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

Fault movement can range from steady creep to seismic stick slip, and a single fault can stick and slip in one section while it creeps in another. Similarly, fault zones can range from discrete planes with fine grained fault gouge to anastomosing shear zones containing large undeformed blocks. We use a physical model to test how differences in size and size distribution of fault material can affect the dynamics of deformation.

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 force. We systematically vary grain size distribution and record displacement and force with time. Early results show that when only one grain size is used 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 while the frequency of events is governed by the smallest grain size. Understanding how grain mechanics affect fault behavior will contribute to our overall understanding of why faults do or do not generate earthquakes.