Subduction zones generate the most earthquakes, and the most destructive earthquakes often nucleate at the plate boundary near the base of the seismogenic zone, located at a depth of 30-40 km. This active region correlates with evidence of near lithostatic pore pressure from geophysical imaging methods, indicating conditions that significantly impact seismic activity and slip mode. Additionally, the pressure-temperature conditions at 30-40km correspond to rocks undergoing greenschist facies metamorphism, although little is known about their rock properties under high fluid pressure. As a result, it remains unclear how the rock properties of greenschists under high fluid pressure control seismic waves, possibly leading to misinterpretation in geophysical imaging. This thesis aims to collect data on a greenschist metapelite, the Orocopia schist, to better understand how the microstructure and pore fluids control the rock properties and seismic waves. We measured velocities, elastic moduli, and attenuation of the Orocopia schist in the laboratory to determine the effect of pore fluids. Then, we extracted the critical microstructure, like pore shape, porosity, and permeability, with models that helped us extrapolate to the geologic scale. Chapter 2 presents ultrasonic wave measurements to explore the effect of mineralogy, anisotropy, and pore network on velocities. Although mineralogy and mineral anisotropy influence the wave velocities, thin elongated pores aligned within the foliation are necessary to explain the anomalous velocities in subduction zones. The forced oscillations technique is introduced in Chapters 3 and 4 to explore wave-induced fluid flow at a small scale, which causes frequency-dependent attenuation and elastic moduli. Models in Chapter 3 reveal that the microstructure is fundamental for generating fluid-flow mechanisms and a single property control dispersion and attenuation in saturated rocks. The attenuation results in Chapter 3 are extrapolated to the scale of subduction zones in Chapter 4 to understand the effect on earthquakes. We find that high fluid pressure causes enough attenuation to deplete high frequencies, consistent with amplitude spectra of low frequencies of earthquakes. This thesis demonstrates that metapelites under high fluid pressure can represent regions of low velocities and high attenuation in subduction zones but also significantly impact seismic waves.