Rupture to the trench? Frictional properties and fracture energy of incoming sediments at the Cascadia subduction zone

摘要

The mechanical properties of sediment inputs to subduction zones are important for understanding rupture propagation through the accretionary prism during megathrust earthquakes. Clay minerals strongly influence the frictional behavior of fault gouges, and the clay content of subduction input materials varies through a stratigraphic section as well as for subduction margins globally. To establish the frictional properties of the shallow Cascadia subduction zone and place the results in a global context, we conducted high velocity rotary shear experiments on ODP core samples retrieved from Cascadia input sediments (35-45% clay) and a suite of individual clay species. We compared our results to a compilation of published high velocity experiments conducted on samples of wet gouge, dry gouge, and intact rock. For each sample type, three trends were identified with increasing normal stress: 1) the stress drop (tau(p) - tau(ss)) increases linearly, 2) the characteristic thermal weakening distance (D-th) decreases as a power law function except for wet clay-rich gouges, and 3) the fracture energy (W-b) shows no dependence. However, fracture energy does vary with sample type. Clay-rich gouges under wet conditions have the lowest fracture energy, and fracture energy for both dry and wet gouges is at least an order of magnitude lower than estimates from intact rocks. Therefore when clay-rich lithologies are present, they may minimize spatial variations in frictional behavior, allowing earthquakes to propagate to the trench. For Cascadia input sediments, there is little variation in the fracture energy between lithologies, but the fracture energy of Cascadia sediments is around an order of magnitude higher than input sediments from other subduction margins. The high fracture energy of Cascadia sediments relative to other subduction margins may inhibit large amounts of shallow earthquake slip and dynamic overshoot. (C) 2020 Elsevier B.V. All rights reserved.