Attenuation of Seismic Waves in the Mantle: ABSTRACT

Abstract

Field and laboratory measurements of attenuation in metals, non-metals, and rocks over a wide range of frequencies indicate that the specific attenuation factor, 1/Q, is substantially independent of frequency in homogeneous material, whereas it varies as the first power of frequency in liquids. This suggests that the mechanism End_Page 346------------------------------ for attenuation in solids is substantially different from that in liquids; a non-linear mechanism for attenuation has been proposed. An inversion method can be used to compute the intrinsic Q in shear of the earth's mantle from available data on attenuation of the surface waves and free oscillations. The restrictions and assumptions in the calculation are: (1) Q must be positive; (2) Q is assumed to be independent of frequency; and (3) the mechanism of energy dissipation is through a complex modulus. The results show that, in shear, the upper mantle has a much higher attenuation than the lower mantle. Q for the upper mantle, from the surface to a depth of 650 km., is estimated at 110; for the lower mantle, below 650 km., it is much higher than this, but the exact value cannot be estimated with precision. There are indications of fine variations of Q in the upper mantle, but present accuracy of the data and the assumptions used do not permit the literal use of these indications. Partial melting in a low velocity layer at shallow depth is considered and a small amount of partial melting is not inconsistent with the above result and the data. End_of_Article - Last_Page 347------------