2011 Articles

Experimental study of attenuation and dispersion over a broad frequency range: 2. The universal scaling of polycrystalline materials

McCarthy, Christine; Takei, Yasuko; Hiraga, Takehiko

In order to extend the range of conditions that can be obtained in experiments, we have measured the viscoelastic properties of polycrystalline organic borneol, as an analogue to mantle rock. Using a custom fabricated apparatus, the Young's modulus E and attenuation QE−1 were measured accurately over a broad frequency range (10−4 ≤ f (Hz) ≤ 2.15) and at low strain amplitude (10−5–10−6). Creep experiments were performed with the same apparatus to measure the steady state viscosity. Anelasticity and viscosity were measured at high homologous temperatures (T = 22–48°C; T/Tm = 0.61–0.67) and various grain sizes (3–22 μm), the growth of which was controlled by annealing. Using the measured viscosities η and the unrelaxed modulus EU determined from ultrasonic experiments, the frequency of the entire data set was normalized by the Maxwell frequency fM = EU/η, resulting in E and Q−1 master curves. The Q−1 data from previous studies on olivine-dominated samples also collapse onto the same curve when scaled by fM,, demonstrating the universality of anelasticity for polycrystalline materials. The similitude by the Maxwell frequency scaling indicates that the dominant mechanism for the anelasticity observed in this study and in previous studies is diffusionally accommodated grain boundary sliding. A generalized formulation for this similitude is provided to extrapolate the experimental data to velocity and attenuation of seismic shear waves.