Quantifying Near-Surface Patterns of Bedrock Fractures and Assessing Controls on Fracture Formation.

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

The primary goal of this study is to further develop, calibrate, and test a seismic refraction-based methodology to investigate subsurface rock-strength properties and fracture distributions. The strength and coherence of rocks play key roles in shaping landscapes, resisting erosion, and modulating landslide hazards. The key factor, however, is not the strength of intact rock, but rather the effective strength of the entire rock-mass at the surface where it interacts with climatic, hillslope, and biotic variables. This effective strength is modulated by the development of fractures that weaken the rock mass and make it more susceptible to erosion, physical and chemical weathering, biologic activity, or collapse. To delineate variations in fracture density in the shallow subsurface, a promising, but largely unexplored methodology combines shallow seismic refraction surveys of bedrock outcrops with laboratory analyses of ?intact? samples. Initial results indicate two common fracture patterns versus depth: rock that is uniformly fractured (apparently by large-scale tectonic forces); and rock with a distinct fracture gradient in an upper layer (apparently due to climatic and biotic fracturing processes) that overlies a much stronger, less fractured lower layer. Although very promising, this methodology needs to be refined, tested, and explored more thoroughly. Hence the goal of this study. By focusing the method development and calibration on artificial and natural bedrock exposures that permit detailed observation, measurement, and sampling of fracture properties, the seismically-derived results can be tested and validated by direct comparison to field observations. With an improved calibration in hand, two fundamental questions about near-surface fracturing will be investigated: how do near-surface fracture patterns vary both with depth and spatially across the landscape; and what are the dominant controls on near-surface fracture formation? In a mountainous field site in Colorado, the relative importance of (i) freeze-thaw processes in causing rock fracturing versus (ii) gravitational forces that cause fracturing due to hillslope steepness and curvature will be investigated. Why do landslides occur on some hillslopes, but not on others of equal steepness? Why do parts of the landscape erode much more quickly than other, similar appearing areas? One key control on erosion or collapse of hillslopes is the strength of the underlying rock. Whereas various rock types typically have different intrinsic strengths (a granite versus a mudstone, for example), the density of fractures in a rock also exerts a fundamental control on its strength: higher fracture densities and greater connectivity among the fractures weaken a rock and increase its susceptiblility to landsliding or erosion. Recent research suggests that fracture densities are at least as important as intrinsic rock strength in controlling hillslope stability. Despite the importance of rock fracturing for hillslope stability, methodologies for quantifying fracture densities have remained elusive: commonly the bedrock is hidden under a layer of soil, and even when exposed, only fractures on the topmost surface of the bedrock are visible. A promising, new approach uses shallow seismic surveys to probe the top 10-20 m of a hillslope and convert variations in seismic velocity with depth into changes in fracture density with depth. This research will explore this nascent technology by testing and calibrating it in natural and artificial bedrock exposures where the fracture density has been previously quantified. Subsequently, this shallow seismic methodology (which uses backpack-able portable arrays) will be used to test how variations in (i) the intensity and frequency of freeze-thaw processes and (ii) hillslope curvature and steepness influence the density and depth of rock fracturing. The overall goal is to improve our ability to efficiently assess both hillslope vulnerability to erosion or failure by landsliding and the impact of plants, weather, and topography on hillslope stability and bedrock fracturing.