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  • 1
    Series available for loan
    Series available for loan
    Washington, DC : United States Gov. Print. Off.
    Associated volumes
    Call number: SR 90.0003(985)
    In: U.S. Geological Survey circular
    Type of Medium: Series available for loan
    Pages: IX, 52 S.
    Series Statement: U.S. Geological Survey circular 985
    Language: English
    Location: Lower compact magazine
    Branch Library: GFZ Library
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  • 2
    ISSN: 1420-9136
    Keywords: Crustal model ; Central U.S.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Earthquake investigations in the central United States require a realistic crustal model for accurate location of seismic events, for the determination of earthquake source parameters, and for theoretical ground motion prediction for an assumed earthquake source. A number of crustal models have been proposed which were based on first arrival refraction data obtained from artificial and natural seismic sources. The reflectivity method for generating realistic theoretical seismograms is used together with recently acquired refraction survey data to distinguish among several of the proposed crustal models. The data used in this study preclude a definitive statement about the nature of the lower crust in the region. However, a crust with two or more layers in the upper 20 km immediately below the basement is preferred. The absence of prominent near yertical reflections suggests that the layers may be separated by rapid velocity gradients rather than by sharp discontinuities.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 128 (1988), S. 7-42 
    ISSN: 1420-9136
    Keywords: Surface waves ; scattering ; layered media
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Malin's (1980) first-order single scattering theory has been extended to study the scattering of surface waves as well as body waves by distributed point scatterers in a layered medium. The scattered waveform itself is generated and examined instead of its energy envelope. The theory used allows 1) mode conversion, 2) wave type conversion, 3) finite scatterer distribution, and 4) the effect of attenuation from scattering as well as intrinsic absorption. The cases studied are for elastic or slightly attenuative media with any kind of source and receiver at any place in the layered structure. This direct calculation of coda waves provides us an immediate description of the relation of coda and scattering. The objectives are to find 1) the effect of layering on scattering, 2) the effect of scatterer distribution on recorded vertical and horizontal motion, 3) the relation of scatteringQ to intrinsicQ, 4) the scattering behavior of surface and body waves, and 5) the superposition of scattering waves to form the coda. The generation of body waves by ‘locked mode’ approximation, which makes the body-wave scattering a subset of the ‘surface-wave’, scattering. Preliminary results explain some observed coda behavior surprisingly well. We find a larger geometrical spreading for near scatterers, which is caused by mode conversion or wave type conversion because of the wide angle scattering. This makes the speading correction higher for early part of coda which may account for the lowQ observed in early coda of regional earthquakes. This study is of practical value as an effort to understand the complicated coda phases.
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  • 4
    Publication Date: 1997-12-01
    Description: Rayleigh waves generated by a 10-kg dynamite explosion were recorded along a dense, linear array of 47 seismic stations. The array had a total length of 1.4 km, and it was deployed on the sediments of the Po River floodplain near the city of Ferrara (Northern Italy). The recorded signals were dominated by fundamental-mode Rayleigh waves. Several techniques, including multiple-filter analysis, phase velocity stacking, and single-station cross-correlation, were used to obtain group and phase velocity dispersion from subsets for the data set. Synthetic seismograms were computed for one velocity structure, showing a good match to the recorded waveforms. Late Rayleigh-wave codas were characterized by persistent, monochromatic ringing, interpreted as due to surface waves scattered by weak lateral heterogeneities. Scattering within the alluvium of the great Po River Valley likely occurred across a reticular system of ancient, secondary river beds. Compared to the rest of the floodplain, these preferential directions of local drainage are characterized by different sedimentary environments. The availability of in situ, cross-hole measurements of shear-wave velocities allowed the comparison with our results from surface observations, demonstrating that the analysis of explosion-induced Rayleigh waves can be used for the geophysical characterization of flat-layered structures at a local scale for seismic engineering purposes.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 5
    Publication Date: 2000-08-01
