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

The spatial distribution of effective normal stress, sigma e ${\sigma }_{\mathrm{e}}$, is essential for understanding the fault motion. Although Rice (1992, ) 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, ) 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 sigma e ${\sigma }_{\mathrm{e}}$ and slow earthquakes is often discussed. To quantitatively examine the causes of a local decrease in sigma e ${\sigma }_{\mathrm{e}}$ 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, ): the gradient of sigma e ${\sigma }_{\mathrm{e}}$ remarkably decreases with the increase in depth, whereas the realistic fluid properties rule out nearly constant sigma e ${\sigma }_{\mathrm{e}}$ at depth. We obtained a monotonic increase in sigma e ${\sigma }_{\mathrm{e}}$ with the increase in depth for the physically sound solutions and failed to generate a local decrease in sigma e ${\sigma }_{\mathrm{e}}$. The presence of a splay fault and fluid leakage though it cannot decrease sigma e ${\sigma }_{\mathrm{e}}$ locally. We found that a local decrease in permeability decreased sigma e ${\sigma }_{\mathrm{e}}$ 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.

Automated summary

The spatial distribution of effective normal stress, sigma e, is important for understanding fault motion. However, models based on the concept proposed by Rice (1992) and applicable to other tectonic settings have not been developed. Through model calculations considering the characteristics of subduction zones, it was found that a local decrease in permeability may lead to a decrease in sigma e locally and the occurrence of shallow slow earthquakes.