DYNAMIC FRACTURING IN FAULT TIP DAMAGE ZONES? AN OUTCROP STUDY OF THE SEVIER FAULT ZONE, SOUTHERN UTAH

Researchers have hypothesized that branching fracture geometries in rocks adjacent to faults may be evidence for past earthquakes. In this model, unstable, dynamic propagation at or above a critical velocity within the rock causes branching and could be used as a field-based fingerprint for past seismicity. We present a new model for fracture branching that leverages field, experimental, and numerical modeling to explain the evolution of fracture branching within a fault tip zone of the Sevier normal fault, Utah.

We investigate NE-striking fractures in the footwall of a NNE-striking, west-dipping fault segment, where incision has exposed a cross-sectional view of fractures within the cross-bedded Jurassic Navajo Sandstone. We used field data and drone imagery to document the fracture network, and we used drone-based imagery to build a structure-from-motion (SfM) virtual outcrop model, measuring 100 m high and 260 m wide, to aid our analysis of fracture geometry, intensity, and spacing regularity.

Results reveal an up-section increase in fracture intensity and decrease in spacing regularity accommodated by upward fracture branching. These fracture branching geometries are remarkably similar to those documented in previous field-based and experimental studies of dynamic fracturing. Our fractures strike 30 degrees from dominant fracture orientations in other structural positions, suggesting propagation in a perturbed stress field near the fault tip.

We propose mixed mode I-III generation of twist hackles that propagate from the tip line of an upward propagating parent fracture. Because these hackle fractures propagated upward with the parent fracture at spacings that appear to violate classic stress-shadow dampening, we posit that strain energy associated with earthquakes may have dissipated within the rock volume causing rapid, unstable fracture propagation. This is consistent with previous studies that showed concentration of earthquake ground motion within the footwalls of normal faults.