Crustal and Upper Mantle Velocity Gradients in the Vicinity of the East Kazakh Test Site; Ray Synthesis of Lg in Three Dimensional Crustal Models
Final rept. 1 Sep 1990-19 Oct 1992
CONNECTICUT UNIV STORRS DEPT OF GEOLOGY AND GEOPHYSICS
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Using a simple homogeneous crust and a first order Moho discontinuity, Lg waveforms are synthesized in canonical models as well as models chosen for selected Eurasian paths. Lg blockage by basins and Moho transitions is a strong function of the slope of the Moho surface and is only a weak function of the angle of source-receiver path relative to the strike of the structure. Synthetic waveforms are often in good qualitative agreement with observed waveforms, including details in the shape of the Lg coda. Poorer agreement is seen in cases in which either a the inferred Moho from surface topography disagrees with that determined from deep seismic sounding observations or b sedimentary basin structure is not included in the synthetic modeling. Synthetics that allow single scattering incorporated by the Born approximation show that scattering by heterogeneity within the crustal waveguide can fill up the Lg coda between multiple SmS arrivals and lengthen the Lg coda. The trajectories and geometric spreading of rays contributing to Lg suggest that the most important effects of scattering will be concentrated in the Moho transition and in the upper several kilometers of the crust.... Lg, Synthetic seismograms, Crustal structure.
- Geology, Geochemistry and Mineralogy