Physical Limits on Damaged Fault Zone Thickness: the Role of Seismogenic Depth

Abstract:

Natural faults are surrounded by materials damaged through quasi-static and dynamic processes, forming fault zones whose thickness and degree of damage may vary along strike and depth, and as a function of fault maturity. Fault zones can affect earthquake ruptures in observable ways. In particular, we have found that waves trapped in tabular fault zones with reduced seismic wave velocities can generate pulse-like rupture and oscillatory rupture speed, facilitate supershear rupture transition and allow for steady rupture propagation at speeds that are unstable or inadmissible in homogeneous media. Fault zones also enhance near-field ground motions. These effects can appear in pre-existing fault zones but also in damaged zones generated dynamically during rupture, and crucially depend on the fault zone thickness and velocity reduction. Here we attempt to understand what controls fault zone thickness.

A synthesis of field observations indicates a dependence of fault zone thickness on cumulated slip characterized by linear scaling at small slip and saturation at large slip. These observations combined with empirical slip-length relations suggest that the maximum fault zone thickness is controlled by (proportional to) the depth extent of the seismogenic zone. We will present fracture mechanics theoretical arguments and dynamic rupture simulations with off-fault plasticity and damage that predict this saturation of fault zone thickness. In essence, the stress intensity factor at the front of a rupture in 3D, which controls the distance reached by the large off-fault stresses that cause damage, scales with the shortest characteristic length of the slipping zone. This length is limited by the seismogenic depth.

Further implications and effects of seismogenic depth will be presented, including limits on the fault stepover offset that can be breached by a dynamic rupture.