Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

Abstract

AbstractThe ultrasonic technique for measuring travel times of compressional and shear waves using dual‐mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single‐mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp = 5.60 km/s, ∂Vp/∂P = 0.090 (km/s)/GPa, Vs = 3.37 km/s, ∂Vs/∂P = 0.05 (km/s)/GPa, σ = 0.216, and Vp/Vs = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂Vp/∂T = −0.518 × 10−3 (km/s)/K and ∂Vs/∂T = −0.182 × 10−3 (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual‐mode transducers, it is possible to accurately determine Vp/Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.