A continuum of stress, strength and slip in the Cascadia subduction zone

As oceanic lithosphere subducts beneath continental lithosphere it experiences variable degrees of interaction with the overriding plate and movement is accommodated by a continuum of slip modes(1). At shallow depths, the plates are locked and movement occurs intermittently as earthquakes. By contrast, at large depths the down-going plate slips into the mantle continually. In the transition zone between locked and stable slip, plate movement is accommodated by slow slip(2), which generates tectonic tremor(3). Here we use tectonic tremor to infer the location and duration of slow slip in the Cascadia subduction zone from 2006 to 2011. We find that individual slow-slip events are initiated deep on the plate interface and migrate upwards. With decreasing depth, we observe a gradation from small, frequent slip, to large, infrequent slip. These observations fill in the transition zone with a continuum of slip size and periodicity, and indicate that the fault weakens with depth, which we attribute to lower friction. We suggest that stable sliding loads the fault at depth and transfers stress to the base of the transition zone, causing the initiation of slow slip. In a self-similar process, slow slip migrates upwards and ratchets stress up the fault, towards the shallower seismogenic zone. Our conceptual model provides an intuitive understanding of subduction zone dynamics.