Frequency-Dependent Q in the Earth's Outer Core from Short-Period P4KP/PcP Spectral Ratio.

Abstract

The attenuation of P-waves and its frequency dependence in the Earth's outer core are investigated using the P4KP/PcP spectral amplitude ratio. We analyze PcP and P4KP phases recorded on short-period seismographs in the Japan Islands for three large intermediate-depth and deep earthquakes from March 1991 through July 1994. The observed P4KP/PcP spectral amplitude ratio is corrected for responses of the mantle and the core, and attenuation in the lower mantle. The responses for P4KP and PcP are considered for the frequency dependence of their reflection/transmission coefficients at the core-mantle boundary. After the corrections, we estimate QP, values in the outer core in the frequency range of 0.15-2 Hz, and find that the QP increases with increasing frequency (f) in the form of QP=(4, 600±100) f(0.55±0.07). The observed frequency-dependent QP values increase more slowly with increasing frequency than those predicted by the model of the bulk attenuation based on the relaxation mechanism of immiscible particles in the outer core.