Natural Fault Systems Research Articles

Earthquake activity related to seismic cycles in a model for a heterogeneous strike-slip fault

We investigate the evolution of seismicity within large earthquake cycles in a model of a discrete strike-slip fault in elastic solid. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction and dislocation creep along the fault, and 3D elastic stress transfer. The fault consists of brittle parts which fail during earthquakes and undergo small creep deformation between events, and aseismic creep cells which are characterized by high ongoing creep motion. This mixture of brittle and creep cells is found to generate realistic aftershock sequences which follow the modified Omori law and scale with the mainshock size. Furthermore, we find that the distribution of interevent times of the simulated earthquakes is in good agreement with observations. The temporal occurrence, however, is magnitude-dependent; in particular, the small events are clustered in time, whereas the largest earthquakes occur quasiperiodically. Averaging the seismicity before several large earthquakes, we observe an increase of activity and a broadening scaling range of magnitudes when the time of the next mainshock is approached. These results are characteristics of a critical point behavior. The presence of critical point dynamics is further supported by the evolution of the stress field in the model, which is compatible with the observation of accelerating moment release in natural fault systems.

Automated interpretation of geological fault surfaces imaged in 3D seismic data

A novel approach to the automatic extraction of geological faults from 3D seismic data is described and comparisons of manually and automatically picked fault geometries interpreted from a high-quality 3D seismic dataset are presented. Based on the premise that faults introduce discontinuities to otherwise coherent layers (horizons) in seismic data volumes, prototype software has been developed that employs a coherency measure to detect points of significant discontinuity. An iterative highest confidence first merging strategy then groups clusters of ‘faulty’ points to form many planar segments, or patches, of larger fault surfaces. Because a planar model cannot reflect the curvature and localized irregularities that characterize real fault surfaces, the 3D fault surface geometry is generated using a combined parametric and residual field model to link the fault patches. The parametric model, in our case a planar model, represents the basic structure of the fault surface whilst the residual field models the surface deviation from the planar model. This is one of the few projects of this kind that combines expertise from computer vision and structural geology. We conclude with a discussion on how empirical structural geological criteria describing the geometry of natural fault systems will be integrated with the autopicking software to optimize the geological validity of the interpreted fault geometry.

Ergodicity in Natural Fault Systems

— Attempts to understand the physics of earthquakes over the past decade generally have focused on applying methods and theories developed based upon phase transitions, materials science, and percolation theory to a variety of numerical simulations of extended fault networks. This recent work suggests that fault systems can be interpreted as mean-field threshold systems in metastable equilibrium (Rundle et al., 1995; Klein et al., 1997; Ferguson et al., 1999), and that these results strongly support the view that seismic activity is highly correlated across many space and time scales within large volumes of the earth’s crust (Rundle et al., 2000; Tiampo et al., 2002). In these systems, the time averaged elastic energy of the system fluctuates around a constant value for some period of time and is punctuated by major events that reorder the system before it settles into another metastable energy well. One way to measure the stability of such a system is to check a quantity called the Thirumalai-Mountain (TM) energy metric (Thirumalai and Mountain, 1993; Klein et al., 1996). In particular, using this metric, we show that the actual California fault system is ergodic in space and time for the period in question, punctuated by the occurrence of large earthquakes, and that, for individual events in the system, there are correlated regions that are a subset of the larger fault network.

The nature and tectonic significance of fault-zone weakening: an introduction

Abstract Fault zones control the location, architecture and evolution of a broad range of geological features, act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards. In general, the repeated localization of displacements along faults and shear zones, often over very long time scales, strongly suggests that they are weak relative to their surrounding wall rocks. Geophysical observations from plate boundary faults such as the San Andreas fault additionally suggest that this fault zone is weak in an absolute sense, although this remains a controversial issue. Our understanding of fault-zone structure and mechanical behaviour derive from three main sources of information: (1) studies of natural fault zones and their deformation products (fault rocks); (2) seismological and neotectonic studies of currently active natural fault systems; (3) laboratory-based deformation experiments using rocks or rock-analogue materials. These provide us with a basic understanding of brittle faulting in the upper crust of the Earth where the stress state is limited by the frictional strength of networks of faults under the prevailing fluid-pressure conditions. Under the long-term loading conditions typical of geological fault zones, poorly understood phenomena such as subcritical crack growth in fracture process zones are likely to be of major importance in controlling both fault growth and strength. Grain-size reduction in highly strained fault rocks produced in the plastic-viscous and deeper parts of frictional regime can lead to changes in deformation mechanisms and relative weakening that can account for the localization of deformation and repeated reactivation of crustal faults. Our understanding the interactions between deformation mechanisms, metamorphic processes and the flow of chemically active fluids is a key area for future study. An improved understanding of how fault- or shear-zone linkages, strength and microstructure evolve over large changes in finite strain will ultimately lead to the development of geologically more realistic numerical models of lithosphere deformation that incorporate displacements concentrated into narrow, weaker fault zones.

