Three-dimensional Fault Geometry Research Articles

Three-dimensional fault geometry and kinematics of the 2008 Mw 7.1 Yutian earthquake revealed by very-high resolution satellite stereo imagery

Three-dimensional (3-D) fault geometry and surface slip are key factors for understanding fault behaviour and earthquake mechanics. Previous studies of the 2008 Yutian earthquake occurred on the northwestern Tibetan Plateau, and are limited by a lack of detailed measurements of fault structures and near-fault motion due to the inaccessibility and extreme weather of the area and severe decorrelation of InSAR. In this paper, we use a very-high resolution (VHR) DEM of the 2008 rupture zone extracted from the Pléiades satellite stereo imagery to measure the 3-D fault geometry and on-fault slip. The maximum vertical and horizontal coseismic displacement is 3.6 m and 2.8 m respectively. Despite the overall normal faulting, near-fault reverse vertical motion was observed, ranging from 0.5 m to 1.9 m, caused by the transformation of strike-slip motion into localised shortening due to local fault geometric variations. Measurements of the near-surface dip for the three fault segments (55°, 62°, and 58°) show good consistency, different from previous studies (43°, 60°, and 52°). Joint inversion of InSAR and the near-fault measurements of surface slip also show that the fault geometry does not change much along strike. The local ratio of the cumulative to the coseismic vertical offset recorded on an old alluvial fan and an ice tongue ranges from 1.86 to 2.79, suggesting at least 1–2 historical events occurred here before the 2008 earthquake.

Modeling earthquake sequences along the Manila subduction zone: Effects of three-dimensional fault geometry

To assess the potential of catastrophic megathrust earthquakes (MW > 8) along the Manila Trench, the eastern boundary of the South China Sea, we incorporate a 3D non-planar fault geometry in the framework of rate-state friction to simulate earthquake rupture sequences along the fault segment between 15°N-19°N of northern Luzon. Our simulation results demonstrate that the first-order fault geometry heterogeneity, the transitional-segment (possibly related to the subducting Scarborough seamount chain) connecting the steeper south segment and the flatter north segment, controls earthquake rupture behaviors. The strong along-strike curvature at the transitional-segment typically leads to partial ruptures of MW~8.3 and MW~7.8 along the southern and northern segments respectively. The entire fault occasionally ruptures in MW~8.8 events when the cumulative stress in the transitional-segment is sufficiently high to overcome the geometrical inhibition. Fault shear stress evolution, represented by the S-ratio, is clearly modulated by the width of seismogenic zone (W). At a constant plate convergence rate, a larger W indicates on average lower interseismic stress loading rate and longer rupture recurrence period, and could slow down or sometimes stop ruptures that initiated from a narrower portion. Moreover, the modeled interseismic slip rate before whole-fault rupture events is comparable with the coupling state that was inferred from the interplate seismicity distribution, suggesting the Manila trench could potentially rupture in a M8+ earthquake.

Influence of a pre-existing basement weakness on normal fault growth during oblique extension: Insights from discrete element modeling

We use discrete element modeling to investigate three-dimensional fault geometry and the three-dimensional evolution of a fault network that develops above a 60° dipping planar pre-existing weakness striking 60° relative to the extension direction. The evolution of the fault network comprises three stages: (i) reactivation of pre-existing structure and nucleation of new faults (0–10% extension); (ii) radial propagation and interaction between reactivated structure and new faults (15%–20% extension); and (iii) linkage between reactivated structure and adjacent faults (20%–25% extension). During the first stage, the pre-existing structure mostly reactivates, forming a long and under-displaced fault. New faults are mainly extension-perpendicular and dip at 60°. During the second stage, ‘saw-tooth’ fringes grow upwards from the upper tip of the reactivated structure (which becomes the major fault) and influence the density and orientation of surrounding faults. During the third stage, the reactivated structure links laterally and vertically with adjacent faults, creating non-planar fault geometries. Following linkage, the reactivated structure enhances the displacement of linked faults along branch lines. Our study demonstrates that pre-existing weak faults can be reactivated, propagating upwards in an irregular (‘saw-tooth’) pattern, and affecting fault density, orientation, dip and displacement, and providing the nucleation site of new faults.

