Steady-State Effective Normal Stress in Subduction Zones Based on Hydraulic Models and Implications for Shallow Slow Earthquakes

Abstract

The spatial distribution of effective normal stress, (Formula presented.), is essential for understanding the fault motion. Although Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) proposed a steady-state solution for a vertical strike-slip fault zone with constant fluid properties, models that are based on the concept by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) and are applicable for other tectonic settings have not yet been developed. Such a model is particularly important in subduction zones because the relationship between low (Formula presented.) and slow earthquakes is often discussed. To quantitatively examine the causes of a local decrease in (Formula presented.) on a shallow region of the subduction zone, we performed model calculations that incorporated mechanisms characteristic to subduction zones. Our basic model, which considers the effect of smectite dehydration and the mechanical effect of subduction, yields results that are consistent with those reported by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1): the gradient of (Formula presented.) remarkably decreases with the increase in depth, whereas the realistic fluid properties rule out nearly constant (Formula presented.) at depth. We obtained a monotonic increase in (Formula presented.) with the increase in depth for the physically sound solutions and failed to generate a local decrease in (Formula presented.). The presence of a splay fault and fluid leakage though it cannot decrease (Formula presented.) locally. We found that a local decrease in permeability decreased (Formula presented.) locally around an impermeable zone and, thus, possibly led to the occurrence of shallow slow earthquakes. The water release caused by the dehydration reaction may not be the dominant factor, although smectite dehydration releases silica and promotes its precipitation.