    Description: We used broadband waveforms collected at short hypocentral distances (r〈 or =40 km) during the Umbria-Marche (Italy) seismic sequence of September-November, 1997, in order to calculate the scaling relationships for the ground motion within the meizoseismal area, in the 0.5-16.0 Hz frequency band. Data were collected by a 10-station portable seismic network deployed by the Istituto Nazionale di Geofisica (Rome, Italy) shortly after the occurrence of the first mainshock of the sequence, on 26 September 1997. Among the thousands of events recorded, we selected 142 earthquakes characterized by good signal-to-noise ratios at all frequencies, and by the absence of multiple shocks within the time window spanned by each recording. The data set of the selected waveforms was made of 2030 horizontal-component seismograms. The logarithm of the peak values of narrow bandpass-filtered versions of the velocity time histories are modeled at each frequency as AMP(f, r) = EXC(f, r (sub ref) ) + SITE(f) + D(r, r (sub ref) , f). EXC(f, r (sub ref) ) is the excitation term at an arbitrary reference hypocentral distance, r (sub ref) ; SITE(f) is a site term. The empirical attenuation functional, D(r, r (sub ref) , f), represents an estimate of the average crustal response for the region, at the hypocentral distance r, at the frequency f. It is modeled by using the following functional form: D(r, r (sub ref) , f) = log g(r) - log g(r (sub ref) ) pi f(r-r (sub ref) )/beta Q (sub 0) (f/f (sub ref) ) (super eta ) ; (f (sub ref) = 1.0 Hz, r (sub ref) = 10 km). g(r) = r (super -1) is the body-wave geometrical spreading function; beta = 3.5 km/sec is the shear-wave velocity in the crust. Due to the constraints applied to the system prior to the regressions, the excitation term represents the expected peak ground motion at the reference distance, as it would be observed at a site representative of the average site response of the network. The random vibration theory (RVT) is used to obtain a theoretical prediction of the attenuation functional. For reproducing D(r, r (sub ref) , f) we use the crustal attenuation parameter Q(f) = 130(f/f (sub ref) ) (super 0.10) obtained by Malagnini et al. (2000) from the analysis of a regional data set representative of the entire Apennines, in the (0.24-5.0 Hz) band. Two parameters are used to predict shapes and levels of the seismic spectra, the stress drop Delta sigma , and a high-frequency attenuation parameter kappa (sub 0) . The values used to reproduce the observed velocity spectra are Delta sigma = 200 bars; kappa (sub 0) = 0.04 sec. The indicated stress drop was estimated in this region by Castro et al. (2000), on recordings of the largest shock of the Umbria-Marche sequence.
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  • 6
    Publication Date: 2000-08-01
    Description: Regressions over a data set of broadband seismograms are performed to quantify the attenuation of the ground motion in the Apennines (Italy), in the 0.25-5.0 Hz frequency band. The data set used in this article consists of over 6000 horizontal-component seismograms from 446 events, with magnitude ranging from M (sub w) nearly equal 2 to M (sub w) = 6.0. Waveforms were collected during recent field experiments along the Apennines. Data from two MedNet broadband stations, located in central and southern Apennines, were also used. Seismograms are bandpass-filtered around a set of sampling frequencies, and the logarithms of their peak values are written as AMP(f, r)-EXC(f, r (sub ref) )+SITE(f)+D(r, r (sub ref) , f). EXC(f, r (sub ref) ) is the excitation term for the ground motion at the hypocentral distance r (sub ref) . SITE(f) represents the distortion of the seismic spectra induced by the shallow geology at the recording site. D(r, r (sub ref) , f) includes the effects of the geometrical spreading, g(r), and of a frequency-dependent crustal attenuation Q. It is determined as a piecewise linear function, allowing to consider complex behavior of the regional attenuation. A first estimate of D(r, r (sub ref) , f) is obtained using a coda normalization technique (Aki, 1980; Frankel et al., 1990) and used as a starting model in the inversion of the peak values. Then, by trial and error, the empirical D(r, r (sub ref) , f) is fitted using a trilinear geometrical spreading, with crossover distances at 30 and 80 km, and the crustal parameter Q(f) = 130 (f/f (sub ref) ) (super 0.10) ; f (sub ref) = 1.0 Hz. These results suggest a low-Q crust in the entire Apennines in the 0.25-5.0 Hz range, implying that the seismic hazard in the region may be dominated by the local seismicity. The final section is devoted to highlight the limitations of the formula proposed by Console et al. (1988) to estimate duration magnitudes M (sub d) in Italy.
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  • 7
    Publication Date: 2001-12-01
    Description: Distance scaling of earthquake-induced ground motion is studied in the Erzincan region, located in the eastern part of the North Anatolian Fault zone. The data set used in this study consists of 170 aftershocks of the M (sub s) = 6.8 Erzincan earthquake of 13 March 1992, with moment magnitudes between 1.5 and 4.0. In order to empirically obtain the scaling relationships for the high-frequency S-wave motion, regressions are carried out on 352 horizontal-component short-period seismograms, all recorded within a hypocentral distance of 40 km, to empirically obtain the scaling relationships for the high-frequency S-wave motion. Peak ground velocities are measured in selected narrow-frequency bands, in the frequency range of 1.0-16.0 Hz, and are subsequently regressed to define a piecewise linear attenuation function, a set of excitation terms, and a set of site terms. Results are modeled in the framework of random vibration theory, using a bilinear geometrical spreading function, g(r), characterized by a crossover distance at 25 km: g(r) = r (super -1.1) is used for r〈 or =25 km, whereas g(r) = r (super -0.5) is used for larger distances. An extremely low-quality factor, Q(f) = 40(f/f (sub ref) ) (super 0.45) , is used to describe the anelastic crustal attenuation in the region, consistently with the independent results of Akinci and Eyidogan (1996, 2000). Excitation terms are well matched by using a Brune spectral model with stress drop Delta sigma = 10 MPa (taken from the recent literature, Grosser et al., 1998). An effective high-frequency, distance-independent rolloff spectral parameter, kappa (sub eff) = 0.02 sec, is obtained in this study. Peak ground acceleration predictions based on these parameters show a much more rapid decrease with distance than the relations usually used in Turkey indicating that our results should only be applied to the Erzincan region itself.
