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Experimental Study of Dislocation Damping Using a Rock Analogue

In order to explore the effects of dislocations on seismic velocity and attenuation, we conducted a series of forced oscillation and ultrasonic tests on rock analogue samples (polycrystalline borneol) that were predeformed under various differential stress Δσ. Additionally, creep experiments were conducted to determine the steady‐state flow law for borneol. The dominant deformation mechanism of polycrystalline borneol changes from diffusion to dislocation creep at about Δσ=2 MPa. At high stresses, power law creep with a stress exponent of ∼4 was measured. Microstructure of the deformed samples showed wavy grain boundaries due to dislocation‐induced migration and the occasional existence of microcracks. A borneol sample deformed in the dislocation creep regime showed a significant reduction in Young's modulus E and a slight increase in attenuation Q−1 at frequencies lower than 100 Hz, whereas E at ultrasonic frequency (106 Hz) did not reduce. Therefore, a major part of the dislocation creep‐induced anelastic relaxation is a peak with a characteristic frequency between 100 and 106 Hz, which is much higher than the range of grain boundary‐induced anelasticity of this material. Further experiments under higher confining pressure are needed to assess the relative contribution from dislocations and microcracks to this peak.