Viscosity Determination in the Earth's Outer Core from ScS and SKS Phases

Abstract

Attenuation of seismic waves in the Earth's outer core is investigated by using pairs of transmitted waves through the outer core and reflected waves at the core-mantle boundary, which have common ray paths in the mantle. Taking into account the conditions: that the angle of incidence at the core-mantle boundary is small so that ray theory is applicable; that the phase to be analyzed should be clear and not contaminated by other phases; and that the ray does not penetrate into the inner core; pairs of SKS and ScS are used to determine the viscosity coefficient in the outer part of the Earth's outer core. Long-period seismograms of three deep-focus earthquakes supplied by the U.S.C. G.S. are used in this analysis. Ratios of the Fourier spectral amplitudes of SKS to those of ScS in the frequency range of 0.04Hz to 0.15Hz, after eliminating the effects of crustal layering and the seismograph response, yield a mean viscosity coefficient ν in the outer core. Using 16 pairs of SKS and ScS waves ν in the above frequency range is estimated to be 3×10 10 to 7×10 10 dyne·sec/cm 2, which is consistent with the ν determination from multiply-reflected ScS's at the core-mantle boundary.