The Evolution of Static and Dynamic Elastic Properties Under Triaxial Conditions

Elastic properties can be measured statically where strain data are recorded and related to stress during slow unloading of a specimen, or dynamically, where the elasticity can be calculated from the seismic wave velocity. Knowledge of the elastic properties is important in fault zone studies as microcracks change the elastic properties of rock and hence the stress state surrounding faults. In order to understand the elastic properties of the crust at depth using seismology, the relationship between the static and dynamic properties must be known. Previous studies have been performed under uniaxial stress and it has been unclear as to how the results might relate to triaxial stress states. These results have shown that: (1) an increase in the density of microcrack damage causes a decrease in static and dynamic Young's modulus and an increase in static and dynamic Poisson's ratios; (2) stress-dependency of static and dynamic elastic properties exist due to opening of cracks; (3) there is a linear relationship between static and dynamic Young's moduli and a significant discrepancy between the static and dynamic Poisson's ratios; and (4) the static and dynamic properties differ due to the size distribution of the crack population relative to the amplitude and frequency of the applied stress, frictional sliding on closed cracks during loading/unloading, and the assumption of isotropic elasticity in the sample. From these studies, some key questions that need addressing are: Is the evolution of static and dynamic properties the same under triaxial stress? Is the correlation between static and dynamic properties better under triaxial stress? We have developed a high pressure triaxial apparatus that can measure static and dynamic properties simultaneously. Here, results of the evolution and comparison of static and dynamic are represented based a more realistic crustal stress state.