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  • 8
    Publication Date: 2002-06-01
    Description: Cong et al. (2000) used P-wave dispersion from small earthquakes in the New Madrid Seismic Zone to estimate QP through the use of a continuous-relaxation model. This technique was used as an alternative to the usual spectral-slope techniques and employed an innovative step of comparing group delays relative to those at a reference frequency in order to reduce the bias in the dispersion due to errors in the assumed distance and velocity. The resultant Qm values showed an increasing trend with distance, which they interpreted as due to an increase of crustal Q with depth and the portion of the wave propagation through an active seismic zone...
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  • 9
    Publication Date: 2008-04-01
    Description: Large data sets of vertical and horizontal seismograms from the Pacific Northwest Seismic Network, Northern California Seismic Network, and Berkeley Digital Seismic Network are used to study the high-frequency (0.25-16 Hz) ground-motion scaling characteristics in Washington-Oregon, northern California (39 degrees N to 42 degrees N and 119 degrees W to 124 degrees W), and central California (35 degrees N to 39 degrees N and 118 degrees W to 123 degrees W). We used peak filtered ground velocities to characterize the propagation, excitation, and site terms. The regression results for propagation were modeled using a geometrical spreading function, g(r), and a frequency-dependent attenuation, Q(f) = Q (sub 0) f (super eta ) . For the Pacific Northwest, the best Q model that fits the observation is expressed by Q(f) = 280f (super 0.55) . The eastern central California and western central California results are parameterized with Q(f) = 280f (super 0.50) and Q(f) = 240f (super 0.35) , respectively. The northern California results are not easy to model and require using a frequency-dependent eta and a frequency-dependent geometrical spreading. The geometrical spreading effect for the frequencies higher than 5 Hz is very strong in that region. The excitation terms for the small events studied were modeled using a Brune's source model. An average stress drop of 30 bars was obtained for the Pacific Northwest. The northern California average value for stress drop is 90 bars. The observations of eastern central California were modeled with a stress drop of 49 bars, while the best fit for the western part required Delta sigma = 80 bars. The range of the values obtained for Q, g(r), and Delta sigma indicates that the ground-motion parameters for one region should not be used for another. As an example, our results show that the ground-motion amplitude due to an M (sub w) 5.0 earthquake at a distance of 50 km is different by a factor of 1.5 between the Pacific Northwest and eastern central California.
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  • 10
    Publication Date: 2007-06-01
    Description: By using small-to-moderate earthquakes located within approximately 200 km of San Francisco, we characterize the scaling of the ground motions for frequencies ranging between 0.25 and 20 Hz, obtaining results for geometric spreading, Q(f), and site parameters using the methods of Mayeda et al. (2005) and Malagnini et al. (2004). The results of the analysis show that, throughout the Bay Area, the average regional attenuation of the ground motion can be modeled with a bilinear geometric spreading function with a 30-km crossover distance, coupled to an anelastic function exp(-pi fr/[capital greek beta]Q(f), where: Q(f) = 180 f (super 0.42) . A body-wave geometric spreading, g(r) = r (super -1.0) , is used at short hypocentral distances (r〈30 km), whereas g(r) = r (super -0.6) fits the attenuation of the spectral amplitudes at hypocentral distances beyond the crossover. The frequency-dependent site effects at twelve of the Berkeley Digital Seismic Network stations were evaluated in an absolute sense using coda-derived source spectra. Our results show the following. (1) The absolute site response for frequencies ranging between 0.3 Hz and 2.0 Hz correlate with independent estimates of the local magnitude residuals (delta M (sub L) ) for each of the stations. (2) Moment magnitudes (M (sub w) ) derived from our path and site-corrected spectra are in excellent agreement with those independently derived using full-waveform modeling as well as coda-derived source spectra. (3) We use our weak-motion-based relationships to predict motions regionwide for the Loma Prieta earthquake, well above the maximum magnitude spanned by our data set, on a completely different set of stations. Results compare well with measurements taken at specific National Earthquake Hazards Reduction Program site classes. (4) An empirical, magnitude-dependent scaling was necessary for the Brune stress parameter to match the large-magnitude spectral accelerations and peak ground velocities with our weak-motion-based model.
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