Seismic-frequency laboratory measurements of shear mode viscoelasticity in crustal rocks I

Abstract

In the first phase of a seismic-frequency study of the viscoelasticity of cracked and fluid-saturated crustal rocks, forced torsional oscillation experiments have been conducted on specimens of Carrara marble subjected to multiple thermal cycles up to 560°C, under confining pressure of 50 MPa. The experimental results showed distinctly different behaviour of both the shear modulus and internal friction between the first and subsequent thermal cycles. A pronounced reduction in shear modulus (G) observed during the first series of thermal cycles at temperature T < 300°C is attributed to thermal cracking. This large modulus deficit is recovered at higher temperatures, presumably through crack closure by plastic deformation processes that are also reflected in time dependence (over hours) of the shear modulus at temperatures above 300°C. Following the first excursion to 500°C, the shear modulus varies reproducibly with thermal cycling with a monotonic temperature dependence dominated by intrinsic (anharmonic) effects. It is inferred that thermal cracking in Carrara marble is suppressed by plastic flow following the first cycle to 500°C. The experimental results suggest that thermal history has strong effects on mechanical properties of calcite rock (Carrara marble) measured at relatively low temperatures. Internal friction or attenuation of Carrara marble is generally low and temperature-insensitive at low temperatures (< 400°C), but increases dramatically at 400-500°C. The frequency-dependence of internal friction at high temperatures (> 400°C), presumably due to dislocation relaxation, contrasts with frequency-independent Q-1 at lower temperatures. The relatively sudden onset of markedly frequency-dependence seismic wave attenuation might prove to be a useful diagnostic of temperature under crustal conditions. Show more