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Development of Wide, Asymmetric Damage Zones Early in Normal Fault Displacement Accumulation: Implications for Fluid Flow Prediction in Subsurface Systems

Session: Faults, Fractures, and Geomechanics for the Energy Transition

Presenting Author:

Benjamin Surpless

Authors:

Surpless, Benjamin E.¹, Palmer, JP², Rueda, Isabella³

(1) Earth and Environmental Geosciences, Trinity University, San Antonio, TX, USA, (2) Department of Earth and Environmental Geosciences, Trinity University, San Antonio, Texas, USA, (3) Department of Earth and Environmental Geosciences, Trinity University, San Antonio, TX, USA,

Abstract:

Fault damage zones develop due to fault tip propagation, fault core dynamics, flexure of rock adjacent to the fault plane, and dissipation of energy in earthquakes. We integrate outcrop data, 3D photogrammetry, and numerical modeling to assess damage zone development associated with a low-displacement normal fault. We suggest that normal fault damage zones may widen rapidly early in fault-displacement accumulation and display strong spatial asymmetry.

The steeply dipping Sevier fault zone near Orderville, Utah, displays multiple fault segments that accommodate extension. We focused on outcrop exposures of the Jurassic Navajo Sandstone near the tip of the Spencer Bench fault segment, where erosion has exposed a 2.5m-displacement cross-section. Access to outcrops was impossible on foot, so we used drone-based imagery to construct virtual outcrop models (VOMs) for analysis.

VOM analysis reveals a wide, asymmetric, four-zone fault architecture, including a ~40-m wide footwall damage zone, a narrow core (<1m), and a ~70-m hanging wall damage zone that transitions (~40 m) to a transfer zone, where fracturing was generated by strain transfer between this segment and a nearby fault. Scanline and network topologic analysis show that the hanging wall and transition zones display the highest fracture intensities, geometric complexities, and greatest network connectivity, while footwall and transfer zone fracturing is planar, vertically continuous, less connected, and less intense. Average fracture intensity increases up section across all zones, suggesting possible fracture branching during network development.

We also modeled a steeply-dipping, elliptical fault plane with displacement tapering in the Fault Response Module of MOVE 2023 (by Petex). Our results, using mechanical properties of the Navajo Sandstone, reveal that flexure associated with displacement tapering results in strong asymmetries in strain above the centroid (location of maximum displacement). There, strain is higher and more widely distributed in the hanging wall. This modeling shows that flexure associated with displacement tapering may strongly affect the distribution and intensity of damage zone development.

Our results suggest that even low-displacement normal fault segments may generate wide damage zones, with hanging wall-footwall asymmetry controlled by position relative to the fault centroid. Because damage zone fracturing cannot be imaged by geophysical methods, these findings permit better prediction of subsurface fluid pathways along fault zones.