Slow Slip in the Cascadia Subduction Zone: Thinking Outside the Plane

Abstract

In the Cascadia subduction zone beneath Vancouver Island a major (Mw 6–7) slow slip event and accompanying tremor occur about every 14 months at depths between ∼30 and 45 km. The total slip during an event is between 2 and 4 cm, which is significantly less than the 4.4 cm of plate convergence over the 14-month recurrence interval. We propose that large slow slip events occur within a relatively thin (<500 m) shear zone in the upper layer of the subducting oceanic crust and that the ∼1–2 cm of missing slip is accommodated by creep in an overlying layer of parallel, seismic reflectors (the E-layer) during the 14 months between events. We suggest that this creep is instrumental in the formation of the E-layer. We show that a simple two-layer model, where a viscoelastic E- layer overlies an elastic oceanic crust, can quantitatively match observed constraints if viscosity in the E-layer is in the range $(8.5\pm 2.0)\times {10}^{17}\text{Pa}s$ and the shear strength of the interface is in the range $86\pm 9\text{kPa}$. The low viscosity is consistent with other estimates for shear zones in the lower crust. The low shear strength is consistent with the value inferred from tremor triggering by surface waves from large earthquakes and with a frictional instability where fluid pressures are near-lithostatic.