Damage zone heterogeneity on seismogenic faults in crystalline rock; a field study of the Borrego Fault, Baja California

Complex fracture damage around large faults is often simplified to fit exponential or power law decay in fracture density with distance from the fault. Noise in these datasets is attributed to large subsidiary faults or random natural variation. Through a field study of the Borrego Fault (Baja California) damage zone, combining mm-resolution structural mapping and point sampling, we show that such variations are the expression of systematic damage heterogeneity. The oblique-slip Borrego Fault comprises NW and SE segments that ruptured during the Mw7.2 2010 El Mayor-Cucapah earthquake. Measurements of fracture density along eight linear fault-perpendicular transects and from a high-resolution 68 m² structural map display a power law decay and define footwall damage zone widths of ~85 m and ~120 m for the NW and SE segments respectively. Variance in fracture density decays with distance following an inverse exponential relationship, to background variance at ~16 m. Spatial analysis of the high-resolution fracture map reveals a patchy distribution of high- and low-intensity clusters at metre- and decimetre-scales. We attribute high-intensity clusters at these scales to local complexity caused by interactions between minor subsidiary faults (10¹ m length and 10⁻²-10⁻¹ m displacement). Fracture density differences between high- and low-intensity clusters decrease with distance from the fault, demonstrating a systematic change in outcrop-scale damage heterogeneity. Based on these observations we present a revised model for damage zone growth including growth of heterogeneity.