Constant Dip Research Articles

Impact of Variable Fault Geometries and Slip Rates on Earthquake Catalogs From Physics‐Based Simulations of a Normal Fault

AbstractPhysics‐based earthquake simulators have been developed to overcome the relatively short duration and incompleteness of historical earthquake and paleoseismic records, respectively. These simulators have the potential to be a useful addition to seismic hazard assessment as they produce millions of synthetic earthquakes over thousands to millions of years using predefined fault geometries and slip rates. Due to the sparsity of fault data and the computational expense of the modeling, it is common to simplify earthquake simulator input parameters. Fault surfaces are typically characterized by simplified mapped traces and constant dip with depth, with slip rates that are often assumed to be uniform over the fault plane. This study uses the well‐defined 3D geometry of an active normal fault in offshore Aotearoa New Zealand, the Cape Egmont Fault, to demonstrate the impact of using nonuniform slip rates and nonplanar fault geometries on the resulting synthetic earthquakes from the earthquake simulator RSQSim. Adopting variable slip rates that decrease to zero along the fault tipline (rather than uniform slip rates) reduces unrealistically high nucleation rates of seismicity along fault edges. Introduction of complex 3D fault geometries, including fault segmentation and bends on kilometer scales, and variable slip rates produces less characteristic earthquake populations, increasing the number of M6‐7 moderate‐large magnitude events. Incorporation of variable fault geometries and slip rates in physics‐based simulators may modify the seismic hazards estimated from synthetic earthquake catalogs.

Study of shelly shore ridge formation using wave flume experiments: Morphological evolution and depositional patterns

Coarse shore/berm ridges comprising mollusk shells are peculiar morphological features occurring in some beaches globally, generally wave-built, generated during storms and high tides. Studies regarding the formation of these shelly shore ridges using wave flume experiments (physical modeling) do not provide a detailed analysis of their morphological evolution and depositional features. A bioclastic beach profile with a constant dip, formed by mollusk shells (whole and fragmented ones, DSV-50 = 4.5 mm) of bivalves and gastropods, was subjected to a storm wave condition simulated in a two-dimensional flume with a scale reduction factor of 1/20. A decimeter-scale shelly shore ridge was successfully formed by swash bar emergence. The seaward surface was steeper than the landward slope. The bar was progressively built by swash currents that accumulated shells along the shoreline. According to Dean's function, the equilibrium beach profile displays a good fit for coefficient A, as related to the settling velocity of the bioclasts. The power of m is lesser in the equilibrium beach than that in beaches composed of siliciclastic sand. Three patterns of shell organization and pore framework represent the back, top, and frontal sectors of the ridge. The ridge front includes poorly organized shell particles of dense packing. The crest comprises bioclast beds of uniformly obliquely oriented, densely packed particles. Back ridge sediments exhibit loosely packed moderate shell organization. The morphological evolution of the bioclastic ridge formed in the flume closely resembled bioclastic beach sediments. The depositional pattern in the flume ridge was directly related to the energy of breaking waves and associated swash-current energy.

The tectonic architecture of wrinkle ridges on Mars

The geodynamic history of Mars is written largely in the form of its tectonic structures. A global population of compressional structures records a history of interior cooling and contraction, in combination with volcanic loading and flexure of the lithosphere. The most common of these structures are wrinkle ridges, interpreted as having formed by folding of near-surface layered volcanic materials above blind thrust faults. Topographic profiles of wrinkle ridges are found to dominantly fall into three classes: symmetric ridges, arch-style ridges characterized by broad convex upward backlimbs, and double ridges characterized by a broad arch and narrow ridge. A Monte Carlo boundary element model was used to constrain the underlying geometry of faults associated with the different ridge types. Symmetric ridges are pop-up structures formed between quasi-symmetric thrust and backthrust faults, in some cases above a horizontal décollement or low angle fault at depth. Arch-style ridges formed above a dominant primary thrust fault of near constant dip, with a secondary backthrust playing a lesser role in generating subtle inflections in the backlimb. Double ridges require two backthrust faults to localize strain within the narrow ridge at the ridge crest and to generate the secondary antiform on the backlimb. In these ridges, both the upper primary thrust and backthrust are contained within the layered volcanic materials, while the lower portions of the primary thrust can extend deep into the crust or upper mantle. A shallower depth of fault penetration in areas of thicker crust is consistent with the depth of faulting being limited by the brittle-ductile transition. Dips vary widely, but are typically steep in the upper faults and quite gentle in the lower faults. Thus, while wrinkle ridges do provide important records of tectonic and geodynamic processes on Mars as well as other bodies, the associated tectonic structures are more complex and varied than previously considered.

