Rebecca J. Lunn

How Thick is a Fault? Fault Displacement‐Thickness Scaling Revisited

Fault zone thickness is an important parameter for many seismological models. We present three new fault thickness datasets from different tectonic settings and host rock types. Individual fault zone components (i.e., principal slip zones, fault core, damage zone) display distinct displacement-thickness scaling relationships. Fault component thickness is dependent on the type of deformation elements (e.g., open fractures, gouge, breccia) that accommodate strain, the host lithology, and the geometry of pre-existing structures. A compilation of published fault displacement-thickness data shows a positive trend over seven orders of magnitude, but with three orders of magnitude scatter at a single displacement value. Rather than applying a single power-law scaling relationship to all fault thickness data, it is more appropriate and useful to seek separate scaling relationships for each fault zone component and to understand the controls on such scaling.

Determining analytic solutions of multiple species contaminant transport, with sorption and decay

Analytic solutions play an important role in the testing of models evaluated by numerical methods. This paper presents a straight forward method for determining analytic solutions to multiple species contaminant transport problems in porous media. The method uses Fourier sine transforms (as opposed to Laplace transforms) to obtain the solution of Cho (1970, Can. J. Soil. Sci., 51: 339–350) in a simpler way. It provides a flexible approach to the introduction of new boundary and initial conditions within the model. New solutions are presented for constant and exponential initial contaminant profiles.

Development and application of a nitrogen modelling system for large catchments

The NELUP Nitrogen Modelling System (NMS) has been developed to model the transport of nitrates within large catchments. It is one component of a suite of hydrological models that have been integrated with ecological and agroeconomic models in a Decision Support System. The NMS has been designed to be applied in four stages: determination of nitrogen input categories; simulation of plant uptake and decay using the farm management model EPIC; simulation of initial nitrate profiles in the unsaturated zone with the one-dimensional transport model (MP); and finally a simulation of nitrate transport using the SHETRAN-UK catchment flow and transport modelling system. The NMS has been applied to model the transport of nitrate within the 3000 km2 Tyne basin for the period 1985–1989. Predicted in-stream nitrate concentrations throughout the catchment display a low base concentration with peaks corresponding to overland flow events following fertiliser application. Model results compare favourably with the available monthly NRA nitrate observation data at a number of different locations within the catchment. A short field study has been undertaken within the Tyne which demonstrates the occurrence of the transport of nitrate within overland flow events, a hitherto undocumented process.

Simulating brittle fault growth from linkage of preexisting structures

Many researchers have proposed conceptual models of fault development that are based on the linkage of preexisting structures such as isolated faults, joints or veins. To date, such models largely use theoretical mechanics to explain the detailed damage zone geometries observed in linkage structures. In this paper, we present the first numerical simulations of the temporal and spatial development of geometrically complex fault linkage structures using the finite element model for fault damage zone evolution, MOPEDZ. Simulations show spatial and temporal fault zone evolution for a range of preexisting joint (or fault) geometries and stress conditions. Simulations show that linkage geometries are governed by three key factors: the stress ratio; the original joint geometry, such as contractional or dilational configurations; and the orientation of the principal stress. Simulated linkage structures display close correspondence to field observations of fault zone geometry, with all secondary and tertiary damage features being reproduced. The research also demonstrates that given information on the regional stress conditions, numerical modeling can be used to predict fault zone geometries, and hence, identify the most (and least) likely structures for promoting fluid flow.

How can we improve estimates of bulk fault zone hydraulic properties

Abstract The fluid flow properties of faults are highly variable and spatially heterogeneous. We use numerical simulation of flow through field maps of detailed fault zone architecture to demonstrate that flow across the fault zone is controlled by connected high-permeability pathways, which are highly tortuous in mapped fault outcrops. Such small-scale, geometrically complex, fault zone architectural features can never be resolved for subsurface faults. Consequently, the key to prediction of subsurface bulk fault zone hydraulic properties is a statistical characterisation of the likelihood and frequency of such connected pathways. We demonstrate for a single architectural feature, the fault core, that thickness variation along strike can be well described by a spatially correlated random field with a spherical covariance structure. These data are from a single site in a specific lithology. To enable such statistics to be used to make predictions at other sites, a large number of similar datasets must be pooled. This will enable us to relate such spatial statistics to gross properties such as host rock lithology and fault throw, which are measurable for subsurface faults.

