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  • 2000-2004  (5)
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  • 1
  • 2
    Publication Date: 2003-12-01
    Description: P and S body waves from microearthquakes in the New Madrid Seismic Zone (NMSZ) are investigated at selected sites in an effort to understand wave propagation from future large earthquakes. Earthquake body waveforms display distinctive features that constrain the nature of P- and S-wave local site responses and wave propagation within the unconsolidated Mississippi embayment sediments. Modeling of the waveforms demonstrates that a near- surface low-velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large P- and S-wave resonance effects that can be used to infer near-site conditions. Site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies. Site resonance affects P- and S- wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters. Travel times of observed body-wave phases such as P, PpPhp (the first P-wave reverberation within the entire sedimentary column), Ps, Sp, S, and SsPhp can be used to estimate the average wave slownesses and Poisson's ratio within the embayment sediments; an average Poisson's ratio of 0.44 is obtained for the central NMSZ under station PEBM. Detailed S-wave velocities are derived for a Nafe-Drake sediment model using acoustic well logs and the travel-time constraints of observed seismic phases. V (sub p) /V (sub s) ratios vary from 5.5 near the surface to approximately 2.4 at the base of the sediments. Use of the well-log data in wave calculations also explains much of the nature of P- and S-wave coda within the waveforms and shows that 1D heterogeneity is a first-order influence on seismic-wave propagation within the Mississippi embayment.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2003-12-01
    Description: Elastic, near-surface structure and receiver depth effects are seen to control body-wave amplitude spectra in the band from 2 to 50 Hz for local earthquake observations in the New Madrid Seismic Zone. P- and S-wave spectral nulls are created by interference of direct and free-surface reflections due to receiver depth. These wave propagation effects dominate Sp/S spectral ratios and mimic anelastic attenuation effects by making it appear that S waves are deficient in higher frequencies relative to Sp waves. There is no evidence from Sp/S spectral ratios that unconsolidated sediments of the Mississippi embayment are highly attenuating. Dominance of P- and S-wave resonance suggests that the unconsolidated sediments amplify rather than attenuate seismic waves at high frequency. This has important implications for estimating hazards from strong ground motions in the area. In the absence of any identifiable anelastic attenuation effect in the spectral ratio, the only allowable Q (sub p) -Q (sub s) relationship for embayment sediments is that Q (sub s) = 3Q (sub p) , based on consideration of wave travel times.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 4
    Publication Date: 2001-12-01
    Description: The vertical component P-wave receiver function is an important source of data in studies of the crust/mantle transfer function for determining Earth structure under isolated receivers or under receiver arrays. This waveform illuminates a missing aspect of the wave propagation in receiver function studies that employ only the horizontal components of motion, and yields complementary constraints on near receiver heterogeneity and P-wave propagation. The vertical component P-wave receiver function is formed using an array estimate for the effective teleseismic source function that is then deconvolved from all the vertical and horizontal components of ground motion at each station in the array. One-dimensional, three-dimensional, and stochastic wave-propagation models are used to test the robustness of the technique. Breakdown of single-station receiver function deconvolution occurs because of high levels of noncorrelated noise between the ground-motion components. Receiver functions for stations of the southern California TERRAscope array are investigated using the array technique. Vertical receiver functions for stations in the Los Angeles Basin and Long Valley Caldera show high-amplitude secondary arrivals that cannot be explained by simple 1D structures but probably reflect wave propagation in 3D basin structures. Three-component receiver functions from the station at Mammoth Lakes, California, show pathological behavior where the horizontal components of ground motion exceed the amplitude of the vertical components, suggesting extreme topographic and 3D velocity heterogeneity. Use of all three components of the receiver function in modern passive array experiments is encouraged to reduce the problems of nonuniqueness in determining Earth models.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 5
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    Seismological Society of America
    In:  Bulletin of the Seismological Society of America, 91 (6). pp. 1805-1819.
    Publication Date: 2020-06-19
    Description: The vertical component P-wave receiver function is an important source of data in studies of the crust/mantle transfer function for determining Earth structure under isolated receivers or under receiver arrays. This waveform illuminates a missing aspect of the wave propagation in receiver function studies that employ only the horizontal components of motion, and yields complementary constraints on near-receiver heterogeneity and P-wave propagation. The vertical component P-wave receiver function is formed using an array estimate for the effective teleseismic source function that is then deconvolved from all the vertical and horizontal components of ground motion at each station in the array. One-dimensional, three-dimensional, and stochastic wave-propagation models are used to test the robustness of the technique. Breakdown of single-station receiver function deconvolution occurs because of high levels of noncorrelated noise between the ground-motion components. Receiver functions for stations of the southern California TERRAscope array are investigated using the array technique. Vertical receiver functions for stations in the Los Angeles Basin and Long Valley Caldera show high-amplitude secondary arrivals that cannot be explained by simple 1D structures but probably reflect wave propagation in 3D basin structures. Three-component receiver functions from the station at Mammoth Lakes, California, show pathological behavior where the horizontal components of ground motion exceed the amplitude of the vertical components, suggesting extreme topographic and 3D velocity heterogeneity. Use of all three components of the receiver function in modern passive array experiments is encouraged to reduce the problems of nonuniqueness in determining Earth models.
    Type: Article , PeerReviewed
    Format: text
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