Microphysical controls on the elastic wave speeds of an exhumed greenschist and implications for the interpretation of conditions along the subduction interface.

Abstract:

Seismic tomography is one of few tools available to constrain the environmental conditions in active subduction zones. For instance, elastic wave velocities highlight areas with prominent features like low P-wave velocities (Vp) and high Vp/Vs ratios that are often interpreted as high pore fluid pressures. These seismic signatures are sometimes observed down-dip of the seismogenic zone where episodic tremor and slip (ETS) events occur leading to hypotheses that for ETS that require high pore fluid pressures. Despite these interpretations, experimental data and rock physics models are relatively rare for the seismic wave velocities of relevant plate boundary lithologies. Here, we report experimental measurements and rock physics models of ultrasonic wave velocities in the Orocopia schist, an exhumed chlorite-rich schist. Measurements were made on cylindrical samples 25.4 mm in diameter and 51 mm in length at 1 MHz frequency in a standard triaxial deformation apparatus. Velocities were measured on both dry and saturated samples at effective confining pressures from 3 MPa to 140 MPa, temperatures from 22 ºC to 160 ºC, and at 10 MPa differential stress. At all conditions, seismic velocities increase with increasing effective pressure and decrease with increasing temperature. The presence of pore fluid causes an increase in Vp/Vs ratio up to 2.1 with decreasing effective stress at 3 MPa. We are using Differential Effective Medium (DEM) models constrained by microstructural imaging to understand the controls of mineralogy and pore geometry on seismic waves.