Simulating spatial and temporal evolution of multiple wing cracks around faults in crystalline basement rocks

Fault zones are structurally highly spatially heterogeneous and hence extremely complex. Observations of fluid flow through fault zones over several scales show that this structural complexity is reflected in the hydrogeological properties of faults. Information on faults at depth is scarce, hence, it is highly valuable to understand the controls on spatial and temporal fault zone development. In this paper we increase our understanding of fault damage zone development in crystalline rocks by dynamically simulating the growth of single and multiple splay fractures produced from failure on a pre-existing fault. We present a new simulation model, MOPEDZ (Modeling Of Permeability Evolution in the Damage Zone surrounding faults), that simulates fault evolution through solution of Naviers equation with a combined Mohr-Coulomb and tensile failure criteria. Simulations suggest that location, frequency, mode of failure and orientation of splay fractures are significantly affected both by the orientation of the fault with respect to the maximum principal compressive stress and the conditions of differential stress. Model predictions compare well with published field outcrop data, confirming that this model produces realistic damage zone geometries.

Flood Perception and Mitigation: The Role of Severity, Agency, and Experience in the Purchase of Flood Protection, and the Communication of Flood Information

Protection of human life and property from flooding is a strategic priority in the UK. We examine how to encourage home owners to protect themselves and their residences. A model of factors that influence the decision to buy flood-protection devices is tested using survey data from 2109 home owners. The results show that the majority of respondents have not purchased domestic flood protection (N = 1732; 82.1%). Purchase of flood-protection devices was influenced by age; perceived seriousness; and beliefs about, and trust in, the role of regulators in managing flooding. In younger respondents the perceived seriousness of the dangers of flooding acted as precursors and barriers to action depending on individual sense of responsibility and agency. The second part of the study examines responsiveness to information. Information about flooding alone was insufficient to promote behavioural change, particularly among people who had not experienced a flood or who believed that they were not in a flood zone. Implications for understanding flood protection, managing agency issues, and flood-communication campaigns are discussed.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Brittle structures focused on subtle crustal heterogeneities : implications for flow in fractured rocks

Host rock mechanical heterogeneities influence the spatial distribution of deformation structures and hence predictions of fault architecture and fluid flow. A critical factor, commonly overlooked, is how rock mechanical properties can vary over time, and how this will alter deformation processes and resultant structures. We present field data from an area in the Borborema Province, NE Brazil, that demonstrate how temporal changes in deformation conditions, and consequently processes, exert a primary control on the spatial distribution and geometric attributes of evolving deformation structures. Furthermore, each temporal deformation phase imparted different hydraulic architecture. The earliest flowing structures are localized upon subtle ductile heterogeneities. Following fault formation, both fault core and damage zone were flow conduits. In later stages of faulting pseudotachylyte welding created a low-permeability fault core and annealed high-permeability fractures within the fault damage zone. Modern flow occurs along a zone of later open shear fractures, defined by the mechanical strength contrast between the host rock and annealed fault. This second hydraulically conductive zone extends hundreds of metres from the edge of the annealed fault damage zone, creating a flow zone far wider than would be predicted using traditional fault scaling relationships. Our results demonstrate the importance of understanding successive deformation events for predicting the temporal and spatial evolution of hydraulically active fractures.

Rock fracture grouting with microbially induced carbonate precipitation

Microbially induced carbonate precipitation has been proposed for soil stabilization, soil strengthening and permeability reduction as an alternative to traditional cement and chemical grouts. In this paper we evaluate the grouting of fine aperture rock fractures with calcium carbonate, precipitated through urea hydrolysis, by the bacteria Sporosarcina pasteurii. Calcium carbonate was precipitated within a small-scale and a near field-scale (3.1 m2) artificial fracture consisting of a rough rock lower surfaces and clear polycarbonate upper surfaces. The spatial distribution of the calcium carbonate precipitation was imaged using time-lapse photography and the influence on flow pathways revealed from tracer transport imaging. In the large-scale experiment, hydraulic aperture was reduced from 276 μm to 22 μm, corresponding to a transmissivity reduction of 1.71x10-5 m2/s to 8.75x10-9 m2/s, over a period of 12 days under constantly flowing conditions. With a modified injection strategy a similar three orders of magnitude reduction in transmissivity was achieved over a period of three days. Calcium carbonate precipitated over the entire artificial fracture with strong adhesion to both upper and lower surfaces and precipitation was controlled to prevent clogging of the injection well by manipulating the injection fluid velocity. These experiments demonstrate that microbially induced carbonate precipitation can successfully be used to grout a fracture under constantly flowing conditions and may be a viable alternative to cement based grouts when a high level of hydraulic sealing is required and chemical grouts when a more durable grout is required.