Three-dimensional fault geometries and interactions within experimental models of multiphase extension

ABSTRACT We use experimental (analog) models to examine the three-dimensional (3-D) fault geometries and interactions that develop during two phases of noncoaxial extension. In the models, a homogeneous layer of wet clay undergoes two phases of extension whose directions differ by 45°. The resulting fault pattern varies significantly with depth. At shallow levels, second-phase normal faults accommodate most second-phase extension. At depth, both second-phase normal faults and reactivated, first-phase faults with oblique slip accommodate most second-phase extension. A variety of interactions occurs between first-phase and second-phase faults. One interaction involves the upward propagation of second-phase faults from tips of reactivated, blind, first-phase faults. These hybrid faults have deep segments that strike subperpendicular to the first-phase extension direction and shallow segments whose strike varies with depth, becoming increasingly subperpendicular to the second-phase extension direction at shallow levels. A second interaction involves the nucleation of second-phase normal faults on the surfaces of reactivated, first-phase faults. These splay faults propagate upward and laterally from their nucleation sites into the hanging walls of the first-phase faults. As they propagate, they commonly encounter and link with different first-phase faults. The resulting composite faults have zigzag geometries in both map and cross-sectional views. A third interaction involves either the termination of second-phase antithetic normal faults against or near first-phase faults or the offset of first-phase faults by second-phase antithetic normal faults. The 3-D fault patterns and interactions within our models closely resemble those within the Taranaki basin of offshore New Zealand and Milne Point on Alaska’s North Slope.

Fault damage zones

Damage zones show very similar geometries across a wide range of scales and fault types, including strike-slip, normal and thrust faults. We use a geometric classification of damage zones into tip-, wall-, and linking-damage zones, based on their location around faults. These classes can be sub-divided in terms of fault and fracture patterns within the damage zone. A variety of damage zone structures can occur at mode II tips of strike-slip faults, including wing cracks, horsetail fractures, antithetic faults, and synthetic branch faults. Wall damage zones result from the propagation of mode II and mode III fault tips through a rock, or from damage associated with the increase in slip on a fault. Wall damage zone structures include extension fractures, antithetic faults, synthetic faults, and rotated blocks with associated triangular openings. The damage formed at the mode III tips of strike-slip faults (e.g. observed in cliff sections) are classified as wall damage zones, because the damage zone structures are distributed along a fault trace in map view. Mixed-mode tips are likely to show characteristics of both mode II and mode III tips. Linking damage zones are developed at steps between two sub-parallel faults, and the structures developed depend on whether the step is extensional or contractional. Extension fractures and pull-aparts typically develop in extensional steps, whilst solution seams, antithetic faults and synthetic faults commonly develop in contractional steps. Rotated blocks, isolated lenses or strike-slip duplexes may occur in both extensional and contractional steps. Damage zone geometries and structures are strongly controlled by the location around a fault, the slip mode at a fault tip, and by the evolutionary stage of the fault. Although other factors control the nature of damage zones (e.g. lithology, rheology and stress system), the three-dimensional fault geometry and slip mode at each tip must be considered to gain an understanding of damage zones around faults.

Investigation of the Three-Dimensional Fault Geometry Ruptured by the 1995 Hyogo-Ken Nanbu Earthquake Using Strong-Motion and Geodetic Data

The fault geometry of the 1995 Hyogo-ken Nanbu earthquake in Japan is investigated from the perspective of three-dimensional moment distribution, which in turn is estimated using a linear inversion of strong-motion and geodetic data, without regard to aftershock distribution. The inversion result is evaluated with numerical tests in which intentional modeling errors are given on purpose. In regions shallower than about 9 km, faults on the southwestern and northeastern sides dip toward the southeast and northwest, respectively. Fault segments show a clear stepover at a depth shallower than about 9 km and coalesce into a single fault at a depth of about 14 km. The fault geometry is compared with the distribution of aftershocks that occurred within two hours after the main shock. This shows that aftershocks cluster near the bends and the edges of the main faults. These results imply that the main cause of the aftershocks is the concentration of stresses induced by the main shock. Manuscript received 15 June 1999.

