Evolution of the elastic moduli of seismogenic Triassic Evaporites subjected to cyclic stressing

Abstract

Seismic cycles lead to variations in rock physical properties. Quantifying these changes is of key importance in building reliable crustal deformation models. Here we report laboratory measurements of the uniaxial compressive strength (UCS), Young's modulus and Poisson's ratio, both static (Es, νs) and dynamic (Ed, νd) of the seismogenic Triassic Evaporites of the Northern Apennines. Triassic Evaporites are composed of dolostones, anhydrites and gypsum. Gypsum was the weakest lithology, with UCS values ranging from 10 to 26MPa; anhydrite exhibited intermediate values from 52 to 144MPa; and dolostones were the strongest with a maximum UCS of 228MPa.During uniaxial cyclic stressing experiments, we observed complex variations in Es and νs with: large increases are observed in the early cycles (stage 1), followed by essentially constant values (stage 2), before Es decreases and νs increases approaching failure (stage 3). Complementary microseismicity (acoustic emission, AE) data show no significant AE during stage 1, then the stress needed to induce AE remained essentially constant (stages 2 and 3). Integration of mechanical data with microstructural observations suggests a first stage dominated by compaction and strengthening, a second stage characterised by quasi-elastic behaviour associated with the development of randomly oriented microfractures, and a third stage of weakening due to the growth of macrofractures parallel to the direction of the load. Laboratory dynamic elastic moduli are, on average, in agreement with dynamic elastic moduli used in crustal modelling. However static values of Young's modulus are about 50% lower than dynamic ones, and static values of Poisson's ratio are about 40% higher with respect to dynamic values. These observations suggest that the frequency effect on the difference between laboratory and crustal scale dynamic moduli values is rather small and that static values of modulus are more appropriate for crustal deformation modelling than seismically derived values.