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Radiation patterns of body waves due to the seismic dislocation occurring in an anisotropic source medium

ICHIRO KAWASAKI^* and TOSHIRO TANIMOTO

DEPARTMENT OF EARTH SCIENCES FACULTY OF SCIENCE TOYAMA UNIVERSITY, TOYAMA, Japan
INSTITUTE OF GEOPHYSICS FACULTY OF SCIENCE THE UNIVERSITY OF TOKYO, TOKYO, Japan

Abstract

We investigate body force equivalents for a seismic dislocation occurring in an anisotropic source medium and study radiation patterns of seismic body waves resulting from them. The point source representation of the equivalent body forces is obtained following a result of Kosevich (1962, 1965). Green's tensor for an anisotropic medium is calculated using a far-field approximate method by Kosevich and Natsik (1964). Radiation patterns of seismic body waves are obtained by a straightforward convolution operation on the equivalent forces with the approximate Green's tensor.

The seismic dislocation occurring in an anisotropic source medium is equivalent in general to the sum of three orthogonal dipole forces with different magnitudes, for which the seismic moment tensor has a nonzero trace. Because of the third dipole force which never appears for an isotropic medium, a significant distortion of the radiation patterns occurs in a direction near the null vector. Nodal lines of P-wave radiation patterns are separated into isolated loops and/or secondary nodal lines appear. In directions where group velocity differs from the corresponding phase velocity, the effect of the medium transfer response on the polarities of body waves seems to be larger than that in other directions. The combination of the effects of source forces and medium transfer response distorts the radiation pattern.

Footnotes

^* Present address: Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Present address: Seismographic Station, University of California, Berkeley, California.

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