Background
Pre-existing faults represent both important conduits for fluid flow in the subsurface and the most likely locations of future earthquakes. Understanding the distribution and geometry of faults in the Earth’s crust is therefore critical for predicting both subsurface fluid flow pathways and the potential for induced or natural seismic hazard. Geophysical data (e.g., seismic reflection profiles) lack the resolution to fully resolve faults in rock strata, and previously unrecognized faults or fault segments are periodically discovered when seismic rupture or fluid (e.g., CO2) leakage occurs along “hidden” faults. Our inability to adequately image or predict fault extents in the subsurface represents a substantial knowledge gap with wide-ranging implications, from predicting natural or induced seismicity to quantifying fault penetration probabilities critical to subsurface containment or fluid movement at sites for waste disposal, CO2 sequestration, gas storage, and resource (hydrocarbon, geothermal) management. Outcrop exposures of faulted rock strata provide the unique opportunity to directly observe and quantify heights and displacement patterns of naturally occurring faults, and to use these data to infer fault height in the subsurface. Previous outcrop-based studies of fault lengths, heights, and displacements are limited in that most studies have focused on lateral (map view) rather than vertical fault height, and the influence of mechanical and mineralogical rock properties (“mechanical stratigraphy”) are either not considered or only qualitatively assessed. This project combines fieldwork, close-range remote sensing, and laboratory analysis for quantitative assessment of fault height and displacement upward and downward through mechanically layered rocks.
Approach
Field-based outcrop characterization, high-resolution photogrammetric reconstruction, and laboratory analyses (to determine mineralogy of rock strata) are used to characterize interactions between mechanical stratigraphy and fault geometry at seven outcrop localities across the western United States. Data are analyzed using regression-based and probabilistic analyses, and outputs are leveraged to improve prediction of fault height in the subsurface. This approach is novel in that quantified rock properties are directly compared to fault displacement patterns. Data and predictions have wide-ranging relevance to predicting fault penetration across rock layers in the subsurface.
Accomplishments
Drone imagery, rock samples, Schmidt rebound data, and field measurements of fault properties (e.g., displacement and orientation) were acquired at seven outcrop locations in Texas, Utah, and Nevada. We (i) generated photogrammetric reconstructions with ground pixel resolutions of ca. 2-5 mm for each site, (ii) digitally interpreted each study locality and generated fault displacement vs. height analysis using field and digital datasets, and (iii) compiled and analyzed X-Ray diffraction (XRD) mineralogy data from faulted strata with companion structural data, rebound measurements, and fault displacement analyses. Our analyses of fault displacement gradient vs. XRD mineralogy and rebound show that rock properties fundamentally influence patterns of fault displacement in mechanically layered rocks. Results show generally negative correlations between fault displacement gradient and strong minerals (e.g., calcite, dolomite, quartz), reflecting primarily brittle deformation in rock layers with compositions dominated by strong minerals. Generally positive correlations between displacement gradient and weak minerals (i.e., clay in this study) highlight a tendency for increasingly ductile deformation with increased proportions of weak minerals within sedimentary layers.