Paper No. 10
Presentation Time: 8:00 AM-6:00 PM
DAMAGE ZONE CRACKS FORMED UNDER QUASI-STATIC AND DYNAMIC LOADING CONDITIONS: INSIGHTS FROM FIELD OBSERVATIONS, EXPERIMENTS, AND THEORY
GRIFFITH, W. Ashley1, NEWMAN, Patrick
2 and TOKACH, Nicholas
2, (1)Earth and Environmental Sciences, University of Texas at Arlington, Geoscience Building Room 107, 500 Yates St. Box 19049, Arlington, TX 76019, (2)Department of Geology and Environmental Science, University of Akron, Akron, OH 44325-4101, wagriff@uta.edu
The geometry and distribution of secondary cracks around faults have been used in the past to infer properties of the process zone during fault propagation. Process zones can, however, form during the other stages of fault growth and related activity. For example, if the fault nucleated along a pre-existing source of anisotropy, such as a joint, some cracks will likely reflect process zone characteristics inherited from mode I (opening) crack propagation rather than mode II (shearing) or mode III (tearing) propagation typically associated with faults. It is also necessary to consider that the rate of propagation significantly influences the stresses within the process zone: earthquakes propagating at rates approaching the Rayleigh wave speed of a given material carry stress concentrations for which the principal stresses may be magnified and rotated significantly with respect to the stress perturbation associated with a quasi-static crack tip. Finally, secondary cracks may be formed due to slip along non-planar faults, rather than during fault propagation.
Recent advances in the study of dynamic shear rupture physics have shed light on criteria to distinguish cracks formed by dynamic (seismic) loading from those formed by quasi-static loading related to quasi-static fault propagation and slip as well as deformation along non-planar faults. Combined observations from microcrack distributions in quartz grains around exhumed pseudotachylyte-bearing faults in granitoid rocks, laboratory experiments of tensile crack formation around propagating, sub-Rayleigh mode II ruptures, and continuum-mechanics-based models of rupture propagation and frictional fault slip shed light on criteria to distinguish between secondary cracks formed by different seismic and aseismic processes. Results suggest that the orientation and density distribution of secondary cracks with respect to the main fault, as well as the absolute position of the cracks along the main fault, can help to distinguish the source of loading during crack formation.
