Accession Number:

ADP204460

Title:

The Effects of Anisotropy on Regional Seismic Wave Propagation,

Descriptive Note:

Corporate Author:

YALE UNIV NEW HAVEN CT DEPT OF GEOLOGY AND GEOPHYSICS

Report Date:

1995-08-14

Pagination or Media Count:

10.0

Abstract:

We have examined the effect of seismic anisotropy on the scattering of surface waves in actively-deforming continental regions, many of which occur in areas of nonproliferation concern. We have approached the problem from three angles, 1 we have developed waveform inversion methods for Love-Rayleigh coupling in seismic data from the Tibetan Plateau and Tien Shan region, 2 we have developed a flat-layered surface wave code to study the effect of crust and upper-mantle anisotropy on crustal resonances, and 3 we have advanced wavelet-based signal processing methods to estimate the polarization of coherent seismic energy across a broadband seismic array. In data from the recent portable broadband PASSCAL deployment in Tibet, we found clear evidence of long-period Love-to-Rayleigh scattering that is best explained by lateral gradients of anisotropy in the upper mantle, developed in the course of the compression and uplift of the plateau. We developed a modal summation technique for waveform perturbations that allowed us to infer that both S and P wave anisotropy, with a ratio consistent with the mineral alignment of sheared peridotite, in the depth range 100-300 km are necessary to fit the observed data. Using a plane-layered geometry, we are investigating the effect of crust and upper mantle anisotropy on Love-Rayleigh coupling in the 5-30 second period range. Most previous studies have posited a horizontal or vertical axis of symmetry for seismic anisotropy, but our code can incorporate a tilted axis of symmetry, which enhances coupling and scattering effects. Such dipping geometries could develop in the crust as a result of thrust ramps in compressive tectonic regimes, either in the form of oriented crystals in shear zones or fine-layering of isotropic material.

Subject Categories:

  • Government and Political Science
  • Seismology
  • Seismic Detection and Detectors
  • Nuclear Weapons
  • Geology, Geochemistry and Mineralogy

Distribution Statement:

APPROVED FOR PUBLIC RELEASE