Heterogeneous Elastic Properties of Subduction Fault Zones: Laboratory Insights from a Cascadia Megathrust Analog

Tomographic studies often reveal that the subduction plate interface is a low-velocity zone commonly attributed to anomalous fluid pressure and effective stress, while research on exhumed plate boundary fault zones highlights their geological heterogeneity. To bridge these perspectives, we conducted ultrasonic wave-speed measurements on samples from the Lakes of the Gods fault zone in the Olympic Subduction Complex, an exhumed analog of the modern Cascadia megathrust fault zone. Our results demonstrate that dominant lithology, degree of structural fabric development, and deformation styles exert first-order controls on seismic properties, producing anisotropy of up to 25.6% for Vp and 31.6% for Vs. Composite larger-scale velocities obtained by weighting laboratory measurements according to the mapped abundance of fault-zone units closely match in-situ velocities inferred from geophysical observations. This correspondence indicates that the low-velocity zone observed in tomographic profiles arises from the intrinsic elastic properties of accreted metasediments and their fabric. Furthermore, directional shear moduli derived from our measurements (20.0–30.1 GPa) are lower than geophysically-derived estimates at comparable depths of 13-14 km, suggesting that fabric development acts as a critical weakening mechanism that offsets burial-related stiffening. Collectively, our results show that lithologic and structural heterogeneities within subducted sediments contribute to spatial variations in seismic properties within the seismogenic zone and provide empirically constrained elastic parameters essential for improving earthquake source physics and seismic hazard assessment.