En Echelon Research Articles

Variability in Mechanical Properties and Cracking Behavior of Frozen Sandstone Containing En Echelon Flaws under Compression

Variability in Mechanical Properties and Cracking Behavior of Frozen Sandstone Containing En Echelon Flaws under Compression

En echelon faults reactivated by wastewater disposal near Musreau Lake, Alberta

SUMMARY We use machine-learning and cross-correlation techniques to enhance earthquake detectability by two magnitude units for the earthquake sequence near Musreau Lake, Alberta, which is induced by wastewater disposal. This deep catalogue reveals a series of en echelon ∼N–S oriented strike-slip faults that are favourably oriented for reactivation. These faults require only ∼0.6 MPa overpressure for triggering to occur. Earthquake activity occurs in bursts, or episodes; episodes restricted to the largest fault tend to have earthquakes starting near the southern end (distant from injectors) and progressing northwards (towards the injectors). While most events are concentrated along these ∼N–S oriented faults, we also delineate smaller faults. Together, these findings suggest pore pressure as the triggering mechanism, where a time-dependent increase in pore pressure likely caused these faults to progressively reawaken. Analysis of the ‘next record-breaking event’, a statistical model that forecasts the sequencing of earthquake magnitudes, suggests that the next largest event would be ML ∼4.3. The seismically illuminated length of the largest fault indicates potential magnitudes as large as Mw 5.3.

Coseismic surface rupture characteristics and earthquake damage analysis of the eastern end of the 2021 MS 7.4 Madoi (Qinghai) earthquake

At 02:04 on May 22, 2021, an MS 7.4 earthquake occurred in Madoi County in Qinghai Province, China. This earthquake is the largest seismic event in China since the 2008 MS 8.0 Wenchuan earthquake. Thus, it is critical to investigate surface deformation and damage in time to accurately understand the seismogenic structure of the Madoi earthquake and the seismogenic capacity of the blocks in this region. This study focuses on the Xuema Village, located at the eastern end of the coseismic surface ruptures produced by the event, and assesses the deformation and seismic damage in this area based on field surveys, UAV photogrammetry, and ground penetrating radar (GPR). The results indicate that the rupture scale is substantially smaller at the eastern end of the rupture zone compared to other segments. En echelon type shear tensile fractures are concentrated in a width range of 50–100 m, and the width of single fractures ranges from 20 to 30 cm. In contrast, the degree of seismic damage significantly increases at this site. All of the brick and timber houses are damaged or collapsed, while the steel frame structures and the color steel houses are slightly damaged. More than 80% of the bridge decks on the Changma River Bridge collapse, similar to the terraces along the Youerqu and Changma Rivers and the cut slopes of Provincial Highway S205. We infer that the seismogenic fault of the Madoi earthquake exerts a tail effect in this segment. The tension zone has led to a reduction at the eastern end of the rupture zone, causing shaking damage. Local topography and buildings without earthquake-resistant construction along the strike of the rupture zone have undergone different levels of seismic damage.

Ground Fracturing Related to 7.8Mw Saravan Earthquake, Sistan and Baluchistan, Iran - Evidence for Brittle Deformation

Abstract The eastern parts of the Sistan and Baluchistan province, southeast Iran is a tectono-magmatically active zone that continues to witness numerous small and large earthquakes. The volcanism in this region is related to the subduction of the Oman plate below the Eurasian plate. The study of the spatial distribution of the epicenter and focus of the earthquakes suggests close association with the numerous faults related to collisional tectonics. Evidence of ground fractures and faulting related to Saravan earthquake of 16th April 2013 which is the largest (7.8 Mw) recorded earthquake in the region is documented. This earthquake occurred along a segment of the NW-SE trending Saravan normal fault and its reverberations were felt as far away as New Delhi and Dubai. En echelon, 2-km long, northwest-southeast trending surface fractures and faults are recorded in the field near Hampani village southwest of Gosht city. The epicenter is located in the remote hilly terrain east of Gosht. Many of the fractures are listric. The fault records a downthrow of about half a meter and some segments record brecciation of phyllite, liquefaction of sediments and emergence of saline springs are associated features. Despite the high magnitude only a few casualties and minor damage to civil structures were recorded in Iran due to the sparse population and its remoteness.

