Outer Damage Zone Research Articles

Characteristics of earthquake ruptures and dynamic off-fault deformation on propagating faults

Abstract. Natural fault networks are geometrically complex systems that evolve through time. The evolution of faults and their off-fault damage patterns are influenced by both dynamic earthquake ruptures and aseismic deformation in the interseismic period. To better understand each of their contributions to faulting we simulate both earthquake rupture dynamics and long-term deformation in a visco-elasto-plastic crust subjected to rate- and state-dependent friction. The continuum mechanics-based numerical model presented here includes three new features. First, a 2.5-D approximation is created to incorporate the effects of a viscoelastic lower crustal substrate below a finite depth. Second, we introduce a dynamically adaptive (slip-velocity-dependent) measure of fault width to ensure grid size convergence of fault angles for evolving faults. Third, fault localization is facilitated by plastic strain weakening of bulk rate and state friction parameters as inspired by laboratory experiments. This allows us to simulate sequences of episodic fault growth due to earthquakes and aseismic creep for the first time. Localized fault growth is simulated for four bulk rheologies ranging from persistent velocity weakening to velocity strengthening. Interestingly, in each of these bulk rheologies, faults predominantly localize and grow due to aseismic deformation. Yet, cyclic fault growth at more realistic growth rates is obtained for a bulk rheology that transitions from velocity-strengthening friction to velocity-weakening friction. Fault growth occurs under Riedel and conjugate angles and transitions towards wing cracks. Off-fault deformation, both distributed and localized, is typically formed during dynamic earthquake ruptures. Simulated off-fault deformation structures range from fan-shaped distributed deformation to localized splay faults. We observe that the fault-normal width of the outer damage zone saturates with increasing fault length due to the finite depth of the seismogenic zone. We also observe that dynamically and statically evolving stress fields from neighboring fault strands affect primary and secondary fault growth and thus that normal stress variations affect earthquake sequences. Finally, we find that the amount of off-fault deformation distinctly depends on the degree of optimality of a fault with respect to the prevailing but dynamically changing stress field. Typically, we simulate off-fault deformation on faults parallel to the loading direction. This produces a 6.5-fold higher off-fault energy dissipation than on an optimally oriented fault, which in turn has a 1.5-fold larger stress drop. The misalignment of the fault with respect to the static stress field thus facilitates off-fault deformation. These results imply that fault geometries bend, individual fault strands interact, and optimal orientations and off-fault deformation vary through space and time. With our work we establish the basis for simulations and analyses of complex evolving fault networks subject to both long-term and short-term dynamics.

Acoustic evidence for a broad, hydraulically active damage zone surrounding the Alpine Fault, New Zealand

We present an analysis of the wireline sonic logs acquired through Alpine Fault hanging wall rocks during the second phase of the Deep Fault Drilling Project (DFDP-2). Due to inaccuracy in the initial tool-processed slowness logs, a more robust reprocessing of the data using the slowness–time coherence method was required. The reprocessing of the data yielded slowness logs that provide a significantly better estimate of compressional and shear slowness. The average slowness values of compressional and shear waves are 68 μs/ft and 110 μs/ft, respectively, corresponding to velocities of 4.5 km/s and 2.8 km/s, with some excursions related to fractures, borehole conditions, and tool settings, but no systematic downhole trends. The log values are compared to laboratory measurements of P- and S-wave velocity for samples of Alpine schist and mylonitic rocks to examine the scale dependence of the measurement. The average log velocity is 2 to 18% slower than the laboratory measurements due to the presence of mesoscale fractures that are present in situ but are not sampled at the laboratory scale. Using effective medium theory, we estimate the crack density in the hanging wall host rock to be ≤0.4. Comparison to field-scale seismic studies indicates that there is a broad (500–600 m wide) outer damage zone surrounding the intensely deformed inner damage zone of the Alpine Fault. This fractured outer zone facilitates fluid flow through the hanging wall and around the Alpine Fault.

