EXPERIMENTAL ANALYSIS OF THE EFFECT OF GRAIN SIZE DISTRIBUTION ON STICK-SLIP AND CREEP BEHAVIOR

Faults accommodate displacement through a continuum of aseismic steady slip or creep to earthquake-generating stick-slip events. The mode of slip can change along the length of a single fault, as is the case of the San Andreas Fault. Field observations along the San Andreas Fault have shown that fault gauge in a locked section of the fault is dominated by small grains ~100 μm in diameter, while sediment at a creeping section of the fault has a wider distribution of grain sizes. This difference in grain size distribution is interpreted as the result of different slip in the creeping zone compared to the locked zone. Here we investigate whether different grain size distributions lead to different modes of slip.

We test the effect of grain size distribution on fault behavior using elliptical, acrylic disks of three sizes and a simple shear apparatus with a localized shear plane and energy conserving boundary conditions that do not prescribe the strain rate or the force. We record displacement and force with time and vary grain size distribution. Early results show that a uniform grain size generates stick slip behavior and that slip events scale with the diameter of the grains such that our smallest grains approximate creep. When mixed grain sizes are used, slip events fall between the expected magnitude for the smallest and the largest grain size present. Understanding how grain mechanics affect fault behavior will contribute to our overall understanding of why faults do or do not generate earthquakes and can give insight into the behavior of the San Andreas Fault.