Throw variations and strain partitioning associated with fault-bend folding along normal faults

Abstract. Normal faults have irregular geometries on a range of scales arising from different processes including refraction and segmentation. A fault with constant dip and displacement on a large-scale will have irregular geometries on smaller scales, the presence of which will generate fault-related folds and down-fault variations in throw. A quantitative model is presented which illustrates the deformation arising from movement on irregular fault surfaces, with fault-bend folding generating geometries reminiscent of normal and reverse drag. Calculations based on the model highlight how fault throws are partitioned between continuous (i.e. folding) and discontinuous (i.e. discrete offset) strain along fault bends for the full range of possible fault dip changes. These calculations illustrate the potential significance of strain partitioning on measured fault throw and the potential errors that will arise if account is not taken of the continuous strains accommodated by folding and bed rotations. We show that fault throw can be subject to errors of up to ca. 50 % for realistic down-dip fault bend geometries (up to ca. 40∘), on otherwise sub-planar faults with constant displacement. This effect will provide irregular variations in throw and bed geometries that must be accounted for in associated kinematic interpretations.

Relationship between Growth Faults, Subsidence, and Land Loss: An Example from Cameron Parish, Southwestern Louisiana, USA

O'Leary, M. and Gottardi, R., 2020. Relationship between growth faults, subsidence, and land loss: An example from Cameron Parish, Southwestern Louisiana, USA. Journal of Coastal Research, 36(4), 812–827. Coconut Creek (Florida), ISSN 0749-0208.This study investigates the relationship between faulting, subsidence, and land loss in coastal Louisiana. A methodology that integrates three-dimensional (3D) seismic data, well logs, high-resolution topographic mapping (LIDAR), and historical aerial photography is successfully developed to identify fault-related geomorphic changes in southwestern Louisiana's Chenier Plain. Analysis of a 3D seismic survey and well logs reveals the presence of 10 normal faults that form an east-west graben in the middle of the study area. Well logs were used to further constrain the geometry of the faults. Shallow water well logs were used to map the faults at shallow depth, below the resolution of the seismic survey. Fault traces were extrapolated to the surface by maintaining constant dip and projected on LIDAR data. Elevation profiles derived from the LIDAR were conducted across the different faults, and results show that a distinct difference between the upthrown and downthrown sides of the faults occurs. Historical aerial photographs were used to investigate any change in geomorphology from 1953 to 2017 within the study area. Results reveal the occurrence of water bodies on the immediate downthrown sides of suspected fault traces. These findings suggest that faulting influences and focuses areas where subsidence is happening and subsequent land loss may occur, and detailed understanding of active shallow faulting in coastal areas can be used to identify regions that are at risk of land loss.

Robust Estimation of Multiple Local Dips via Multidirectional Component Analysis

We propose a novel method for robust estimation of multiple local dips in seismic data via signal decomposition. The decomposition is achieved by regularizing each separated component to have a single dip. The single-dip regularization is built based on the fact that piecewise constant dips indicate that local gradient vectors lie on the same directions. The method is formulated as an optimization problem, which is solved in the framework of a block coordinate descent method. The proposed method is robust to random noise and can be used to estimate multiple dips accurately. As the proposed method has clear physical meaning, it is easy to be applied for other tasks, such as random and coherent noise attenuation, signal separation, and so on. We test the feasibility of our method in estimating local dips with synthetic and field data and compare with the structural tensor method and the plane-wave destruction method. Results demonstrated that the proposed method can estimate multiple dips and is more robust to noise than the other two methods. Applications for random noise attenuation, coherent signals separation, and unconformity detection are also tested.

