Strains and Stresses Near Explosions and Earthquakes.
Rept. for 5 Mar-5 Sep 86,
WOODWARD-CLYDE CONSULTANTS PASADENA CA
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Laboratory evidence indicates that when strains exceed 10 to the -6 in media under low confining pressure, certain nonlinear processes begin to occur. They are manifested by a reduction in apparent Q. Since direct measurement of near field strain are rare, it has been difficult to evaluate whether or not this phenomenon is significant near to realistic seismic sources. A high frequency theory for computing stresses and strains using generalized ray theory has been developed. It can be shown using this theory that near field body wave strain pulses are closely related to near field velocity pulses. Large data bases of near field velocity records and crustal structures for modeling them are available. Transfer functions for transforming velocity pulses into estimated strain pulses can be computed by deconvolving theoretical versions of the former from the latter. In most cases they are delta-like functions. These transfers operators have been computed for a suite of velocity records from 5 Pahute Mesa Nuclear explosions ranging in yield from 155 to 1300 kt and for an Imperial Valley earthquake of M sub 0 0.6 times 10 to the 24 dyne-cm. For the explosion data base the strains were as high as 10 to the -3 3 orders of magnitude higher than the level at which the laboratory data suggests that nonlinear effects become important. The earthquake data base indicated strains levels between 10 to the -6 and 10 to the -5. Because of the pressure dependence of the nonlinear phenomenon, it is probably only important in a thin layer near the surface of the earth.