Mapping a geothermal anomaly using apatite (U‐Th)/He thermochronology in the Têt fault damage zone, eastern Pyrenees, France

Abstract(U‐Th)/He ages on apatite obtained in the vicinity of the Têt fault hydrothermal system show a large variability. In the inner damage zone adjacent to the fault core, where fluid flows are concentrated, AHe ages display a large scatter (3–41 Ma) and apatite ageing. Samples from the outer damage zone show young ages with less dispersion (0.9–21.1 Ma) and apatite rejuvenation. Outside the damage zone, ages are consistent with the regional exhumation history between 20 and 12 Ma. The important age dispersion found in the damage zone is interpreted as the result of 4He mobility during fluid infiltration. Our results show that thermochronological data close to a fault should be interpreted with caution, but may offer a new tool for geothermal exploration.

Fault zone architecture and its scaling laws: where does the damage zone start and stop?

Abstract Damage zones of different fault types are investigated in siliciclastics (Utah, USA), carbonates (Majella Mountain, Italy) and metamorphic rocks (western Norway). The study was conducted taking measurements of deformation features such as fractures and deformation bands on multiple 1D scanlines along fault walls. The resulting datasets are used to plot the frequency distribution of deformation features and to constrain the geometrical width of the damage zone for the studied faults. The damage-zone width of a single fault is constrained by identifying the changes in the slope of cumulative plots made on the frequency data. The cumulative plot further shows high deformation frequency by a steep slope (inner damage zone) and less deformation as a gentle slope (outer damage zone). Statistical distributions of displacement and damage-zone width and their relationship are improved, and show two-slope power-law distributions with a break point at c. 100 m displacement. Bleached sandstones in the studied siliciclastic rocks of Utah are associated with a higher frequency of deformation bands and a wider damage zone compared to the unbleached zone of similar lithology. Fault damage zones in the carbonate rocks of Majella are often host to open fractures (karst), demonstrating that they can also be conductive to fluid flow.

Controls on fault zone structure and brittle fracturing in the foliated hanging wall of the Alpine Fault

Abstract. Three datasets are used to quantify fracture density, orientation, and fill in the foliated hanging wall of the Alpine Fault: (1) X-ray computed tomography (CT) images of drill core collected within 25 m of its principal slip zones (PSZs) during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections up to 500 m from the PSZs, and (3) CT images of oriented drill core collected during the Amethyst Hydro Project at distances of ∼ 0.7–2 km from the PSZs. Results show that within 160 m of the PSZs in foliated cataclasites and ultramylonites, gouge-filled fractures exhibit a wide range of orientations. At these distances, fractures are interpreted to have formed at relatively high confining pressures and/or in rocks that had a weak mechanical anisotropy. Conversely, at distances greater than 160 m from the PSZs, fractures are typically open and subparallel to the mylonitic or schistose foliation, implying that fracturing occurred at low confining pressures and/or in rocks that were mechanically anisotropic. Fracture density is similar across the ∼ 500 m width of the field transects. By combining our datasets with measurements of permeability and seismic velocity around the Alpine Fault, we further develop the hierarchical model for hanging-wall damage structure that was proposed by Townend et al. (2017). The wider zone of foliation-parallel fractures represents an outer damage zone that forms at shallow depths. The distinct < 160 m wide interval of widely oriented gouge-filled fractures constitutes an inner damage zone. This zone is interpreted to extend towards the base of the seismogenic crust given that its width is comparable to (1) the Alpine Fault low-velocity zone detected by fault zone guided waves and (2) damage zones reported from other exhumed large-displacement faults. In summary, a narrow zone of fracturing at the base of the Alpine Fault's hanging-wall seismogenic crust is anticipated to widen at shallow depths, which is consistent with fault zone flower structure models.

