Grain‐size‐sensitive seismic wave attenuation in polycrystalline olivine

Abstract

[1] In order to investigate the processes responsible for the attenuation of seismic shear waves in the Earth's upper mantle, four olivine polycrystals ranging in mean grain size d from 3 to 23 mm have been fabricated, characterized, and mechanically tested in torsion at high temperatures and seismic frequencies. Both the shear modulus, which governs the shear wave speed V S , and the dissipation of shear strain energy Q À1 have been measured as functions of oscillation period T o , temperature T, and, for the first time, grain size. At sufficiently high T all four specimens display similar absorption band viscoelastic behavior, adequately represented for 1000 < T < 1200 or 1300°C and 1 < T o < 100 s, by the expression] a with A = 7.5 Â 10 2 s Àa mm a , a = 0.26 and E = 424 kJ mol À1 . This mildly grain-size-sensitive viscoelastic behavior of melt-free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding, the latter becoming progressively more important with increasing T and/or T o . Extrapolation to the larger (mm-cm) grain sizes expected in the Earth's upper mantle yields levels of dissipation comparable with those observed seismologically, implying that the same grain-size-sensitive processes might be responsible for much of the observed seismic wave attenuation. The temperature sensitivity of V S is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations.

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

“…The interpretation of η K in the Burgers DEC models is unclear, and its relation to rock deformation experiments is less clear as most of those experiments use more complicated transient rheologies (e.g., Chopra, 1997;Gribb and Cooper, 1998;Jackson et al, 2002). We find that the relatively simple Burgers viscoelastic model is capable to phenomenologically explain observed interseismic deformation.…”

QP and QS in the upper mantle beneath the Iberian peninsula from recordings of the very deep Granada earthquake of April 11, 2010

“…[7] The importance of the physical properties on the seismic attenuation parameters have been demonstrated in experimental studies applied to environmental and petrological conditions of the upper mantle [e.g., Jackson et al, [Jackson et al, 2002;Faul and Jackson, 2005], partial melting and the water content [Aizawa et al, 2008] play a crucial role in the attenuation of the seismic waves propagating in the upper mantle. The Q P M calculated in the frequency range between 0.25 and 8 Hz and the Q S M values for frequencies from 0.25 to 2 Hz (Figure 3) are low (Q P M = 74 and Q S M = 96 at 1 Hz), similar to the quality factors measured in high attenuation crustal domains.…”

Temperature and rheological properties of the mantle beneath the North American craton from an analysis of heat flux and seismic data

“…Grain sizes in the upper mantle can be determined from data on xenoliths and massif peridotite samples and from seismic wave attenuation. According to Jackson et al [2002], the range allowed by seismic data is 0.1-10 mm, but most authors consider the 1-10 mm range to be more realistic. We first fix the grain size to a value of 1 cm as in the work by Korenaga and Karato [2008].…”

“…Whatever the water content of the mantle is (within a 100-4000 ppm H/Si range), small-scale convection is able to supply enough heat to the Canadian Shield lithosphere in this deformation regime. Diffusion creep can work, too, but it requires grain sizes of 0.1 mm or less that are not considered realistic [Jackson et al, 2002].…”