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

En Echelon Faults and Basin Structure in Huizhou Sag, South China Sea: Implications for the Tectonics of the SE Asia

The Huizhou sag is situated on the continental shelf of the northern continental margin of the South China Sea. In this paper we present a grid of reflection seismic and well data to characterize the basin structure and prominent unconformities. We employ EBM and 2DMOVE softwares to explore the subsidence history and stratigraphic development history of the basin. We found a rapid subsidence period since 15.5 Ma. Moreover, we calculated the stretching factors of the upper crust and the whole crust in the Huizhou sag. The results show the values are 1.10–1.13 and 1.08–1.31, respectively, indicating faulting in Huizhou sag is relatively small. It is noteworthy that the faults map reveals en echelon distribution at the north and south margins of the basin. We suggest en echelon faults here are caused by the subduction of Proto-South China Sea toward NW Borneo block and cease of the South China Sea. Considering the pronounced unconformities, subsidence rates, fault activities and sediment thickness, the Cenozoic tectonic evolution of the basin can be divided into rifting (49–32 Ma), post-rifted (32–15.5 Ma) and rapid subsidence (15.5–0 Ma) stages. Our study will shed new light on the tectonics of SE Asia and petroleum exploration in the South China Sea.

Fault patterns associated with extensional fault-propagation folding

Extensional fault-propagation (drape) folds are a common type of structure in rift basins, and a number of oil and gas fields produce from these structures. The detailed geometry and trap volumes are dependent on the relationship between the basement faults and the overlying folds and associated secondary faults in the sedimentary cover. Experimental models were used to study the 3-D geometry and secondary fault patterns of drape folds associated with single faults and two or more intersecting faults. Two-layer clay models, consisting of stiff and soft clay were used to model the deformation zones in the basement and sedimentary cover, respectively. Extension on single basement faults normal to the direction of extension results in the development of a deformation zone with expanding width, in which the fault density initially increases and then remains approximately constant. Extension on terminating faults results in conical drape folds with faults oblique to the fold axis along the propagating tip. Basement faults oblique to the direction of extension are characterized by oblique secondary faults along the boundaries of the deformation zone, which curve into a direction normal to the extension direction in the center.Trapdoor fault geometries associated with two or more intersecting faults result in a triangular uplift with maximum relief at the apex. The difference in relief between the flanks and the apex is greatest in the case of propagating basement faults. En echelon secondary faults initiate on the flanks and extend into the apex area, curving into orientations that are closer to normal to the regional direction of extension. The width of the fault zone and the total fault length increase with increasing extension. The fault density initially increases and then remains approximately constant with increasing extension.

An Experimental Study on Evolution of the Thermal Field of En Echelon Faults During the Meta‐Instability Stage

AbstractDetecting earthquake precursors is a difficult task in earthquake prediction at home and aboard. Carrying out rock deformation experiments in the laboratory and observing the evolution of the characteristics and rules of related physical parameters are fundamental to exploring of earthquake precursors. In this study, a sample of a compressive en echelon fault set was used to perform deformation experiments. An infrared thermal image system recorded temperature variations of the rock sample during the deformation stage. The deformation process with precursory significance is divided into strongly‐off‐linearity stage, meta‐ instability stage and instability stage based on the stress‐time curve. The purpose of this work is to study the spatial and temporal variations of the thermal field in each stage especially the meta‐instability stage and summarize the possible precursory phenomena. The experimental results show that the temperature in the jog area increases because of compressive stress state during the strongly‐off‐linearity stage. The temperature inside the fault decreases due to the crack in the jog area and the high temperature points on the fault spread and connect due to the converting of fault dislocation from local to overall during the stage of meta‐instability. And the synergy of temperature increases on the fault and the synergy of temperature decreases between the faults (including the jog area), which are two important signals before the instability. The temperature of the whole sample decreases because of stress release except for increasing on the faults caused by friction during the instability stage. In short, there are evident characteristics of the thermal field in the stage of meta‐instability, which can help identify the meta‐instability state. The temperature varies in different deformation stages and different tectonic positions, which means the stage of the deformation and the tectonic position of anomalies should be taken into consideration when we attempt to find thermal anomalies related to tectonic activity.