Predictive permeability model of faults in crystalline rocks; verification by joint hydraulic factor (JH) obtained from water pressure tests

In the present study, a new model is proposed to predict the permeability per fracture in the fault zones by a new parameter named joint hydraulic factor (JH). JH is obtained from Water Pressure Test (WPT) and modified by the degree of fracturing. The results of JH correspond with quantitative fault zone descriptions, qualitative fracture, and fault rock properties. In this respect, a case study was done based on the data collected from Seyahoo dam site located in the east of Iran to provide the permeability prediction model of fault zone structures. Datasets including scan-lines, drill cores, and water pressure tests in the terrain of Andesite and Basalt rocks were used to analyse the variability of in-site relative permeability of a range from fault zones to host rocks. The rock mass joint permeability quality, therefore, is defined by the JH. JH data analysis showed that the background sub-zone had commonly <3 Lu (less of 5 ×10−5 m 3/s) per fracture, whereas the fault core had permeability characteristics nearly as low as the outer damage zone, represented by 8 Lu (1.3 ×10−4 m 3/s) per fracture, with occasional peaks towards 12 Lu (2 ×10−4 m 3/s) per fracture. The maximum JH value belongs to the inner damage zone, marginal to the fault core, with 14–22 Lu (2.3 ×10−4–3.6 ×10−4 m 3/s) per fracture, locally exceeding 25 Lu (4.1 ×10−4 m 3/s) per fracture. This gives a proportional relationship for JH approximately 1:4:2 between the fault core, inner damage zone, and outer damage zone of extensional fault zones in crystalline rocks. The results of the verification exercise revealed that the new approach would be efficient and that the JH parameter is a reliable scale for the fracture permeability change. It can be concluded that using short duration hydraulic tests (WPTs) and fracture frequency (FF) to calculate the JH parameter provides a possibility to describe a complex situation and compare, discuss, and weigh the hydraulic quality to make predictions as to the permeability models and permeation amounts of different zone structures.

Contributions of In-Situ Stress Transient Redistribution to Blasting Excavation Damage Zone of Deep Tunnels

With the background of construction of the headrace tunnels with the deepest buried depth in China at present, by means of carefully acoustic velocity detection and analysis of Excavation Damage Zone (EDZ), the contributions to damage zones made by the effect of in situ stress transient redistribution are studied and compared with the extent of damage caused by the explosive load. Also, the numerical simulation was adopted to verify detecting the results. It turned out that the in situ stress transient redistribution during blasting has great influence on the development of EDZ of deep tunnels. The blasting excavation-induced damage zone of deep tunnels can be divided into the inner damage zone and the outer damage zone from the excavation surface into surrounding rocks. Although this damage zone dividing method is similar to the work of Martino and Chandler (2004), the consideration of developing a mechanism of the inner damage zone, especially the contribution of in situ stress transient redistribution, is totally different. The inner damage zone, which accounts for 29–57 % of the total damage zone, is mainly caused by explosive load and in situ stress transient adjustment, while the outer damage zone can be mostly attributed to the static redistribution of in situ stress. Field tests and numerical simulation indicate that the in situ stress transient redistribution effect during blasting contributes about 16–51 % to the inner damage zone in the 2# headrace tunnel of Jinping II Hydropower Station. For general cases, it can be concluded that the in situ stress transient redistribution is one of the main contributors of an excavation damage zone, and damage caused by in situ stress transient redistribution effect may exceed the damage caused by explosion load and become the main inducing factor for damage with the rise of in situ stress levels.

Determination of volume loss and element mobility patterns associated with the development of the Copper Basin fault, Picacho State Recreation Area, SE California, USA

The fault zone architectural elements of the Copper Basin fault include a fault core, inner damage zone, and outer damage zone. Utilizing Yb as a reference frame element, significant elemental mass changes were documented across all three architectural zones, with the greatest change in the fault core. Porosity and volumetric strain both increase toward the principal slip surface reaching a maximum in the inner damage zone and then dropping significantly in the fault core. In the damage zone and unaffected wall rocks, smectite is the most prominent clay species, whereas illite dominates the fault core suggesting that temperatures may have reached ∼100–150 °C or more during the production of the fault core. A number of studies have interpreted similar mass changes and clay mineral reactions to indicate high water/rock ratios. Such interpretations imply that during rupture, permeability increased within the fault core allowing it to act as a pathway for hot, chemically reactive fluids. Then during and after rupture, CO2 and SiO2 rich fluids moved into the inner and outer damage zones where calcite and quartz precipitation healed the fractured and permeable rock.

