Localization in the high-strain domains is a pre-requisite for numerous weakening mechanisms such as thermal pressurization 3, 4 and shear heating 5, 6 that may induce weakening and may lead to an earthquake instability 7. The interactions and feedbacks between the domains ultimately determine the mode of fault slip, from earthquakes, slow slip to viscous creep 2. This type of instability is distinct from the frictional instability used to describe crustal earthquakes.įault zones are complex geological structures where deformation partitions and localizes into high-strain and low-strain domains over a wide range of length scales 1, 2. Failure of the fault zone nevertheless occurs once these weak layers coalesce in a kinematically favored network. However, it is difficult to produce “rate weakening” behavior due to the low measured stress exponent, n, of 1.3 ± 0.4 and the low activation energy, Q, of 16,000 ± 14,000 J/mol implying that the material will be strongly “rate strengthening” with a weak temperature sensitivity. Weakening of the fault rocks is hence intrinsic, it occurs once nanocrystalline layers form. Here, we show that such fault rocks are an order of magnitude weaker than their microcrystalline counterparts when deformed at identical experimental conditions. However, the rheology of nanocrystalline fault rocks remains poorly constrained.
![derivative of log base a of x derivative of log base a of x](http://1.bp.blogspot.com/_X1HiqovpZd0/S0qHHw5tWOI/AAAAAAAAAMY/AddVf10VdNM/s280/DerivativesOfExponentialLogarithmicInverseTrig.jpg)
Nanocrystalline fault rocks are ubiquitous in “principal slip zones” indicating that these materials are determining fault stability.
![derivative of log base a of x derivative of log base a of x](https://www.aazios.com/sites/default/files/egpp2.jpg)
Fault zones accommodate relative motion between tectonic blocks and control earthquake nucleation.