1 Rupture to the trench? Frictional properties and fracture energy of incoming 1 sediments at the Cascadia subduction zone 2 3 C. E. Seyler 1,* , J. D. Kirkpatrick 1 , H. M. Savage 2 , T. Hirose 3 , and D. R. Faulkner 4 4 1 Department of Earth and Planetary Sciences, McGill University, Montréal, QC, Canada 5 2 Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA, USA 6 3 Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and 7 Technology, Kochi, Nankoku, Japan 8 4 School of Environmental Sciences, University of Liverpool, Liverpool, UK 9 * Corresponding author: Caroline Seyler (caroline.seyler@mail.mcgill.ca) 10 11 Abstract 12 The mechanical properties of sediment inputs to subduction zones are important for understanding 13 rupture propagation through the accretionary prism during megathrust earthquakes. Clay minerals 14 strongly influence the frictional behavior of fault gouges, and the clay content of subduction input 15 materials varies through a stratigraphic section as well as for subduction margins globally. To 16 establish the frictional properties of the shallow Cascadia subduction zone and place the results in 17 a global context, we conducted high velocity rotary shear experiments on ODP core samples 18 retrieved from Cascadia input sediments (35-45% clay) and a suite of individual clay species. We 19 compared our results to a compilation of published high velocity experiments conducted on 20 samples of wet gouge, dry gouge, and intact rock. For each sample type, three trends were 21