Accession Number:

ADA170017

Title:

Development of Ultrasonic Modelling Techniques for the Study of Seismic Wave Scattering due to Crustal Inhomogeneities.

Descriptive Note:

Final rept. 13 Apr 83-30 Sep 85,

Corporate Author:

MASSACHUSETTS INST OF TECH CAMBRIDGE EARTH RESOURCES LAB

Report Date:

1986-03-01

Pagination or Media Count:

86.0

Abstract:

Scattering of Rayleigh waves from surface features was investigated using three-dimensional ultrasonic models at frequencies near 1 MHz and two dimensional finite difference calculations. Models were constructed of aluminum blocks and an aluminum powder-epoxy composite of lower density and seismic velocity. The aluminum has similar seismic velocities and density to igneous and metamorphic rocks while the composite is similar to sedimentary materials. Kilometers in the earth are scaled to millimeters in the models which makes 1 MHz in the models equivalent to 1 Hz in the earth, typical of the frequencies observed in regional seismograms. Relief on the models which was a few millimeters, the order of a wavelength, was restricted to an isolated circular mesa of composite on a metal block and an isolated circular depression both unfilled and filled with composite. Calculations were made for similar situations by the finite difference method comparisons indicated a general similarity in the seismograms, allowing us to use the finite difference calculations to gain physical insight into the scattering process. Topography alone produces attenuation due to scattering into reflected surface waves and body waves. Adding low velocity composite to the model considerably changes this picture Rayleigh wave energy is strongly trapped in the low velocity material on the surface and produces strong reverberation as it bounces around in the mesa or valley. This effect is suppressed to a certain extent by the attenuating properties of the composite. Analogs of this type of behavior seem to exist in observed regional seismograms.

Subject Categories:

  • Seismology

Distribution Statement:

APPROVED FOR PUBLIC RELEASE