Degree Type


Date of Award


Degree Name

Master of Science


Geological and Atmospheric Sciences



First Advisor

Jacqueline E. Reber


Crustal faults can creep continuously or lock and slip violently producing earthquakes. Several mechanisms have been proposed to explain this variation in slip dynamics ranging from changes mineralogy to the changes in pore fluid pressure or fault normal stresses. As faults deform they produce a granular wear material that accommodates the majority of strain. The impact of the distribution of the grain sizes in fault rock on slip dynamics of faults has not received much attention so far. Here I propose that slip dynamics can be linked to the grain size distribution in a fault material and that this may have direct implication for the generation of earthquakes.

In this study, granular assemblages of different grain sizes and grain size distributions are sheared. Boundary conditions allow locking and elastic buildup of stress. Tightly packed granular systems lock and then dilate locally when sheared. This allows slip and requires rapid reorganization of the grains. Larger grains have more and larger slip events than smaller grains. When grains have a long axis longer than one tenth the width of the shear apparatus, dilation is inhibited by the boundaries of the shear apparatus and the effect of increasing grain size is inhibited. When grains of different sizes are sheared together the relative abundance of each grain size controls how much it contributes to the shear dynamics of the system.

Although the experiments are performed in 2D and use uniform grain shapes without comminution they have direct implications for natural faults. As fault systems develop over time the size distribution of fault rocks and the internal structure of the fault can change. Understanding the role of grain size in controlling slip dynamics can help us anticipate the evolution of faults

Copyright Owner

Jeremy Randolph-Flagg



File Format


File Size

65 pages

Included in

Geology Commons