Numerical Modeling of Rock Flexure Caused by Displacement Tapering Along a Normal Fault Plane: a New Explanation for the Generation of Asymmetric Normal Fault Damage Zones

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Authors:

Abstract:

The development of normal fault damage zones is well-documented, but to our knowledge, no one has modeled how rock flexure affects damage zone distribution and geometry. Damage zones are important because they commonly display volumes of high intensity fracturing and brittle deformation adjacent to a fault plane, and these zones may have permeabilities several orders of magnitude greater than adjacent intact rock. Because damage zone fracturing cannot be detected by geophysical imaging, research results that permit prediction of the spatial distribution of damage zone fracturing could benefit geothermal energy exploration and hydrogeologic applications.

In this study, we applied Fault Response Modeling within the MOVE software suite (by PETEX) to investigate how displacement, lateral fault segment length, and lithology impact stress and strain in the volume of rock surrounding a normal fault. We modeled a steeply dipping elliptical fault plane with maximum displacement at the centroid and displacement decreasing to zero at the fault tipline. We primarily modeled the well-studied Jurassic Navajo Sandstone, which, as a porous sandstone, could serve as an excellent geothermal reservoir. We focused on assessing maximum strain, dilation, maximum stress, and maximum Coulomb shear stress (MCSS), because they inform the spatial distribution of damage zone development. 

We tested fault length-displacement ratios based on real-world fault populations. Changes in displacements and lengths resulted in similar mathematical relationships with similar damage zone asymmetries but with different extrema. To test variations in lithology, we varied Young’s modulus and Poisson’s ratio consistent with real-world rock properties. Shale, the Navajo Sandstone, and basalt have different Young’s moduli, and we tested three values of Poisson’s ratio for the Navajo Sandstone. Shale and basalt have large differences in Young’s moduli and display the most extreme differences in their modeled stress values. Low and high Poisson’s ratio sandstones displayed the most extreme strain differences. The strongest lithologic control on volumetric distribution of damage zone development is based on Poisson’s ratio. As with displacement, similar damage zone asymmetries were observed for all lithologies.

We conclude that displacement tapering sets up footwall and hanging wall flexure resulting in strong asymmetry relative to the displacement centroid. Wide damage zones are most likely to form in the hanging wall above the fault slip centroid and in the footwall below the fault slip centroid.