Insights on frictional processes in sheared clastic marine sediments using ultrasonic nondestructive testing

We investigate changes in the elastic properties of deforming core materials recovered from the Nankai Trough Accretionary Prism along the IODP NanTroSEIZE transect. We shear clastic marine sediments while simultaneously making ultrasonic velocity measurements across the deforming layers. Examining the resulting changes in elastic moduli at the laboratory scale allows us to identify characteristic “fingerprints” of deformation style during direct-shear experiments, which may then be compared with measurements conducted at the field scale to infer how deformation is localized within the accretionary prism. Identifying relationships between hold time and attenuation may also shed light on fault healing mechanisms taking place immediately following a rupture. Together the effects of sliding rate and hold time on elastic moduli provide us with new ways of constraining the mechanical behavior of large plat-boundary settings throughout the seismic cycle. We tested intact core material, remolded layers, and disaggregated granular powders derived from a range of depths from IODP sites C0007 and C0004 penetrating the frontal thrust and a large out-of-sequence thrust in the outer prism, respectively. Samples were deformed in a double-direct shear configuration, varying first the strain rate and then subjecting the gouge layer to a series of slides and holds of increasing duration. Elastic wave propagation depends on micromechanical interactions and gouge layer strength, so as the shear zone evolves changes in elastic wavespeed provide a means to interrogate strain materials non-destructively, providing insight into frictional processes and mechanics as that deformation is taking place. Our results suggest that there are characteristic changes in P and S-wave velocity and attenuation for marine clastic to hemipelagic sediments as a function of sliding rate and hold time. Ultimately, examining variation in elastic moduli during a simulated seismic cycle may provide insight into the nature of deformation taking place within original or synthetic fault gouge.