Stick-slip behavior in saturated fault gouge; insights from grain-scale models
Abstract
Many natural faults are filled with gouge, an unconsolidated collection of granular material, saturated with fluid. The interactions between fluid pressure and fault stability may exert an important control on fault-stability. Fluids may inhibit or retard earthquakes, due to dilatancy hardening - an increase in the effective stress on grain contacts caused by a drop in pore pressure resulting from porosity increase that accompanies shear. Also, externally-imposed variations in fluid pressure (either natural or man-made) may trigger earthquakes. To explore this dynamic system, we use a grain-scale model based on the discrete element method, coupled with a continuum model of fluid pressure that assumes pressure-driven flow through a permeable material (Goren et al., JGR, 2011). This model allows us to explore the feedbacks between porosity changes arising from rearrangement of grains, and local pressure and permeability variations due to changing pore configurations. We conducted a series of numerical experiments consisting of circular two-dimensional grains trapped between two parallel rough boundaries, which represent fault gouge and the surrounding fault blocks, respectively. The system is taken to be periodic in the layer-parallel direction. We impose a fixed confining stress on the boundary walls, and apply a variable shear stress to one of the boundaries. This rate of change of the shear stress is proportional to the difference between a constant applied driving velocity and the actual velocity of the boundary. With this setup, we can observe a range of fault behavior, from continuous or oscillatory creep near the driving velocity (stable sliding) to stick-slip behavior (unstable sliding). Unstable sliding is promoted when confining stresses are large or the driving velocities are small, as predicted by a simple block-slider theoretical model. We also vary the permeability of the boundaries (which depends on the properties of the surrounding wall rock) and the internal permeability (which depends on grain size of the gouge particles). We have compared wet simulations with permeable boundaries to dry simulations conducted under the same conditions. Slip events in the wet simulations are, on average, smaller (in energy released and stress drop) and more frequent. In particular, wet simulations have fairly common events with only moderate slip velocity (on the order of the driving velocity) that are rare in the dry simulation. These small events are usually associated with localized dilation and pressure reduction. Larger slip events are accompanied by compaction and pressure increase. These slip events have smaller peak velocities and longer durations than events in dry simulations that have the same energy release. This is, in part, due to some events being preceded by precursory slow slip. The overall effect of fluid may be to strengthen or weaken the layer, depending on conditions. Small slow slip events may be an effect of dilatancy hardening. However, any small compaction of a fragile system may enhance pressure enough to trigger the larger events. We will illustrate the spatial distributions and temporal evolution of shear and fluid pressure for a variety of slip events.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T24A..04S
- Keywords:
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- 8045 STRUCTURAL GEOLOGY / Role of fluids;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting