Preexisting Fault Planes Research Articles

A conjugated structure discloses interaction between two fault systems in eastern Taiwan during 2022 Guangfu earthquake

The Longitudinal Valley (LV) in the eastern Taiwan is located at the convergent boundary between the Philippine Sea Plate and the Eurasian Plate. Within the LV, two significant fault systems, the Longitudinal Valley Fault (LVF) in the eastern part and the Central Range Fault (CeRF) in the western part, have been recognized. However, understanding the precise mechanisms and spatial relationships of the two fault systems in the middle segment of the LV has remained a challenge. In this study, we integrate various data sources, including geodetic measurements, seismic data, field investigations, and simulation results, all obtained during the 2022 Guangfu earthquake event. Our goal is to shed light on the complex structural interactions between the two fault systems. We introduce a coupled structural model consisting of a main fault plane and a coupled ramp plane. The deep-seated fault plane is constrained using seismic data, while the shallow ramp plane is determined through field investigations. Geodetic measurements are utilized to constrain the surface deformation resulting from dislocation model. The simulation outcomes reveal that the 2022 Guangfu earthquake was primarily triggered by the activity along the CeRF. The CeRF experienced an eastward reverse slip of approximately 58 cm along the fault plane. This fault not only intersects the LVF beneath the Coastal Range but also initiates the development of a ramp plane along pre-existing fault planes of the LVF. This intricate interplay results in approximately 20 cm of vertical surface deformation within the Coastal Range. The linear connection observed in the simulation residuals is associated with pre-existing faults within the LVF system. This alignment corresponds to the ground distribution of ramp plane, suggesting that older fault systems within this area exhibit characteristics of reactivation in response to earthquake-induced processes. Our study not only pinpoints the potential location of the CeRF intersecting with the LVF in the middle segment of the LV but also provides insight into one of the interaction mechanisms between the CeRF and the LVF. These findings hold significant implications for assessing seismogenic structural activity and enhancing the mitigation of regional earthquake hazards in the future.

Characteristics of land subsidence, earth fissures and related disaster chain effects with respect to urban hazards in Xi’an, China

The ancient capital of Xi’an is a typical city in China with a shortage of water resources. With an urban population of 7 million, its domestic and industrial use of water depends almost entirely on groundwater. Large-scale long-term extraction of groundwater has caused severe land subsidence and triggered the reactivation of active normal faults in the Quaternary under-consolidated sediments underlying the city. This has further led to ground ruptures that have gradually evolved into earth fissure zones. During this process, there has been a close spatiotemporal relationship between land subsidence and the formation of earth fissures. From field monitoring data, the subsidence bowls are typically located in loess depressions between earth fissure zones within structural blocks of sediment cut by active Quaternary faults and preexisting fault planes. The subsidence bowls are generally elliptical in shape, with the long axes consistent with the preferred orientation of the earth fissures. The underlying Quaternary active faults and preexisting fault planes predispose the intervening sediment blocks to formation of earth fissures, and intense groundwater extraction has led to accelerated subsidence and earth fissuring during the past 50 years. Mining groundwater, land subsidence and earth fissures occur in a sequence and constitute a disaster chain with respect to urban hazards in Xi’an. Thus, it is important to understand the relationships between these events. This paper summarizes the characteristics of land subsidence and earth fissures in Xi’an and discusses the chain of relationships that connects them.

Laboratory evidence of strength recovery of a healed fault: implications for a mechanism responsible for creating wide fault zones

Fault zones consist of a high-strain fault core and a surrounding damage zone of highly fractured rock. The close, reciprocal relationship between fault zones and earthquake rupture evolution demands better understanding of the processes that create and modify damage zones. This study modeled the evolution of a damage zone in the laboratory by monitoring seismic signals (acoustic emissions) in a specimen of ultramylonite stressed to failure. The result provided evidence supporting the strength recovery of parts of the healed surface. A new fault initiated in an area of heterogeneous structure a short distance from the preexisting fault plane. Repeated cycles of fracture and healing may be one mechanism responsible for wide fault zones with multiple fault cores and damage zones.

Paleozoic fault systems of the Tazhong Uplift, Tarim Basin, China

This report presents interpretations developed from a detailed study of new three-dimensional (3-D) high-resolution reflection seismic data in a portion of Tarim Basin. The tectonic history began with oceanic spreading during the Cambrian–Early Ordovician and continues beyond the Silurian–Devonian time of oceanic closure. Paleozoic faults of the Tazhong Uplift in the hinterland of Tarim Basin are capped by thick and undisturbed Meso-Cenozoic sedimentary sequences. Four Paleozoic fault systems have been recognized: (1) the Cambrian–Early Ordovician extensional faulting, (2) the Late Ordovician NWW trending thrust faulting, (3) the Silurian–Devonian NNE strike-slip faulting and (4) the Permian plutonism influenced by pre-existing fault planes. Zones of weakness created during Cambrian–Early Ordovician extensional faulting influenced subsequent tectonic movements. The Late Ordovician fault system divides the Tazhong Uplift into several deformation zones. Their mechanical characteristics vary across the study area, with stronger thrusting in the east. The Silurian–Devonian strike-slip fault system consists of three components: main faults, subordinate en echelon faults, and fault troughs. The main faults appear as steeply dipping, almost vertical, offsets on seismic map view, with associated flower structures on seismic profiles, together with other levels of faults. The Permian magmatic plugs have a spotty or bandy distribution, and are interpreted to have utilized former faults.

Temporal and spatial evolution of hypocentres and anisotropy from the Darfield aftershock sequence: implications for fault geometry and age

The first four months of aftershocks of the Darfield earthquake have been studied using data from temporary and permanent seismic stations to investigate the fault geometry, stress field and evolution of seismicity and seismic properties. Earthquake relocations illuminate fault segments and show that the majority of aftershocks occurred beyond the areas of highest slip during the Darfield earthquake. Seismic anisotropy shows a mixture of fast directions parallel to the maximum horizontal stress and fault-parallel fast directions. This, combined with the lack of observable growth of seismicity along fault segments, suggests that the Greendale Fault broke a pre-existing fault plane.

Fault inversion vs. new thrust generation: A case study in the Malargüe fold-and-thrust belt, Andes of Argentina

The reverse reactivation of pre-existing normal faults has been documented in many orogens. In other cases, the lack of sub-surface information has allowed the construction of both inversion and non-inversion structural models. We will analyze the possibility of fault inversion in one such case, in the Malargüe fold-and-thrust belt in the Andes of Argentina. In order to address this issue, we modeled fault inversion vs. new thrust generation using the ReActiva 2.4 software, and varying the physical parameters for the pre-existing fault and the host rock. The results of modeling are compared to the possible sub-surface characteristics of faults in the Malargüe fold-and-thrust belt. We show that the orientation of these structures in the Andean stress field makes them likely to reactivate if the pre-existing fault planes present a low coefficient of friction and/or fluid overpressure. Both are expectable in the geological setting in which the structures are found. We conclude that fault inversion cannot be dismissed, and should be taken into account for structural models of the Malargüe fold-and-thrust belt. Our results can be extended to other orogens with similar characteristics.

Long-period, long-duration seismic events during hydraulic fracture stimulation of a shale gas reservoir

We report here a series of long-period and long-duration (LPLD) seismic events observed during hydraulic fracturing in a shale gas reservoir. These unusual events, 10–100 s in duration, are observed most clearly in the frequency band of 10–80 Hz and are remarkably similar in appearance to tectonic tremor sequences first observed in subduction zones. These complex but coherent wave trains have finite moveouts obtained from cross-correlation. The moveout direction of the events confirms that they originate in the reservoir from the area where the fracturing is going on. Clear P- and S-wave arrivals cannot be resolved within the LPLD episodes but, in some cases, small micro-earthquakes occur in the sequences. Whether these micro-earthquakes are causal or coincidental is not known. It has also been observed that in three contiguous frac-stages, all LPLD events appear to come from two distinct places along one of two hypothetical fracture planes. Interestingly, the stages which have the largest number of LPLD events also have the highest observed pumping pressures during fracturing, the highest density of natural fractures, and the greatest number of micro-earthquakes. One possible explanation of these LPLD events is that the high pore fluid pressure during hydraulic fracturing stimulates slow slip on pre-existing fault planes. In the absence of elevated pressure, slip would not be expected on these planes as they are poorly oriented to the stress field. Slip on these fault planes may be occurring because the fluid pressure is close to the magnitude of the least principal stress. We observe a few events between pumping cycles perhaps indicating that, once triggered, these planes continue to slip due to the high transient pressure within the fault planes after pumping has stopped.