Broadband-seismic analysis of a massive landslide in southwestern China: Dynamics and fragmentation implications

We analyse broadband seismic records generated by the massive Xinmo landslide that occurred in southwestern China at 5:39 on 24 June 2017 (Beijing Time, UTC + 8). Characteristics of landslide seismic signals, including propagation velocity, duration and frequency contents, are presented and discussed by comparing them with seismic signals generated by a local earthquake of similar magnitude. We perform a long-period seismic waveform inversion in the frequency band of 0.01–0.1 Hz to acquire the landslide force history using 46 seismic traces from 21 seismic stations with distances from the epicentre extending to 360 km. Based on the inversion, we calculate the dynamic parameters of the landslide and recognize its stages and movement characteristics. Landslide basal friction coefficients are also acquired using both a constant dip and varying dips along the slope. A combined analysis of near-field short-period seismic records and dynamic inversion results indicates that the high-frequency energy of landslide signals is most likely generated by rock fragmentations during collision and scraping.

Testing the deltaic origin of fan deposits at Bradbury Crater, Mars

The stratigraphic architectures of three fan-shaped deposits at Bradbury crater, Mars were investigated to test the hypothesis that the deposits were formed in standing bodies of water. Quantitative stratigraphic methods were applied to the northernmost deposit, which displays meter-thick layering. Planes fit to exposed strata display a constant dip throughout the section, with no obvious slope breaks and a mean dip of 6.1° to the northwest. We conclude that the stratal geometries of this outcrop are most consistent with formation as an alluvial/fluvial or debris flow fan. As this deposit lacks the diagnostic stratigraphic architecture of a delatic system, it presents no conclusive evidence for a sustained standing body of water at Bradbury crater.

Shallow/upper crustal shear wave structure of the Tehran region (Central Alborz, Iran) from the inversion of Rayleigh wave dispersion measurements

Surface wave dispersion curves from microearthquakes are used to obtain group velocity dispersion maps. The calculation of the local dispersion curves for each grid point from these maps then produces the input data to retrieve the 3D shear wave velocity model of the Tehran region. The group velocity maps indicate that the tomographic results agree well with the three main tectonic features and the geological units in the study area. The tomographic maps generally possess high-velocity structures across most of the mountain belts (Central Alborz and east-southeast mountains), whereas the Tehran Basin correlates to a low-velocity structure. Increasing the period in the study area highlights four independent low-velocity zones that reflect faults and fault junction systems. The shear wave velocity profiles indicate that the depth to bedrock exhibits southward variation ranging from ~ 300 m to ~ 1500 m. We also focus our analysis on the existence of faults within the shear wave profiles and discuss the low shear wave velocity anomalies deeper than 2 km result from the main fault structures (e.g., North Tehran, North-South Rey and Parchin). Furthermore, we argue that the dip angle of the North Tehran fault varies along fault strike, whereas the North-South Rey fault possesses a constant dip angle. Moreover, initial model uncertainties and checkerboard resolution tests are used to identify reliable and robust anomaly features in the 3D shear wave velocity model and 2D tomographic maps, respectively. Microearthquake analysis provides an effective approach for studying the upper crustal structure heterogeneity, especially the fault structure, of the Tehran region.

Impact of near-surface fault geometry on secular slip rate assessment derived from uplifted river terraces: implications for convergence accommodation across the frontal thrust in southern Central Bhutan

Vertical velocities obtained from uplifted river terrace dating near mountain fronts are commonly converted into overthrusting slip rates assuming simple geometry of the fault at depth. However, the lack of information on the dip angle of these shallow structures can lead to misinterpretation in the accommodation of convergence, and thus to erroneous conclusions on the transfer of shortening to the emergent thrust faults. Here, to assess the impact of fault geometry, we focus on the eastern Himalayan region in the south Central Bhutan, where the topographic frontal thrust (TFT) has been already documented by GPS, paleoseismic, geomorphic and geological studies. This study is based on high-resolution near-surface geophysical investigations, including electrical resistivity, seismic and gravity measurements. Using a similar stochastic inversion approach for all data sets, new quantitative constraints on both fault geometry and petrophysical parameters are obtained to image shallow depths, in the upper ca. 80 m. The combined results from both surface observations and geophysical measurement provide a TFT geometry that is dipping northwards with a shallow angle at the top (0–5 m), steeply dipping in the middle (5–40 m) and flattening at deeper depths (>40 m). Together, our new constraints on the fault geometry allow us to estimate a minimum overthrusting slip rate of 10 ± 2 mm yr−1, which is only a part of the ca. 17 mm yr−1 GPS convergence. This suggests that, in the study area, significant deformation partitioning on several faults including TFT and the Main Boundary Thrust cannot be ruled out. More importantly, assuming constant slip rate, the obtained dip angle variations lead to uplift rate changes with distance to the TFT. This underlines that taking into account uplift rate from terrace dating only at the front location and assuming a constant dip angle fault geometry based on surface observations may significantly bias the slip rate estimates.

