Creep events at the brittle ductile transition

We present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones. We assume that creep events at the brittle ductile transition (BDT) are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material. These events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids. We show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust, a ductile shear zone and a creeping zone of fractures at the BDT. The time scale of the events varies between seconds to thousands of years depending on the viscous, elastic and brittle-plastic properties of the fractured BDT, the shear zone and the crust. The nature of the events depends on the aspect ratio, γ of the shear zone thickness, H_w to the length of the fractured zone, w. We find that thick shear zones with small fractures at the BDT are stiff and generate creep oscillations. Thin shear zones with well-connected fractures over a large width have very small stiffness and are well lubricated. They generate slow creep events or steady creep event. The former are similar to transient slip events and the latter to creep at the far field tectonic rates. The viscosity of the shear zone, η_w enhances lubrication if it is small and stiffness if it is large.