Constraining the far-field in situ stress state near a deep South African gold mine

We present and test a new technique for determining the far-field virgin state of stress near the TauTona gold mine. The technique we used to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling-induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. Using this approach, we constrained a range of principal stress orientations and magnitudes that are consistent with all the observed failures and other stress indicators. We found that the state of stress is a normal faulting regime ( S v * ⩾ S Hmax * ⩾ S hmin * ) with principal stress orientations that are slightly deviated from vertical and horizontal and, therefore, denoted with a (*). The maximum principal stress, S v * , is deviated ∼10° from vertical plunging towards the NNW with a magnitude gradient of ∼27 MPa/km. The intermediate principal stress, S Hmax * , is inclined ∼10° from horizontal plunging towards an azimuth of ∼156° and has a magnitude gradient of ∼24 MPa/km. The least principal stress, S hmin * , is inclined ∼5° from horizontal plunging towards an azimuth of 247° and has a magnitude gradient of ∼14 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on cohesionless pre-existing fault planes that are optimally oriented to the stress field. Modelling of breakout rotations and gaps in breakout occurrence associated with recent fault slip on critically stressed faults located >100 m from the mine further confirmed this stress state.

The kinematics of reactivation of normal faults using high resolution throw mapping

Normal reactivation of extensional faults offsetting Cenozoic clastic sediments is investigated using high quality 3D seismic data from offshore Brazil. These faults form complex crestal collapse grabens and result from elliptical doming of the underlying Cretaceous sequence due to Early Cenozoic uplift. The exceptional quality of this dataset allows an extremely detailed analysis of the throw distribution to be conducted on the faults. This, in addition to a reconstruction of the 3D geometry of the fault network, gives insights into the mechanisms and kinematics of reactivation. Two distinct modes of reactivation are recognised from this dataset. The main mode is a classical reactivation by upward propagation of pre-existing structures. A second mode, termed reactivation by dip linkage, is the propagation of an individual fault segment initiated above the pre-existing faults that hard link in the dip direction. For both mechanisms, reactivation processes are selective and only occur on some portions of faults. Factors controlling the preferential reactivation of some segments include: (1) orientation of the pre-existing fault plane relative to the principal stresses responsible for the reactivation, (2) segmentation of the pre-existing network and (3) maximum dimensions and throw values of pre-existing faults and basal tip line geometry associated with a detachment. Reactivation is an important process that may account for part of the scatter in fault-scaling relationships and should be included in fault-growth models.

Constraining the Far-Field Stress State near a Deep South African Gold Mine

The deep gold mines in the Witwatersrand Basin of South Africa are some of the deepest underground mines in the world. For over a century, numerous, often deadly, mining induced earthquakes have been observed. In this work, we develop and test a new technique for determining the virgin state of stress near the TauTona gold mine. This work was completed as part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project. The technique we use to constrain the far-field stress state follows an iterative forward modelling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modelling of the mining-induced stress perturbations, and forward modelling of borehole failures based on the results of the boundary element modelling. The final result is a well constrained stress state consistent with all the observed stress indicators. We find that the stress state is a normal faulting regime with principal stress orientations that are slightly deviated from vertical and horizontal (denoted with a *). The maximum principal stress, Sv*, is deviated 0-20o from vertical towards the NNW and has a magnitude gradient of 27 ± 0.3 MPa/km. The intermediate principal stress, SHmax*, is inclined 0-20o from horizontal with an azimuth of 145o to 168o and has a magnitude gradient of 21.5 to 26 MPa/km. The least principal stress, Shmin*, is inclined 0-10o from horizontal with an azimuth of 235o to 258o and has a magnitude gradient of 13 to 15.5 MPa/km. This stress state indicates that the crust is in a state of frictional faulting equilibrium, such that normal faulting is likely to occur on pre-existing fault planes that are optimally oriented to the stress field. The stress concentrations caused by the mining activities can dramatically alter the range of fault orientations upon which fault slip could occur.

