
    Description: The stochastic method of ground-motion simulation assumes that the energy in a target spectrum is spread over a duration D T . D T is generally decomposed into the duration due to source effects ( D S ) and to path effects ( D P ). For the most commonly used source, seismological theory directly relates D S to the source corner frequency, accounting for the magnitude scaling of D T . In contrast, D P is related to propagation effects that are more difficult to represent by analytic equations based on the physics of the process. We are primarily motivated to revisit D T because the function currently employed by many implementations of the stochastic method for active tectonic regions underpredicts observed durations, leading to an overprediction of ground motions for a given target spectrum. Further, there is some inconsistency in the literature regarding which empirical duration corresponds to D T . Thus, we begin by clarifying the relationship between empirical durations and D T as used in the first author’s implementation of the stochastic method, and then we develop a new D P relationship. The new D P function gives significantly longer durations than in the previous D P function, but the relative contribution of D P to D T still diminishes with increasing magnitude. Thus, this correction is more important for small events or subfaults of larger events modeled with the stochastic finite-fault method.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
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  • 9
    Publication Date: 2015-04-01
    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.
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    Electronic ISSN: 1943-3573
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  • 10
    Publication Date: 2016-04-15
    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.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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