The new Developed System for Elastic-Wave Velocity Measurement Under High P-T Condition of Seismogenic Zone

We have developed a new experimental system that can measure elastic-wave velocities under high-temperature, confining pressure (Pc) and pore pressure (Pp) conditions with in the argon-gas pressure vessel. The elastic-wave velocities of fault rocks are important clues to reveal the rock composition of seismogenic zone. The detailed rock composition model in the deep part of seismogenic fault zones is very important to understand the structure of fault zones and the earthquake process. We measured elastic-wave velocities of cylindrical Westerly granite and Berea sandstone (20mm diameter and 20mm length) under high-pressure and high-temperature in the wet and dry conditions. The experiments were carried out with the gas-medium high-pressure and high temperature deformation apparatus at AIST, Japan. Temperature varied from room temperature to 200° C. In the dry conditions, experiments were conducted under the confining pressure up to 200MPa. In the wet conditions, pore water pressure was applied up to 200MPa. The rock sample was put between upper and lower WC and pistons. The lithium-niobate transducers (LiNbO₃, 1.0mm thick) set at the end surface of the lower piston were used to emit and receive the ultrasonic signals. We used the lower WC piston as a buffer rod to isolate the transducers from the high-temperature environment. The elastic waves reflected at the WC piston-Sample boundary and sample - WC plate boundary are recorded. We measured the difference of arrival times of these two reflected waves. The elastic-wave velocities are calculated from these measured travel times and the sample core length. Our preliminary results of Vp measurement show that, Vp of Westerly granite increased rapidly from 4.92 km/s at 20 MPa to 5.78 km/s at 80 MPa in the dry condition. Over 100 MPa up to 160 MPa, the Vp increased slightly up to 160 MPa from 5.80 km/s to 5.83 km/s. This Vp -Pressure curve is similar to those of many previous studies. The Vp of Berea sandstone increased form 3.71 km/s at 5 MPa (Pp=5MPa) to 4.13 km/s at 25MPa (Pp=20MPa).