Influence of bedding dip on glacial erosional landforms, uinta mountains, usa

We explore the relationships amongst bedding dip, basin aspect, and glacial landforms using field observations and analyses of the northwestern inta ountains of tah. We examine basins on opposing sides of three ice divides in which quartzite beds of the Mount Watson Formation maintain a near constant dip. These areas provide contrasting relationships between ice flow and bedding dip directions while holding rock type and climate constant. We map the occurrence of three glacial erosional landforms: cliffs showing evidence of quarrying, scoured surfaces polished by abrasion, and overdeepenings. Cliffs and overdeepenings are more common in basins where bedding dips up‐basin, while scoured surfaces are more prevalent where bedding dips down‐basin. The significance of jointing in controlling glacial erosional forms is well established and we propose that bedding, as well as joints, dictates the geometry of quarried blocks and influences the spatial patterns of process dominance. Where bedding dips up‐basin, the geometry of pre‐existing weaknesses favours quarrying creating both cliffs and overdeepenings. In contrast, where bedding dips down‐basin, block geometry does not favour the creation of overdeepenings via quarrying and exposed bedding planes are subjected to glacial abrasion, producing scoured surfaces.

A detailed analysis of microseismicity in Samos and Kusadasi (Eastern Aegean Sea) areas

A detailed investigation of microseismicity and fault plane solutions are used to determine the current tectonic activity of the prominent zone of seismicity near Samos Island and Kusadasi Bay. The activation of fault populations in this complex strike-slip and normal faulting system was investigated by using several thousand accurate earthquake locations obtained by applying a double-difference location method and waveform cross-correlation, appropriate for areas with relatively small seismogenic structures. The fault plane solutions, determined by both moment tensor waveform inversions and P-wave first motion polarities, reveal a clear NS trending extension direction, for strike slip, oblique normal and normal faults. The geometry of each segment is quite simple and indicates planar dislocations gently dipping with an average dip of 40–45°, maintaining a constant dip through the entire seismogenic layer, down to 15 km depth.

A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)

The island of Boa Vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world (50% of the total outcropping igneous rocks of Boa Vista). The felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years.Three consecutive felsic episodes are distinguished: the first occurred between 14.5Ma and 13.5Ma, the second between 13.6Ma and 12.0Ma, and the third and last between 12.0Ma and 10.4Ma. Their geochemical composition reveals the presence of crystal fractionation processes with participation of feldspars and amphibole as well as different accessory phases involved in each episode.The Boa Vista felsic cone-sheet swarm, the youngest third episode, is formed by several hundreds of peralkaline ne-phonolitic sheets distributed within a circular surface about 11km in radius. The cone-sheets conform a single general pattern characterized by a decreasing dip outward the structure, together with a constant dip of each individual sheet. The average inclination of sheets is around 40° in the sectors next to the centre of the structure whereas it is only about 30° next to its periphery. The magmatic focus of the sheets is located over 3km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm.

Selecting an optimal aperture in Kirchhoff migration using dip-angle images

We present a method for selecting a migration aperture in Kirchhoff migration. We first split migrated data into constant-dip-angle partial images. Then, in every partial image, we estimate the consistency between each event and the constant dip of the analyzed section. We filter out events whose slope is far from the corresponding dip. Stacking of the filtered partial images corresponds to migration having an optimal aperture. Synthetic and real data examples demonstrate that the proposed approach to migration-aperture optimization is able to reduce migration noise while preserving diffraction energy, which characterizes small geologic objects and brings additional resolution to the image.

