Abstract
Short-period teleseismicP waves from the Nevada Test Site (NTS) show systematic variations in amplitudes and travel times, with low amplitudes corresponding to fast travel times, suggesting elastic focussing-defocussing effects. Also, the azimuthal amplitude and travel time patterns for events at the Pahute Mesa subsite are systematically different from those at the Yucca Flat subsite, indicating the presence of a near-source component in both the amplitude and travel-time variations. This component is isolated by removing the mean station pattern for all of NTS from the observations. A very-near-source component in the Pahute Mesa observations is also isolated by removing subsite station means from the measurements, whereas the Yucca Flat observations exhibited no coherent very-near-source component. These anomalies are back-projected through laterally homogeneous structure to form thin lens models at various depths. Travel-time delays are predicted from the amplitude variations using the equation for wavefront curvature. The long-wavelength components of the predicted and observed time delays correlate well, at depths of 25 km for the very-near-source component under Pahute Mesa and 160 km for the regional component under NTS. The time delay surfaces predicted by the amplitudes at these depths are mapped into warped velocity discontinuities suitable for the calculation of synthetic seismograms using the Kirchhoff-Helmholtz integral formulation. Both the intersite (near-source) and intrasite (very-near-source) differences in amplitudes are qualitatively predicted very well, although the range of variation is somewhat underpredicted. This deficiency is likely due to the destructive interference of anomalies inherent in back-projection to a single layer.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Burdick, L. J., andHelmberger, D. V. (1978),The Upper Mantle P Velocity Structure of the Western United States, J. Geophys. Res.83, 1699–1712.
Burger, R. W., Lay, T. andBurdick, L. J. (1986),Estimating the Relative Yields of Novaya Zemlya Tests by Waveform Intercorrelation, Geophys. J. R. Astr. Soc.,87, 775–800.
Butler, R. (1983),P Wave Travel Time and Amplitude in Western North America., Nature306, 677–678.
Butler, R. (1984),Azimuth, Energy, Q, and Temperature: Variations on P Wave Amplitudes in the United States, Reviews of Geophysics and Space Physics22, 1–36.
Chang, A. C., andvon Seggern, D. H. (1980),A Study of Amplitude Anomaly and mb Bias at LASA Subarrays, J. Geophys. Res.85, 4811–4828.
Cormier, V. F. (1987),Focusing and Defocusing of Teleseismic P Waves by Known 3-d Structure Beneath Pahute Mesa, Nevada Test Site, Bull. Seism. Soc. Am.77, 1688–1703.
Haddon, R. A. W., andBuchen, P. W. (1981),Use of Kirchhoff's Formula for Body Wave Calculations in the Earth, Geophys. J. R. Astr. Soc.67, 587–598.
Haddon, R. A. W., andBuchen, P. W. (1981),Joint Interpretation of P Wave Time and Amplitude Anomalies in Terms of Lithospheric Heterogeneities, Geophys. J. R. Astr. Soc.55, 19–43.
Helmberger, D. V., andHadley, D. M. (1981),Seismic Source Functions and Attenuation from Local and Teleseismic Observations of the NTS Events JORUM and HANDLEY, Bull. Seismol. Soc. Am.71, 51–67.
Lay, T., Lynnes, C. S., andWelc, J. (1986),Analysis of Teleseismic P Wave Amplitude and Coda Variations for Underground Explosions at U. S. and Soviet Test Sites, Scientific ReportAFGL-TR-86-0038, University of Michigan, Ann Arbor, Michigan.
Lay, T., Wallace, T. C., andHelmberger, D. V. (1984),The Effects of Tectonic Release on Short-period P Waves from NTS Explosions, Bull. Seismol. Soc. Am.74, 819–842.
Lynnes, C. S., andLay, T. (1988a),Analysis of Amplitude and Travel Time Anomalies for Short-period P Waves from NTS Explosions, Geophys. J. R. Astr. Soc.92, 431–443.
Lynnes, C. S., andLay, T. (1988b),Observations of teleseismic P wave coda for underground explosions, InScattering and Attenuation of Seismic Waves, Pure Appl. Geophys.128, 231–249.
Minster, J. B., Savino, J. M., Rodi, W. L., Jordan, T. H., andMasso, J. F. (1981),Three-dimensional Velocity Structure of the Crust and Upper Mantle Beneath the Nevada Test Site, Final Technical ReportSS-R-81-5138, Science, Systems and Software, La Jolla, California.
Scott, P., andHelmberger, D. V. (1985),Applications of the Transmitted Kirchhoff-Helmholtz Method to Transmitted Body Wave and Possible Structural Effects at NTS, Bull. Seismol. Soc. Am.75, 131–156.
Spence, W. (1974),P Wave Residual Differences and Inferences on an Upper Mantle Source for the Silent Canyon Volcanic Center, Southern Great Basin, Nevada, Geophys. J. R. Astr. Soc.38, 505–523.
Swartztrauber, P. N., andSweet, R. A. (1975),Efficient FORTRAN Subprograms for the Solution of Elliptic Partial Differential Equations, NCAR Technical NoteNCAR-TN/IA-109.
Sweet, R. A. (1974),A Generalized Cyclic Reduction Algorithm, SIAM J. Numer. Anal.11, 506–520.
Taylor, S. R. (1983),Three.-dimensional Crust and Upper Mantle Structure at the Nevada Test Site, J. Geophys. Res.88, 2220–2232.
Thomson, C. (1983),Ray-theoretical Amplitude Inversion for Laterally Varying Velocity Structure Below NORSAR, Geophys. J. R. Astr. Soc.74, 525–558.
Thomson, C. J., andGubbins, D. (1982),Three-dimensional Lithospheric Modelling at NORSAR: Linearity of the Method and Amplitude Variation from the Anomalies, Geophys. J. R. Astr. Soc.71, 1–36.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Lynnes, C.S., Lay, T. Effects of lateral velocity heterogeneity under the Nevada Test Site on short-periodP wave amplitudes and travel times. PAGEOPH 132, 245–267 (1990). https://doi.org/10.1007/BF00874365
Received:
Revised:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00874365