Predictive permeability model of faults in crystalline rocks; verification by grouting in Seyahoo dam

This paper deals with quantitative fault zone descriptions, qualitative fracture and fault rock properties, and engineering data in the study of the permeability structure of fault zones. Datasets include scan-lines, drill cores and cement grouting from Seyahoo dam in andesite and basalt rocks, from which systematic grouting volumes can be used to analyze the in-site relative permeability both in host rocks and fault zones. Dam-scale injection of cement reveals patterns that can be ascribed to the impact of faulting; there is an increase in cement injection in fault zones compared to areas with background fracturing away from faults. In detail, there is an innate division of the rock volume into sub- zones characterized by distinct structural style and permeability, with a background level and three fault related sub- zones (fault core, inner damage zone, and outer damage zone). Injection data shows that the background sub-zone commonly can be injected with less than 0.05 m 3 cement per meter dam (commonly not injected), whereas the fault

Predictive permeability model of faults in crystalline rocks; verification by grouting in Seyahoo dam

This paper deals with quantitative fault zone descriptions, qualitative fracture and fault rock properties, and engineering data in the study of the permeability structure of fault zones. Datasets include scan-lines, drill cores and cement grouting from Seyahoo dam in andesite and basalt rocks, from which systematic grouting volumes can be used to analyze the in-site relative permeability both in host rocks and fault zones. Dam-scale injection of cement reveals patterns that can be ascribed to the impact of faulting; there is an increase in cement injection in fault zones compared to areas with background fracturing away from faults. In detail, there is an innate division of the rock volume into sub- zones characterized by distinct structural style and permeability, with a background level and three fault related sub- zones (fault core, inner damage zone, and outer damage zone). Injection data shows that the background sub-zone commonly can be injected with less than 0.05 m 3 cement per meter dam (commonly not injected), whereas the fault

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

We analyze the structure of the Archaean Pretorius fault in TauTona mine, South Africa, as well as the rupture-zone that recently reactivated it. The analysis is part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project that utilizes the access to 3.6 km depth provided by the mining operations. The Pretorius fault is a ~10 km long, oblique-strike-slip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in TauTona mine. We identify here three structural zones within the fault-zone: (1) an outer damage zone, ~100 m wide, of brittle deformation manifested by multiple, widely spaced fractures and faults with slip up to 3 m; (2) an inner damage zone, 25–30 m wide, with high density of anastomosing conjugate sets of fault segments and fractures, many of which carry cataclasite zones; and (3) a dominant segment, with a cataclasite zone up to 50 cm thick that accommodated most of the Archaean slip of the Pretorius fault, and is regarded as the ‘principal slip zone’ (PSZ). This fault-zone structure indicates that during its Archaean activity, the Pretorius fault entered the mature fault stage in which many slip events were localized along a single, PSZ. The mining operations continuously induce earthquakes, including the 2004, M2.2 event that rejuvenated the Pretorius fault in the NELSAM project area. Our analysis of the M2.2 rupture-zone shows that (1) slip occurred exclusively along four, pre-existing large, quasi-planer segments of the ancient fault-zone; (2) the slipping segments contain brittle cataclasite zones up to 0.5 m thick; (3) these segments are not parallel to each other; (4) gouge zones, 1-5 mm thick, composed of white ‘rock-flour’ formed almost exclusively along the cataclasite-host rock contacts of the slipping segments; (5) locally, new, fresh fractures branched from the slipping segments and propagated in mixed shear-tensile mode; (6) the maximum observed shear displacement is 25 mm in oblique-normal slip. The mechanical analysis of this rupture-zone is presented in Part II (Heesakkers et al., Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona mine, South Africa 2011, this volume).