The Plausibility of Long-wavelength Stress Correlation or Stress Magnitude as a Mechanism for Precursory Seismicity: Results from Two Simple Elastic Models

Observations of accelerating seismic activity prior to large earthquakes in natural fault systems have raised hopes for intermediate-term eartquake forecasting. If this phenomena does exist, then what causes it to occur? Recent theoretical work suggests that the accelerating seismic release sequence is a symptom of increasing long-wavelength stress correlation in the fault region. A more traditional explanation, based on Reid's elastic rebound theory, argues that an accelerating sequence of seismic energy release could be a consequence of increasing stress in a fault system whose stress moment release is dominated by large events. Both of these theories are examined using two discrete models of seismicity: a Burridge-Knopoff block-slider model and an elastic continuum based model. Both models display an accelerating release of seismic energy prior to large simulated earthquakes. In both models there is a correlation between the rate of seismic energy release with the total root-mean-squared stress and the level of long-wavelength stress correlation. Furthermore, both models exhibit a systematic increase in the number of large events at high stress and high long-wavelength stress correlation levels. These results suggest that either explanation is plausible for the accelerating moment release in the models examined. A statistical model based on the Burridge-Knopoff block-slider is constructed which indicates that stress alone is sufficient to produce accelerating release of seismic energy with time prior to a large earthquake.

Pull-apart stepover structures in an asphalted road surface—a geological curiosity

An unusual and perhaps unique example of strike-slip displacement and pull-apart stepover structures is described from a relatively newly asphalted road surface in Central Norway. The structures, which include pull-apart ‘basins’, show a remarkable similarity to those developed both in natural strike-slip fault systems and in physical analogue models. A surface of décollement is believed to have developed at the interface between the new asphalt layer and the old, bitumen-emulsified, planed metalled surface, perhaps aided by the presence of excess rainwater. Gravitationally induced slippage of the asphalt on this 1 in 15 gradient hill-road actually occurred several months after the asphalt had been laid.

CHARACTERISING THE FULL STRESS TENSOR BASED ON OBSERVATIONS OF DRILLING-INDUCED WELLBORE FAILURES IN VERTICAL AND INCLINED BOREHOLES LEADING TO IMPROVED WELLBORE STABILITY AND PERMEABILITY PREDICTION

To minimise wellbore failures in unstable environments, knowledge of the complete stress tensor is crucial to designing optimally-stable borehole trajectories, selecting suitable mud weights, and determining appropriate casing points. Understanding how the in situ stress field interacts with the drilling and production of a well enables one to design for maximum stability and to facilitate intersecting the greatest population of hydraulically-conductive fractures for efficient production. Knowledge of the in situ stress field is also important to reduce uncertainties in sand production prediction to allow more aggressive completion designs and production schedules.A new interactive software system, Stress and Failure of Inclined Boreholes (SFIB) (Peska and Zoback, 1995a) is used to demonstrate how observations of drilling-induced compressive and tensile wellbore failures from acoustic and electrical images in vertical and inclined boreholes can be integrated with routinely-collected drilling data (leak-off and drill stem tests) to construct a well-constrained stress tensor. These techniques can also exploit wellbore image data to constrain in situ rock strength in vertical and inclined wells. This paper illustrates how to apply this knowledge to limit wellbore instability, design optimally stable wellbores, develop constraints that help mitigate problems associated with sand production, and optimise productivity of fractured reservoirs.In addition to mapping drilling-induced wellbore features, image data can also be used to determine the distribution, orientation, and apparent aperture of natural fractures and fault systems. With knowledge of the orientations and magnitudes of the in situ stresses it is possible to identify the subset of fractures that are likely to be hydraulically conductive.Examples of recent applications in the North Sea, Gulf of Mexico, California, and Puerto Rico illustrating how this integrated approach can be used in a variety of tectonic settings.