Computerized X-ray tomography analysis of three-dimensional fault geometries in basement-induced wrench faulting

Computerized X-ray tomography applied to sandbox experiments makes it possible to analyze the kinematic evolution, as well as the three-dimensional geometry, of faults in basement-controlled wrench faulting. With increasing basement displacement, the evolution of shear bands developed in the model sand bed is observed as follows. (1) The right-stepping shears that have a ‘cirque’ or ‘shell’ or ‘ship body’ shape develop on both sides of the basement fault. (2) The shears on the one side of the basement fault join those on the other side, and therefore each shear surface forms a helicoidal shape. Shears reach the surface of the sand near or above the basement fault, thereby, en echelon Riedel shears are observed at the surface of the sand. (3) Lower-angle shears and P-shears, lying within the zone defined by the first Riedels, have steeper dips and a less pronounced helicoidal aspect than the Riedels. Such three-dimensional shape and kinematic evolution of shears in the fault experiments agree well with the similar faults which propagate up through the unconsolidated sediments from a wrench fault in bedrock as observed in the excavated trenches at some active faults. In the fault experiments, the strike of the Riedel shear to the basement fault in dense sand is larger than that in loose sand, in reasonable agreement with the directions predicted by the Coulomb criterion in terms of the angle of internal friction. Likewise, the geometries of the Riedel shears are considered to depend on the overburden materials in natural cases of wrench faulting.

An inverse method for determining three-dimensional fault geometry with thread criterion: application to strike-slip and thrust faults (Western Alps and California)

In order to draw geological structures such as faults, interpolation is generally needed between scattered data. The use of an approximation criterion integrating the kinematic properties of the faults helps to document the fault surfaces by adding a compatibility criterion to the data set. Assuming that two jointed blocks of rocks slipping on each other generate a thread surface, an approximation method has been developed which integrates a thread criterion. This approximation method is used to solve an inverse problem with least-squares criteria including proximity to data points, smoothness and thread criteria. The aim is to find a smooth surface which is as close as possible to a thread and as close as possible to the observed data set. Applications to two corrugated fault surfaces with a dense data set, located in the Western Alps (France) and in the Transverse Ranges (California), confirm the validity of the thread assumption. Despite their difference in mean corrugation wavelength (5 m and 10 km respectively), in the type of fault (strike-slip and thrust fault respectively), and in the nature of the faulted rocks (limestones and sandstones respectively), very similar results are obtained. In both cases the observed data fit well with a thread surface and the computed fault displacement fits well with the measured displacement on the fault (striae, seismic focal mechanism, geodetic data, restoration). The conclusion is that treating a fault as a thread is a valid physical description which gives the slip direction independently of other kinematic indicators. The advantage of using a thread criterion in addition to classical proximity and smoothness criteria is that this physical insight allows information from areas where data are relatively dense to help constrain areas where data are relatively sparse, these last areas being those that are usually not well constrained by proximity and smoothness criteria.

Application of an Inverse Method for Calculating Three-Dimensional Fault Geometries and Slip Vectors, Nun River Field, Nigeria

A general, automatic method for determining the three-dimensional geometry of a normal fault of any shape and size is applied to a three-dimensional seismic reflection data set from the Nun River field, Nigeria. In addition to calculating fault geometry, the method also automatically retrieves the extension direction (i.e., the horizontal component of the slip vector) without requiring any previous information about either the fault shape or the extension direction. Solutions are found by minimizing the misfit between sets of faults that are calculated from the observed geometries of two or more hanging-wall beds. In the example discussed here, the predicted fault surface is in excellent agreement with the shape of the seismically imaged fault. Although the calculated ext nsion direction is oblique to the average strike of the fault, the value of this parameter is not well resolved. Our approach differs markedly from standard section-balancing models in two important ways. First, we do not assume that the extension direction is known, and second, the use of inverse theory ensures that formal confidence bounds can be determined for calculated fault geometries. This ability has important implications for a range of geological problems encountered at both exploration and production scales. In particular, once the three-dimensional displacement field has been constrained, the difficult but important problem of three-dimensional palinspastic restoration of hanging-wall structures becomes tractable.