Modeling the Onset of Gas Entrainment in a Reduced T-Junction With Co-Current Stratified Gas-Liquid Flow

A model was developed to predict the onset of gas entrainment in a single downward oriented branch. The branch was installed on a horizontal square cross-sectional channel having a smooth stratified co-currently flowing gas-liquid regime in the inlet region. The branch flow was simulated as a three-dimensional point-sink while the run flow was treated as a uniform velocity at the critical dip. Experiments were performed to determine the critical liquid flow distribution between the run and the branch. A correlation was developed relating the branch Froude number to the ratio of the superficial liquid mass fluxes in the run and the branch. The correlation was used as a boundary condition in the model. A methodology was developed using digital imaging to record the coordinates of the critical dip at the onset of as entrainment. The dip angle was found to range between 40 to 60 degrees and constant dip angles of 40, 50 and 60 degrees were selected as boundary conditions. The critical height was predicted to within 50% of experiments with the error attributed to differences in the modeled and experimental geometries. A semi-empirical analysis using the experimental geometry yielded a critical height prediction to within 20% of experimental results.

The anatomy of the 2009 L'Aquila normal fault system (central Italy) imaged by high resolution foreshock and aftershock locations

[1] On 6 April (01:32 UTC) 2009 a MW 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. We study the geometry of fault segments using high resolution foreshock and aftershock locations. Two main SW dipping segments, the L'Aquila and Campotosto faults, forming an en echelon system 40 km long (NW trending). The 16 km long L'Aquila fault shows a planar geometry with constant dip (∼48°) through the entire upper crust down to 10 km depth. The Campotosto fault activated by three events with 5.0 ≤ MW ≤ 5.2 shows a striking listric geometry, composed by planar segments with different dips along depth rather than a smoothly curving single fault surface. The investigation of the spatiotemporal evolution of foreshock activity within the crustal volume where the subsequent L'Aquila main shock nucleated allows us to image the progressive activation of the main fault plane. From the beginning of 2009 the foreshocks activated the deepest portion of the fault until a week before the main shock, when the largest foreshock (MW 4.0) triggered a minor antithetic segment. Seismicity jumped back to the main plane a few hours before the main shock. Secondary synthetic and antithetic fault segments are present both on the hanging and footwall of the system. The stress tensor obtained by inverting focal mechanisms of the largest events reveals a NE trending extension and the majority of the aftershocks are kinematically consistent. Deviations from the dominant extensional strain pattern are observed for those earthquakes activating minor structures.

Multiple-beam interference fringes applied to investigate the index profile of the optical fiber dip

A numerical-based phase shift method is presented to study optical fibers having a dip in the refractive index profile at the center of the core. Mathematical expressions for the shape of multiple-beam Fizeau fringes crossing a graded-index optical fiber (GRIN) comprising a central graded-index dip or GRIN fiber with a constant refractive index dip, immersed in a silvered liquid wedge, are derived. The index profile parameters of the central dip are determined by comparing the theoretical fringe shape across the fiber cross-section and the experimental fringe shape obtained from micro-interferograms of the wedge interferometer.

Geometria 3d de falhas de empurrão e dobras associadas como expressões da morfologia do descolamento basal: resultados de uma simulação computacional