Phenomenology of Baneberry Nuclear Event Revisited with 3-D Finite Element Transient Simulation

This paper highlights a three-dimensional (3-D) transient numerical simulation of the Baneberry event of December 18, 1970, with a 10-kT yield and a 278-m source depth, conducted at the Nevada Test Site. This site has complex geological features with preexisting faults and layered geological strata characterized by a hard Paleozoic layer below the source, and saturated tuff on the west side of the source and clay-rich tuff toward the east side, both overlaid by top alluvial layers. In addition, a layer of 50% montmorillonite is sandwiched between two layers of 20% montmorillonite on the east end. This event is reported to have vented because of fault rupture and shock-wave reflections from a closer hard Paleozoic layer near the source. Here, the shock-induced slip along the preexisting fault plane has an important bearing on the containment efficiency of this event. None of the earlier reported simulation studies address the above slip phenomenon and the influence of variation in geological strata in the presence of the preexisting fault in a 3-D framework for underground nuclear events. The paper describes the capabilities of the SHOCK-3D finite element code for simulating short-time shock-wave propagation, fault rupture leading to sliding along the fault plane, and subsequent crater formation at ground zero with a long-duration transient computation to study the quasi-static behavior of the Baneberry event. Precise modeling schemes of the composite geological strata and fault system demonstrate that a dip-slip mechanism had developed for this event, leading to final venting. The present numerical computation results with SHOCK-3D are in excellent agreement with site observations. In addition, the limitations of earlier reported simulation results from the TENSOR two-dimensional axisymmetric code presented by Terhune et al. have also been overcome.

Stress perturbations registered by jointing near strike‐slip, normal, and reverse faults: Examples from the Ebro Basin, Spain

Stress perturbation patterns created by coeval and preexisting fault planes are characterized by means of detailed mapping and analysis of joint sets in Miocene carbonates of the Tudela area. Strike and spacing of the pervasive N‐S joint set have been specially surveyed for this purpose; their spatial variations have been analyzed and interpreted in the light of different theoretical models proposed by other authors. Perturbations inferred near strike‐slip faults correspond to the classical examples related to compressional and extensional fault tips. A normal fault striking oblique to the horizontal stress axes deflects the latter toward making σ2 and σ3 trajectories parallel and perpendicular, respectively, to the fault strike, while causing an abrupt stress release within the footwall. Near a reverse fault with a small dextral component the deflection of σ1 trajectories is gentle, but a significant increase of differential stress occurs in the hanging wall.

Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes

Simultaneous monitoring of He, H2, Ar, N2, CH4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H2, as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H2. We suggest that the selective and preseimic increases of H2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress‐corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H2 production by mechanochemical reaction of fractured rock surface with water in pre‐existing fault planes as groundwater conduits.

Deep Earthquakes: Old Faults or New?

Although deep‐focus earthquakes occur deep in the mantle, where high temperatures and pressures should make impossible brittle failure (the machanism of shallow quakes), seismically, deep quakes look very much like their shallow counterparts. For this reason, researchers have sought mechanisms that resemble shallow faulting, but that involve processes occurring in regions prone to deep earthquakes—that is, in slabs of oceanic plates descending into the mantle. The figure below depicts a cross section of such a slab (dark green and dark blue) as it descends into the mantle (light green and light blue) as it descends into the mantle (light green and ligh blue). According to one theory, the dehydration of magnisium/iron silicated, occurs from 100 to 125 km beneath the surface and liberates water the could weaken and reactivate a preexisting fault plane. This dehydration mechanism probably accounts for earthquaks occuring at depths below 75 km down to 300 km, and possibly down to 670 km.

Frictional coefficient on faults in a seismogenic region inferred from earthquake mechanism solutions

The coefficient of friction associated with faults in a seismogenic region is inferred from numerous focal mechanism solutions. Earthquake slip can occur on a favorably oriented fault plane when preexisting fault planes are oriented in various directions. The fault plane is thought to be selected when the Coulomb failure stress given by σf=τ − μ(σn − p) − S0 has a maximum value along a slip direction on the plane. Here, τ is the shear stress, μ is the coefficient of friction, σn is the normal stress, p is the pore pressure, and S0 is the cohesion. Consequently, the geometry of the plane and the slip direction include information concerning the coefficient of friction for the faults. In the present paper the coefficient of friction is inferred from the distribution of P axis orientations, assuming hydrostatic pore pressure. The method is very simple and has the advantage that the results obtained do not depend on pore pressure, when pore pressure is not proportional to the normal stress. Observed distributions of P axis azimuths and plunges display a very narrow peak having a width of about 20°. The coefficient of friction on the faults can then be estimated from the width to be less than or equal to μ = 0.2. When the stress is balanced by friction, the shear stress can be calculated. Here, the shear stress (in megapascals) has a value of 4.0z, where z is the depth (in kilometers), under the assumption of hydrostatic pore pressure and a coefficient of friction of 0.2. The value obtained for the coefficient of friction is very small compared with those derived from laboratory frictional experiments. It is thought that this small value of μ is attributed to the friction itself, when pore pressure is not proportional to the normal stress.

