THE IMPACT OF SEMI-BRITTLE MATERIAL ON FAULT BEHAVIOR

Semi-brittle deformation is widespread in various flow regimes and fault zones and may also be crucial during crustal thinning, slow-slip events, and other strain transients. To date only very general analytical solutions are available for modeling semi-brittle flow, and numerical models cannot capture detailed flow behavior because of the simultaneously continuous and discrete nature of semi-brittle material. We therefore perform physical pseudo two-dimensional shear experiments to address two questions: (1) How does the semi-brittle material influence the stick-slip dynamics? (2) How does the brittle- ductile behavior influence shear localization?

We are using a silicone/acryl material mixture to represent a semi-brittle material. The acrylic elliptical discs behave rigidly while the silicone follows a linear viscous rheology and fills the spaces between the discs. The material mixture is deformed in a simple shear apparatus to a shear strain of approximately γ = 2. We monitor the force, displacement, and travel path of the discs. We systematically change the fraction of the beads and therefore the ratio between the ductile and brittle material as well as the strength contrast between the beads and the silicone.

When the bead density is high enough the beads jam during deformation. With increasing strain we observe a stick-slip behavior. The shape of the slip-event force curve is highly dependent on the viscosity of the weak material filling the voids between the beads. For the end-member case where the voids between the discs are filled with air, the release events are very sudden and short resulting in a sawtooth-like curve. Using silicone changes the slip recurring interval and damps the stress release resulting in a slower slip and a concave slip release curve. The deformation localization is also dependent on the weak media. While the shear band width is approximately seven beads wide with air as the weak phase it decreases to a width of three beads in the presence of silicone. Our experiments show that the deformation behavior of a semi-brittle material is fundamentally different from a viscous or brittle material and strongly dependent on the ratio between the two phases, an instructive relationship for interpreting natural semi-brittle structures.