Attenuation of compressional waves in the earth's mantle Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v979v515d

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  • Fourier transformed amplitudes of the compressional waves recorded between 9° and 91° (about 1000 to 10,000 km) epicentral distance from Gnome, Shoal, Haymaker,and Bilby underground nuclear explosions have been compared with Fourier transformed amplitudes of P waves measured at distances between 9.0 and 1 3. 0 km from the explosions. Using a formula of the type A= ε AoF(ξ)e - πfr/QV apparent Qs have been computed for frequency ranges of 0.5 to 1.0 cps and 0.7 to 1.0 cps at the various epicentral distances. The geometrical spreading factor F(ξ) and interface loss parameter ε drop out of the computations when assumed to be frequency independent in the narrow frequency pass-bands considered. The computed apparent Q values indicate that the mantle is inhomogeneous with respect to the absorption of seismic energy. Relatively low Q (high absorption) regions are centered around 25°, 42° and 74° and high Q (low absorption) regions are centered around 20°, 31°, 71°, and 78° epicentral distances. These are superimposed on a gradually increasing Q with epicentral distance trend. The average apparent Q value for the upper mantle is found to be 286 ± 38. The highest Q value of this study occurs between 78° and 80° epicentral distance where it is greater than 3000. The high and low Q values obtained in this investigation are of the same order of magnitude as those reported by other investigators from body wave studies and the general variation of the absorption properties with depth agrees reasonably well with those obtained from surface wave studies. The depth of great increase in electrical conductivity and the observed maximum depth of earthquake foci both agree roughly with a region of large increase in Q in the upper mantle. A relatively high absorption zone is found to occur below this depth.
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