[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-08] The role of fault zone heterogeneity in producing weak faults and promoting unstable slip

★Invited Papers

*John D Bedford^1,2, Daniel R Faulkner², Nadia Lapusta³ (1.JAMSTEC, 2.Univ. of Liverpool, 3.Caltech)

Keywords:Fault zone heterogeneity, Friction, Fault weakening

Large tectonic faults often appear frictionally weak when compared to laboratory measurements of frictional strength and display a range of fault slip behaviour, from aseismic creep to slow-slip to stick-slip. Fault zone heterogeneity if often qualitatively invoked to explain fault weakness and the spectrum of fault behaviour observed in nature; however the role of heterogeneity in controlling the mechanical behaviour of faults is poorly understood. Here we perform a series friction experiments at slow slip rates (0.3 to 3 microns/s) on faults comprised of heterogeneously distributed patches of weak, rate-strengthening gouge (clay) and strong, rate-weakening gouge (quartz). We systematically vary the size of the clay and quartz patches and find that these heterogeneous faults are significantly weaker and more unstable than homogenized mixtures of the two gouges. The heterogeneous faults also experience unstable stick-slip behaviour when the size of the quartz patch is sufficiently large. The observed weakening for the heterogeneous faults is greater than predicted if the strength evolution is assumed to be controlled by the relative sizes of the quartz and clay patches. This enhanced weakening is associated with the weaker clay patches promoting localized shear in the strong quartz patches resulting in slip at a lower overall shear stress than the compositionally equivalent homogeneous gouges. Our findings demonstrate how fault zone heterogeneity can cause the apparent weakness of many crustal faults and also reduce the frictional stability, which may lead to slow- or earthquake slip in nature.