Role of Fluid Flow, Silica Kinetics, and Fault Zone Strengthening in Slip Behavior within the Seismogenic zone

Abstract:

Microstructures in subduction mélange exhumed from the seismogenic zone record pressure solution and crack healing in the interseismic period, and these changes of state impact the fluid flow and slip behavior. To evaluate the components of this system, we develop a numerical model that tracks fluid flow within the framework of an earthquake simulator to develop predictions for slip behavior that account for feedbacks between fluid flow, silica kinetics, and fault zone strengthening. The model is motivated by the premise that mineral redistribution impacts both fluid flow (by generating variations in crack porosity, permeability, and fluid pressure) and earthquake physics (by generating variations in strength along the interface). Therefore, there is coupling between fluid processes and chemical processes, with positive and negative feedbacks that determine the mechanics of slip behavior through the seismic cycle. In the seismogenic zone, these feedbacks can lead to fluid generated at depth to episodically pass through via fault-valve behavior. The numerical model for slip behavior is a 2-D block slider, or cellular automaton. Healing is treated as a temperature-dependent process of both stochastic nucleation of asperity cells and linear strengthening of static friction for those cells with log time. The length scale is imparted by the distance downdip along the slab-top geotherm in thermal models. The plumbing system is governed by the distribution of fluid production and permeability within the cellular network, and the model quantifies temporal and spatial variations in effective stress on the plate interface. The block slider and the fluid flow models are coupled through the effect of cementation: increases in strength are accompanied by decreases in permeability. This model of coupled fluid flow and slip reproduces several key observations from real subduction zones and leads to the conclusion that interseismic changes to porosity and permeability, imparted by cementation, along with varying fluid production at depth, can result in changes to earthquake magnitude distributions, recurrence intervals, fluid flow behavior, and aftershock propensity.