Structures in segment boundary zones of the Lost River and Lemhi Faults, east central Idaho

Geologic mapping and structural analysis of the Lost River and Lemhi normal faults in east central Idaho suggest that many rupture segment boundaries, previously inferred from geomorphic data coincide with two classes of structural complexities: (1) zones of distributed faults and steps or jogs in the range front faults, and (2) intersections with preexisting Eocene to Oligocene(?) cross faults at 10–15 km depth. Segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5–6 km wide (in plan view). In contrast, simple fault traces characterize the range front faults in the interiors of segments. Thus segment boundaries of normal faults are broad zones of intersecting faults, not discrete points. The structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults. Intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the Lost River and Lemhi faults. Downdip projections of preexisting Eocene to Oligocene(?) normal faults in the footwall of the Lost River and Lemhi faults may coincide with four or five segment boundary zones at about 15 km depth, the depth at which large earthquakes nucleate in the Basin and Range province. Structural analyses also show that some cross faults, which coincide with segment boundary zones at the surface, are distant from these zones at depth. Conversely, some potentially significant structures present at depth are far removed from the segment boundary zone at the surface. Thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near‐surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones. Where cross faults define segment boundaries, the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults.

Stress field constraints on intraplate seismicity in eastern North America

Focal mechanisms of 32 North American midplate earthquakes (mb = 3.8–6.5) were evaluated to determine if slip is compatible with a broad‐scale regional stress field derived from plate‐driving forces and, if so, under what conditions (stress regime, pore pressure, and frictional coefficient). Using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes (±10°), the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameter ϕ = (S2 ‐ S3)/(S1 ‐ S3) from the fault/stress geometry. As long as the focal mechanism has a component of oblique slip (i.e., the B axis does not coincide with the intermediate principal stress direction), this calculation identifies which of the two nodal planes is a geometrically possible slip plane (Gephart, 1985). Slip in a majority of the earthquakes (25 of 32) was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad‐scale uniform regional stress field. Slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity. Significantly, all five of these events occurred prior to 1970 (when many regional networks were installed), and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events. The frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory. The ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field. Regional stress magnitudes were determined from (1) S1/S3 ratios based on the frictional strength of optimally oriented faults (the basis for the linear brittle portion of lithospheric strength profiles), (2) the computed relative stress magnitude (ϕ) values, and (3) a vertical principal stress assumed equal to the lithostat. Two end‐member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes. First, the frictional coefficient was held constant on all faults, hydrostatic pore pressure was assumed regionally, and the fault zone pore pressure was determined. Since pore pressure is a measurable quantity with real limits in the crust (P0 < S3), this end‐member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes. Alternately, pore pressure was fixed at hydrostatic everywhere, and the required relative lowered frictional coefficient of the fault zone was computed. Slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate‐driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults (assuming hydrostatic pore pressure regionally). The conditions for slip on these 23 relatively “well‐oriented” earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure (between about 0.4–0.8 of lithostatic, hydrostatic is about 0.37 of lithostatic) or, alternately, moderate lowering (<50%) of the frictional coefficient on the faults which slipped. Superlithostatic pore pressures are not required. Focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field, however, did require pore pressures far exceeding the least principal stress (or extremely low coefficients of friction). These events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms. The analysis also confirmed a roughly north to south contrast in stress regime between the central eastern United States and southeastern Canada previously inferred from a contrast in focal mechanisms between the two areas: most central eastern United States earthquakes occur in response to a strike‐slip stress regime, whereas the southeastern Canadian events require a thrust faulting stress regime. This contrast in stress regime, with a constant maximum horizontal stress orientation determined by far‐field plate‐driving forces, requires a systematic lateral variation in relative stress magnitudes. Superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation, but maximum computed rebound‐related stress magnitude changes are quite small (about 10 MPa) and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths.

