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 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, 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 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 analysis 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 analysis, 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
Project efforts to date include: (i) field work and data acquisition at five outcrop locations in Texas and Utah, with drone imagery, rock samples, Schmidt rebound data, and field measurements of fault properties (e.g., displacement and orientation) collected at each site, (ii) photogrammetric reconstruction of outcrops, with ground pixel resolutions of ca. 2-5 mm achieved for all study sites, (iii) digital interpretation of study localities and initial fault displacement vs. height analysis using field and digital datasets, and (iv) X-Ray diffraction analysis (XRD) and compilation of mineralogic data with structural data, rebound measurements, and fault displacement analyses. Initial project results suggest that fault displacement gradient is systematically related to rock mineralogy, specifically clay content. This finding has implications for predicting fault height beyond the limits of geophysical data resolution in the subsurface.