Effects of internal structure and local stresses on fracture propagation, deflection, and arrest in fault zones

The way that faults transport crustal fluids is important in many fields of earth sciences such as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For understanding the permeability evolution and maintenance in a fault zone, its internal structure and associated local stresses and mechanical properties must be known. This follows because the permeability is primarily related to fracture propagation and their linking up into interconnected clusters in the fault zone. Here we show that a fault zone can be regarded as an elastic inclusion with mechanical properties that differ from those of the host rock. As a consequence, the fault zone modifies the associated regional stress field and develops its own local stress field which normally differs significantly, both as regard magnitude and orientation of the principal stresses, from the regional field. The local stress field, together with fault-rock heterogeneities and interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability development. We provide new data on the internal structure of fault zones, in particular the fracture frequency in the damage zone as a function of distance from the fault core. New numerical models show that the local stress field inside a fault zone, modelled as an inclusion, differ significantly from those of the host rock, both as regards the magnitude and the directions of the principal stresses. Also, when the mechanical layering of the damage zone, due to variation in its fracture frequency, is considered, the numerical models show abrupt changes in local stresses not only between the core and the damage zone but also within the damage zone itself. Abrupt changes in local stresses within the fault zone generate barriers to fracture propagation and contribute to fracture deflection and/or arrest. Also, analytical solutions of the effects of material toughness (the critical energy release rate) of layers and their interfaces show that propagating fractures commonly become deflected into, and often arrested at, the interfaces. Generally, fractures propagating from a compliant (soft) layer towards a stiffer one tend to become deflected and arrested at the contact between the layers, whereas fractures propagating from a stiff layer towards a softer one tend to penetrate the contact. Thus, it is normally easier for fractures to propagate from the host rock into the damage zone than vice versa. Similarly, it is easier for fractures to propagate from the outer, stiffer parts of the damage zone to the inner, softer parts, and from the stiff host rock to the outer damage zone, than in the opposite directions. These conclusions contribute to increased understanding as to how fractures propagate and become arrested within fault zones, and how the fault zone thickness is confined at any particular time during its evolution.

Predictive permeability model of extensional faults in crystalline and metamorphic rocks; verification by pre-grouting in two sub-sea tunnels, Norway

This paper link quantitative fault zone descriptions, qualitative fracture and fault rock properties, and engineering data in the study of the permeability structure of fault zones. Datasets include scan-lines, drill cores and cement pre-grouting from two sub-sea tunnels in gneissic and granitic rocks, from which systematic pre-grouting volumes can be used to analyse the in-site relative permeability both in host rocks and fault zones. Major extensional faults intersected by the tunnels reveal common fault rocks surrounding intensively fractured rock lenses in the core. Fracture frequencies in these lenses can reach 100 fractures/metre (f/m). In the bounding damage zones, networks of fracture sets make up an inner zone of fairly high frequency (20–30 f/m) of fault-parallel, long fractures connected by shorter fractures. An outer zone has lower frequencies (<20 f/m) and more diverse fracture orientations and lengths. There is a general increase in fracture frequency from the background level of the protolith towards the fault core. Tunnel-scale injection of cement reveals patterns that can be ascribed to the impact of faulting; there is an increase in cement injection in fault zones compared to areas with background fracturing away from faults. In detail, there is an innate division of the rock volume into sub-zones characterized by distinct structural style and permeability, with a background level and three fault related sub-zones (fault core, inner damage zone, and outer damage zone). Injection data shows that the background sub-zone commonly can be injected with less than 0.05 m 3 cement per metre tunnel (commonly not injected), whereas the fault core has permeability characteristics nearly as low as the outer damage zone, represented by 0.1–0.2 m 3 cement per metre tunnel, with occasional peaks towards 0.5 m 3. The maximum of cement injection lies in the inner damage zone, marginal to the fault core, with 0.3–0.7 m 3 cement per metre tunnel, locally exceeding 1 m 3. This gives a relative relationship for cement injection of approximately 1:2:1 between fault core, inner damage zone, and outer damage zone of extensional fault zones in crystalline and metamorphic rocks.

