Experimental Approach Using Ice to Understand the Effect of Dislocations on Anelasticity: New Cryo-Forced-Oscillation Apparatus and Improved Method of Microstructural Observation

Dislocations are crystalline defects that are ubiquitous in rocks and can cause dispersion and attenuation of seismic waves propagating in the Earth. Previous experimental studies have shown that anelastic relaxation is significantly enhanced by dislocations (Guéguen et al., 1989; Farla et al., 2012; Sasaki et al., 2019), but its quantitative assessments are limited. In these studies, two-stage deformation tests were performed: (1) a creep test, called "pre-deformation", to introduce dislocations in a sample and (2) a forced oscillation test to measure anelasticity of the pre-deformed sample. However, because both amplitudes of cyclic and offset stresses applied during the forced oscillation tests of these studies were small, dislocations were annealed at high temperature. Hence, the steady-state dislocation density could not be maintained during the forced oscillation test, which makes it difficult to quantify the effect of dislocations on anelasticity.

We aim to address this problem by using ice polycrystals as a rock analogue and to expand the understanding of the effects of dislocations and deformation on anelasticity. Ice has an advantage that dislocation creep can be achieved relatively easily with moderate stress (Goldsby & Kohlstedt, 2001; McCarthy & Cooper, 2016). We are planning to pre-deform samples from 2% to 20% strain using the cryogenic triaxial apparatus at U.Penn followed by forced oscillation tests using a newly fabricated apparatus at Lamont. Although this study also performs two-stage deformation tests, we can apply an offset stress corresponding to the dislocation-creep regime during the forced oscillation test, and hence, annealing of dislocations can be prevented. As another advantage, the strong plastic anisotropy in ice crystals means that slip dominantly occurs on the basal plane (0001), which makes it easy to interpret the experimental data obtained with different strains by relating to the development of CPO. We are now building a cryo-forced-oscillation apparatus by adapting the TKY-type apparatus (Takei et al., 2014) to low temperatures and developing a method to estimate dislocation density and the extent of CPO in ice samples by improving the etch-pitting replication method (e.g., Sinha, 1977). These preliminary results will be presented.