Dynamical segmentation and rupture patterns in a "toy" slider-block model for earthquakes

Abstract. Lattice models for earthquakes have been shown to capture some of the dynamical properties possessed by natural fault systems. These properties include: 1) a scaling (power low) region in the curve representing event frequency as a function of magnitude and area; and 2) space-time clustering of events. To understand the physical origin of these and other effects, we examine the simplest kind of "toy" slider block model. We obtain results indicating that this model displays several additional kinds of phenomena seen in real earthquake faults, even though "realistic" physics is missing from the model. Asperity-like slip distributions in this model arise from strong elastic coupling, rather than the spatially heterogeneous frictional strength often inferred for real faults. Simulation results indicate that "characteristic" earthquakes can be produced as a consequence of the nonlinear dynamics. Thus segmentation on faults may be a result of the nonlinear dynamics as well as being due to geometric properties of fault systems. These conclusions may be modified when more "realistic" physics is added to the model, although the presence of dynamical effects in the toy model calculations similar to those observed in nature demonstrates alternative possibilities for the origin of these effects.

Kinematics of secondary synthetic (“P”) faults in wrench systems

Natural and experimental wrench fault systems commonly develop two distinct synthetic minor fault orientations termed primary and secondary synthetic faults. Experiments show that secondary synthetic faults are restricted to cases where overall simple-shear kinematics are attained or closely approximated. The secondary synthetic faults serve to link the primary faults. The formation of secondary synthetic faults is explained in terms of infinitesimal kinematics. Primary synthetic faults alone produce a kinematic pattern incompatible with the requirement of simple shear that there be no longitudinal strain parallel to the principal deformation zone (PDZ). When combined with secondary synthetic faults the resultant kinematic pattern approximates simple shear, thus preserving compatibility. Primary and secondary synthetic faults form to accommodate the fixed displacements imposed on the PDZ in a simple-shear wrench system.

Extensional fault systems in sedimentary basins: a review of analogue model studies

Scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems. This paper reviews recent results of an on-going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems. Four fundamental model styles, scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust, are reviewed. Extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults. These experimental results are directly comparable to structures found in intracontinental rifts. Extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino-style fault arrays in the pre-rift sequence and listric growth faults in the syn-rift sequence. These results compare with highly extended rift basins and terranes such as the Basin and Range Province, USA. Extension above a simple listric fault produces a characteristic hangingwall roll-over and crestal collapse graben system. At high values of extension, superposed crestal collapse grabens occur and a fan of listric growth faults is developed. These structures bear striking similarities to growth faults in progradational delta systems. Ramp-flat listric extensional fault systems using sand-mica models produce a characteristic family of structures with an upper roll-over crestal collapse system, a ramp syncline and reverse fault/fold zone and a lower roll-over crestal collapse system. An example of a ramp-flat fault system from the Gulf Coast, USA, is presented as a comparison with the analogue model results. Preliminary results of three-dimensional extensional model experiments are presented to illustrate the complexities in such fault systems. In all of the experimental models, it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall. The analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins.

Roughness and wear during brittle faulting

In many natural fault systems, the thickness of gouge and breccia increases approximately linearly with displacement. In contrast, many experimental faults show non linear thickness/displacement relationships. The linear relationship for natural faults has been explained in the past using engineering models for adhesive or abrasive wear. Non linear relationships for experimental faults have not been explained. A model for wear during brittle faulting which considers the scaling of surface roughness can successfully describe the difference between wear on experimental faults and wear on natural faults. We suggest the linear relationship for natural faults results from the approximately self‐similar roughness of the fault surfaces. Experimental faults do not generally follow linear relationships because the roughness of ground surfaces normally used in experimental studies scales differently than the roughness of natural rock surfaces. A simple model which assumes that the volume of wear material created is proportional to the volume of mismatch between the surfaces can explain the differences between wear on experimental faults and wear on natural faults. For ground surfaces of experimental samples, the volume of mismatch is independent of the total slip because at the largest scales these surfaces are flat. In contrast, for natural, self‐similar surfaces the volume of mismatch increases with slip, because slip isolates larger and larger asperities from their original positions in the opposite surface. Natural and experimental faults evolve differently because of the difference in scaling of their respective surface roughnesses.