The evolution of the damage zone with fault growth in sandstone and its multiscale characteristics

We document the formation and evolution of the damage zones associated with strike‐slip faults in porous sandstone, through detailed field and statistical studies of faults of increasing slip magnitudes. The faults initiate as sheared joints with discontinuous damage zone located primarily at fault tips and fault surface irregularities. With increasing slip, the damage zone develops by progressive fracture infilling and is organized into two components with different characteristics. The first of these components is the inner damage zone flanking the fault core with a high fracture frequency and a thickness scaling with fault slip. It contributes to the progressive widening of the fault core with increasing deformation. The second of these components is the outer damage zone with a larger and more variable thickness and a lower fracture frequency than the inner zone. The origin and evolution of the inner and outer damage zones are closely related to the history of fault development, as presented in a new conceptual model for damage zone evolution with fault growth. The statistical properties of the fault networks, determined by scan line surveys, evolve with fault growth. This is attributed to the progressive saturation of the fault networks. Well‐developed fault networks have multifractal properties with important consequences in forecasting and characterizing faults from limited data sets. The conceptual model and statistical data presented here may be used as predictive tools to better estimate the geometrical and statistical properties of the damage zones associated with large faults with remotely resolvable slip magnitude in the subsurface.

Excavation-induced damage studies at the Underground Research Laboratory

The Underground Research Laboratory (URL) is situated in the Lac du Bonnet granite batholith, in southeastern Manitoba, Canada. The URL was developed to study issues related to the deep geologic disposal of used fuel from nuclear reactors and has developed into an International Atomic Energy Agency recognized geotechnical center of excellence. Extensive rock mechanics research has been conducted, including work to understand the character and extent of excavation damage. Experience at the URL has shown that damage exists around underground openings and that the damage develops from the energy imparted to the rock by the excavation method and by redistribution of the in situ stress field around the excavated openings. Subsequent near-by excavations, removal of loose material from the existing tunnel, thermal loads and pore pressure changes will all influence the development and extent of rock damage, as does the rock type and its fabric. Studies at the URL have shown that in highly stressed rock, damage will develop around underground openings even when a low-energy excavation method is used and that properly designing the excavation geometry and orientation plays a major role in constructing stable openings. Two zones of excavation-affected rock have been shown to exist from studies at the URL; (1) a zone of irreversibly damaged rock surrounding the excavation, which may include failed zones, inner and outer damage zones and (2) a zone of excavation disturbance where the in situ stresses are influence by the excavation no damage is measurable. Measurements of the properties of the damaged rock (ultrasonic velocity, transmissivity) using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more highly damaged inner zone. The inner damage zone may have zones of failed rock depending on stress concentrations relative to rock strength and damage imparted by excavation method. The inner damage zone is attributed to the effects of the excavation method and stress redistribution and the outer damage zone is attributed to the effects of stress redistribution alone. In lower stressed rock the extent of the outer zone is much less than for an opening in highly stressed rock with the same excavation shape and orientation relative to in situ stresses. The excavation disturbed zone describes the volume of rock surrounding the damage zone where the stress redistribution is too small to permanently change rock properties (unless some other stress increase, such as thermal loading, is added), and it is also characterized non-permanent but potentially long-term changes in hydraulic head.

Energy density theory formulation and interpretation of cracking behavior for piezoelectric ceramics

Any attempt made to separate energy into electrical and mechanical parts may lead to inconsistencies as they do not necessarily decouple. This is illustrated by application of the energy density function in the linear theory of piezoelasticity. By assuming that a critical energy density function prevails at the onset of crack initiation, it is possible to establish the relative size of an inner and outer damage zone around the crack tip; they correspond to the ligaments at failure caused by pure electric field and pure mechanical load. On physical grounds, the relative size of these zones must depend on the relative magnitude of the mechanical and electrical load. Hence, they can vary in size depending on the electromechanical material and damage resistance properties. Numerical results are obtained for the PZT-4, PZT-5H, and P-7 piezoelectric ceramics. These two ligaments for the two damage zones may coincide for appropriate values of the applied electrical field and mechanical load. Explicit expression of the energy density factor S is derived showing the mixed mode electromechanical coupling effects. The factor S can increase or decrease depending on the direction of the applied electric field with reference to the poling direction. This is in contrast to the result obtained from the energy release rate quantity, which remains unchanged for electric field in the direction of poling or against it.