Fault reactivation and structural inversion. Backarc and intraplate compressive deformations. Example of the eastern Sunda shelf (Indonesia)

The tectonic evolution of the eastern Sunda platform in Indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the Cenozoic: extension with graben infill from the Paleogene to the Early Miocene, a tectonically quiet period with only broad subsidence, and a Middle Miocene-Recent period of folding, inversion and thrusting. These structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates: collisions on the northern and eastern sides, and interplate coupling southward along the Java subduction zone. Inversions often occurred without reactivation of pre-existing normal fault planes at the upper levels of these faults. The factors controlling these deformations are discussed, with special interest placed on the conditions for the potential reactivation of cohesionless normal faults. Inversion and reactivations were mostly controlled by the orientation of the pre-existing fault planes in relation with the regional stress field and by the tectonic stress regime. In a strike-slip regime (effective principal stresses σ 1 and σ 3 horizontal, with σ 1 > σ 2 > σ 3), the optimum conditions for reactivation in isotropic rocks are pre-existing faults that strike close to the potential two vertical conjugate failure planes; the same mechanical properties as for pre-existing fault prevail in such conditions. Strike-slip movements and reverse strike-slip movements and associated en-échelon folds were generally observed. In a compressive regime ( σ 1 and σ 2 horizontal, with σ 1 > σ 2 > σ 3), typical steeply dipping normal faults are difficult to reactivate by slip movements. We suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre-existing fault planes and the inversion of grabens. The strongly inverted grabens are those whose orientation is perpendicular to the regional compression. Nevertheless, with strong regional shortening, large uplifts and thrust faults were observed, with reactivation in the form of reverse or reverse strike-slip movements along pre-existing, steeply dipping normal fault planes.

Fault Reactivation and Structural Inversion Physical Models Analyzed with X-Ray Scanner and Seismic Examples: ABSTRACT

Factors controlling the reactivation by compression of preexisting cohensionless faults and especially normal faults are discussed. Some faults will have been reactivated by compression or a new failure will have occurred, depending on their orientation, their coefficient of sliding friction, and the state of stress. The optimum conditions for reactivation are fault planes whose orientation is closer to the failure plane for isotropic rocks with the same mechanical properties as the preexisting fault plane, i.e., subvertical preexisting fault striking oblique to the main regional stress axis {delta}{sup 1} in strike slip tectonic regime ({delta}{sup 1} and {delta}{sup 3} horizontal) and low dipping preexisting fault in compressive tectonic regime ({delta}{sub 1} and {delta}{sup 2} horizontal). Reverse, strike-slip reverse, or strike-slip movement is excpected along the fault plane depending on its orientation relative to the regional stress field. Weakly inverted graben in an intracratonic area demonstrates these principles. The orientation of a fault plane relative to the regional stress field and the amount of shortening will control whether no reactivation or reactivation with reverse or oblique slip movements is to be expected. During the inversion process, rocks of the hanging walls of the main preexisting normal faults were folded and uplifted. Seismicmore » profiles across the eastern Sunda platform in Indonesia display both large- and small-scale uninverted and inverted grabens in different stages of evolution, together with less common features, such as the inversion of an eroded syncline and low-dipping weak zone reactivation During the reactivation by compression of preexisting normal faults, flexuring and folding of the sediment cover generally occurred before the reactivation of the fault plane.« less

Listric normal faulting, stress refraction, and the state of stress in the Gulf Coast basin

The close association of overpressured shales and the coincident flattening of listric normal faults has been noted and is usually explained in terms of gravity-sliding models. However, incorporation of overpressure into Mohr-Coulomb frictional-faulting theory does not predict normal faults dipping at angles less than 45°, even for very low coefficients of friction. We suggest that low-angle normal faults may form because of a rotation of the maximum principal stress away from vertical with depth. Such a rotation occurs if a significant contrast in viscosity exists between two adjacent strata, such as a normally pressured sandstone and an underlying overpressured shale or evaporite. The stress-refraction model renders the presence of a preexisting fault plane or an extremely low coefficient of friction unnecessary to explain listric fault geometry and offers a failure mechanism in agreement with frictional-faulting theory based on rheological arguments.