Active tectonic insights into fault rheology and behaviour

Institute of Geophysics and Tectonics seminar with speaker Dr. Alex Copley (University of Cambridge).

Ever since the realization that faults accommodate the relative motions of parts of the Earth’s lithosphere, there has been controversy about their material properties and behaviour. A major question that has received much attention concerns understanding the friction laws that determine the stresses involved in fault slip, both seismically in earthquakes and during aseismic creep. In this talk I will summarise a range of observational constraints that can be placed upon the properties of faults. A range of geophysical and geological observations suggest that faults fail in earthquakes at shear stresses of less than c. 50 MPa, equivalent to effective coefficients of friction of less than 0.3, and possibly as low as 0.05. These low levels of effective friction are at least partly due to intrinsically low-friction fault rocks, although high pore fluid pressures probably also play a role. Relating the rates of aseismic fault creep to the stresses driving that creep reveals a highly non-linear relationship between these quantities, and provides an observational constraint on the form of the friction law. Finally, I will discuss how spatial variations in fault strength control the large-scale evolution of mountain ranges, using the example of the central Andes.