D THRUST FAULT GEOMETRY AS AN EXPRESSION OF THE ASSOCIATED DETACHMENT MORPHOLOGY: RESULTS OF A COMPUTER SIMULATION Most two-dimensional fault propagation fold models are based upon the development and migration of kink bands and assume constant dips along each band, constant layer thickness and conservation of the section area. Modelling a fault-propagation fold requires the determination of the position and of the angle of each kink line that defines t he structure. Once modelling a series of dip sections of a fault propagation fold, it is possible to generate three dimensional (3 D) geometrical models by linking a large number of these sections. A computer program was created to generate 3D geometrical model s of single thrusts or imbricate fans. Using the program, a series of experiments were run to study the influence of the detachme nt morphology on geometry of the associated fault ramps. After modelling under variable conditions, following results were obtaine d: (a) the fault ramp surface is non-planar if the detachment surface is also non-planar and the ramp surface reflects all irregul arities of the detachment surface; (b) the trace of a thrust fault on a map is similar to the trace of the associated detachment surface a long a strike section; the fault trace is straight when the detachment surface is flat and is curved when the detachment surface is curved. Depressions in the detachment surface correspond to antitaxial curves (salients) in the fault surfaces. Detachment highs, in co ntrary, correspond to sintaxial curves (recesses) in the fault surfaces. The expressions of the deepest and the shallowest points of th e detachment are, respectively, the salient and the recess culminations. It is concluded that if indenters are not active during thrust nucleation and if no later deformation occurs, the 3D shape of single thrust fault and of thrust systems is determined by the morphology of the associated detachment. Simulation of the development of an imbricate fan linked to a trough-shaped detachment surface and its comparison with results of a natural example given by the Dom Bosco thrust system of the southern Quadrilatero Ferrifero, Brazil, resulted in a close correlation between the models and the Dom Bosco system. In both cases, faults exhibit pronounced curvature with increasing dips toward the hinterland. Fault traces converge in the areas of minimal depths of the detachment surface and diverge in the areas of maximal detachment depth.

Fold amplification and style transition involving fractured dip-domain boundaries: buckling experiments in brittle paraffin wax multilayers and comparison with natural examples

Abstract Experiments aiming to study the conditions of fracture localization during buckle folding in brittle sedimentary layers of the upper crust were conducted using brittle paraffin wax multilayers submitted to axial and vertical load. The particular aim was to understand the mechanisms of development of subtle features often observed in apparently rounded natural folds. These consist of scarcely visible discontinuous fractured zones, more or less parallel to the fold axis, that separate domains of constant dip. These zones are of great interest for fluid dynamics in folded fractured reservoirs. It is shown that such axial kink boundaries referred to as dip-domain boundaries (DDBs) play a major role in the development of curvature in fractured buckle folds. DDBs observed in experiments consist of the localization and coalescence of tensile fractures along axial planes after the elastic instability stage. The tested physical parameters were (1) the intensity of the vertical load, (2) the bed thickness within a mechanical unit of constant thickness, (3) the stiff (multilayer)-soft (embedding material) thickness ratio and (4) the interlayer friction. In low interlayer friction conditions DDBs remain relatively localized during shortening, leading to chevron-style folds. With high interlayer friction conditions, DDBs tend to multiply laterally from an initial chevron fold. The latter phenomenon leads towards a more rounded fold, but one that still shows discontinuous curvature comparable to that observed in natural conditions. Fold models of flexural slip and tangential longitudinal strain (orthogonal flexure) are discussed under the influence of tensile failure in multilayer materials.

Autotracking of faults on 3D seismic data

Manual fault mapping in 3D seismic interpretation is labor-intensive and time-consuming. Complex fault geometries and the distortion of the seismic signal close to faults complicate full automation of the fault-mapping process. We present a semiautomatic fault-tracking method for 3D seismic data that consists of fault highlighting followed by model-based fault tracking. Fault highlighting uses log-Gabor filters for emphasizing oriented amplitude discontinuities at faults in the presence of noise. Subsequent fault tracking fits an active contour to the highlighted fault voxels. The active contour searches for a connected, smooth curve which fits the data and disambiguates misleading or missing information. The fault tracker requires the interpreter to place the active contour close to a fault on one initial seismic inline (2D pick). The active contour deforms to the closest amplitude dis-continuity highlighted. This tracking result is then projected forward to the next inline, providing an initial fault pick on this section that is again optimized by the active contour. Tracking results on a series of successive seismic sections, finally, constitute a 3D fault surface. User interaction is solely required for an approximate fault pick on the first inline, and in cases where the fault line is lost due to insufficient signal. Use of the autotracker prototype provides a fast solution for the mapping of complete 3D fault surfaces of constant dip, and for the automated tracking of fault portions within distinct dip domains, if fault surfaces are curved (i.e., listric). The method was applied to a series of high-quality reflectivity sections extracted from a 3D seismic volume from shallow-offshore Nigeria, with the tracking results (generated within seconds) comparing well with manually interpreted fault surfaces.