EFFECTS OF STRAIN RATE AND LOAD PATH ON SCALING ROCK MECHANICS EXPERIMENTS TO NATURAL FAULT DAMAGE ZONES

The rapid drop in frictional resistance at the leading tip of earthquake ruptures is accompanied by impulsive particle accelerations and stresses in the wall rocks. The resulting transient stress and strain rate conditions are associated with brittle damage, including fracture, fragmentation, and shear failure in the adjacent wall rocks and fault damage zone. Recent work in our lab has shown that such high strain-rate fragmentation is characterized by the production of submicron particle sizes and corresponding large new fracture surface areas; consequently, this process is important as a mechanism for changing constitutive and hydraulic properties of fault damage zones and as an energy sink during seismic rupture. The critical strain rate threshold through which brittle deformation transitions from a failure along a few discrete fracture planes to pulverization is a process governed by microcrack growth dynamics, which are sensitive to loading rate, but also load path and pre-existing material heterogeneity. In this presentation we describe a new experimental design using a Split Hopkinson Pressure Bar apparatus, inspired by theoretical fracture mechanics and new field observations, to study coseismic damage zone fragmentation. The results of these experiments are more directly scalable to natural fault damage zones than past dynamic rock mechanics experiments because they more closely honor the load path experienced by damage zone rocks, and therefore provide new tools to study seismic rupture in the rock record.