FORMATION AND EVOLUTION OF DAMAGE ZONES ALONG UPPER-CRUSTAL FAULTS

Understanding the formation and evolution of damage zones along upper-crustal faults is critical to understanding the dynamics of earthquake rupture, and episodic fluid flow and mineralization. Fracture damage represents a cumulative signature of fracture, sealing, and healing during fault growth, stress cycling and incremental slip. Studies have defined the density-distribution of damage along faults, commonly by counting intercepts of microfractures or mesofractures along scan lines. Measurements document that 1) damage density decreases systematically with distance from fault cores (site of concentrated shear), 2) damage zone thickness increases at a decreasing rate with cumulative fault displacement, and 3) maximum magnitude of damage, located at the inner contact with the fault core, is relatively independent of displacement. Although findings have led to successful models of damage development with displacement, the role of specific processes (e.g., growth, rupture, slip, healing) on damage evolution is less clear. Ongoing studies of exhumed faults are combined with borehole and core observations to better inform evolutionary models and permit quantification of damage development and healing with incremental slip in large displacement faults. At SAFOD, quantification of fracture density at five length scales shows a power-law size-frequency scaling consistent with similar measurements from exhumed faults; this relation applies to microfractures, shear fractures, and large secondary faults, and the interior and margins of damage zones. In core, open microcracks constitute less than several percent of the total microcrack population, much less than typically observed in exhumed faults from similar depths, suggesting the vast majority of microcracks at depth heal during fault activity. Cross-cutting relations, fabrics, and kinematic indicators show fractures are repeatedly reactivated, often under different stress states. This is illustrated along a macroscopic bend in the exhumed San Gabriel fault where the orientation distribution and kinematics of the most recently active fractures were reset as they entered the bend and again as they exited. Findings are used to extend models and quantify rate of damage development and energy dissipation as a function of evolutionary stage.