Exploration Opportunities in Inverted Rift Basins Defined by Syntectonic Strata and Axial Surface Mapping: ABSTRACT

Structural highs and syntectonic depositional sites in Central Sumatra rift basins are controlled by three-dimensional normal fault geometries and lateral fault slip distributions. The rift basins formed due to crustal extension accommodated by normal faults that flatten with depth, causing collapse of the hanging wall and formation of inclined rollover panels. The authors model this deformation by coulomb shear of the hanging wall localized along active fold hinges (axial surfaces) that are pinned at depth to fault bends. Three-dimensional fault geometries are predicted and mapped based on the shapes of rollover panels and fault segments imaged in seismic reflection (depth) profiles. Rollover panels widen during progressive fault slip and are bound by anticlinal and synclinal axial surfaces. Rollover panels also narrow upward into syntectonic strata, which record the kinematics and timing of basin formation. They map axial surfaces to define the positions of fault bends and widths of rollover panels, which are related to slip on underlying fault segments. Structural highs form where fault segments decrease in dip along strike and/or in regions of minimum fault slip. These areas are defined on fault maps by shallow slopes or fault segments and on axial surface maps by narrow rollover limb widths.more » Structural highs formed during rifting and later tectonic inversion can be defined on these maps prior to detailed contouring, which is often difficult in frontier areas and in lacustrine sequences. They present examples of producing hydrocarbon fields defined by axial surface maps from the Central Sumatra basin.« less

Analogue models of extensional fault geometries

Summary The progressive development of extensional fault geometries in a sedimentary cover sequence above a rigid or deforming basement has been experimentally investigated using analogue models. Quartz sand (700 μm) was used as a modelling material. The experiments were recorded using time-delay 16 mm cine photography and 35 mm photography. Final models were impregnated with resin and serially sectioned to investigate the three-dimensional fault geometry. Four series of experiments were carried out: (i) extension above a linear basement dislocation; (ii) extension above a uniformly stretching basement; (iii) extension controlled by a planar fault; and (iv) extension controlled by a listric fault. Extension above a linear basement dislocation produced a single graben structure in which initial high-angle bounding faults were cut by later listric faults. Fault nucleation occurred into the hanging wall of the graben and only minor rotation occurred in the fault blocks within the graben. Extension above a uniformly extending basement produced a variety of fault structures. Heterogeneous nucleation of faults occurred with initial planar geometries giving way to more listric faults. Significant fault-block rotation was observed. In some instances rotation of pre-existing fault planes produced a negative (convex upward) listric-fault geometry. Extension controlled by a planar fault produced a single graben structure in which new faults developed into the graben. Little rotation of fault blocks occurred. Extension controlled by a predetermined listric fault showed the progressive development of a rollover anticline. The crest of the anticline collapsed producing a second-order crestal collapse graben. The nucleation of new faults was consistently in the hanging wall above the major detachment faults. In all of the experiments carried out to date, once a major fault had developed the second-order fault nucleation was consistently in the hanging wall fault block. Footwall collapse and hence fault migration into the footwall was not significant. Listric faults produced considerable rotation of hanging wall blocks. In some instances heterogeneous rotation of preexisting faults generated negative listric-fault shapes. In rollover structures collapse of the crestal region of the fold produced a second-order graben structure.

The restoration of thrust systems and displacement continuity around the Mont Blanc massif, NW external Alpine thrust belt

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.