Post-Messinian evolution of the Florence Ridge area (Western Cyprus Arc), Part I: Morphostructural analysis

The Florence Ridge, part of the western Cyprus arc, is a compressional relief that was eroded during the Messinian salinity crisis while deposition of salt occurred North (Antalya basin) and South (Herodotus abyssal plain). In order to better assess the impact of salt-tectonics in the Florence Ridge region deformations, we conducted a morpho-structural analysis of available multibeam and seismic data (Simed and Prismed II campaigns). It is indeed a crucial issue to distinguish crustal and gravity driven structures in the compressional to strike-slip belts of the eastern Mediterranean.Along the Antalya basin, we mainly observed multi-directional tectonic rafts typical of gravity gliding above salt. On the Florence Ridge itself, the base of salt evolves laterally to a Messinian erosional surface that erodes a series of stacked nappes. This surface is involved in recent faulting. South of the Florence Ridge, a nearly 100km wide fold belt characterizes the Herodotus abyssal plain. Three different zones parallel to the Florence Ridge appear within this fold belt. Those are respectively from North to South zones A, B and C. Zone A is characterized mainly by small-wavelength folding and faulting. Approaching Zone B, a long extensional graben deforms the seafloor. Zone B stands ~100 to 200m higher than zones A and C. There, salt welding seems common. In the easternmost zone B deep sub-circular bathymetric depressions are associated with extremely thick and fan-shaped depocenters probably emplaced in relation with active sub-salt thrusts. Many evidences suggest post-Messinian uplift in this zone. Zone C shows medium to high wavelength salt-cored folds. Wavelength of those folds increase approaching the distal Nile deep-sea fan. Within zone C, a nearly undeformed domain exists approaching the Eratosthenes seamount. ‘En echelon’ folds bound this flat domain suggesting lateral salt extrusion at the junction between zone C and the distal Nile deep-sea fan.To conclude, numerous evidences argue for a post-Messinian reactivation of the pre-Messinian Florence Ridge accretionary wedge (numerous folds, uplifted zones). The salt, as on the nearby Mediterranean Ridge, decouples deep and surface deformations and even allow the development of isolated grabens and associated salt ridges in the heart of the Florence compressional zone. We propose in this case that local basal slope of salt increase related to basement fault activity triggered gravity gliding and thus extensional deformation despite the regional compressional regime. Finally, this study shows that the eastern Nile deep-sea fan, submitted to extremely vigorous salt tectonics seems to have collided with the Florence Ridge fold belt and to have generated a “salt extrusion” zone.

Internal Structure of a Lithalsa in the Akkol Valley, Russian Altai Mountains

Perennial frost mounds are present near the edges of ponds located on the terrace between the mountain flank and a valley-bottom lake in the Akkol valley of the Russian Altai Mountains. These mounds are 10–50 m in diameter and 3–6 m in height. We describe one mound, identified as a lithalsa, which had been eroded so as to expose almost its entire vertical cross-section, revealing its internal structure. The frozen core consisted mainly of soil segments suspended in reticulate ice lenses with a mean thickness of 11–48 mm and a maximum thickness of about 160 mm. The shapes of the soil segments matched their neighbours. Other features included soil segments suspended in the ice veins shaped like ‘En echelon gash veins’, and the presence of a radial structure of ice-rich and sediment-rich frozen bands. These features all suggest the greater importance of a differential stress field during heaving of the mound and after ice segregation, compared to the thermal gradient and water supply. Copyright © 2012 John Wiley & Sons, Ltd.

Numerical analysis of overburden soil subjected to strike-slip fault: Distinct element analysis of Nojima fault

A distinct element method analysis is carried out to examine the development of shear bands in overburden soil subjected to a strike-slip fault. About 2.3 million spherical particles are used in the analysis and the results are compared with the shears observed at the Nojima earthquake fault during the 1995 Hyogoken Nanbu earthquake. En echelon shears and secondary shears which strike at lower angles to the basement fault – typical in strike-slip faults – are observed in the numerical analysis. Simple shear in the horizontal plane and drag due to the dependence of velocity on depth are confirmed to control the helicoidal shape of Riedel shears. Rotation of the compressional direction toward the fault strike as a result of slip along Riedel shears is also verified. It is found that the compressional direction is more horizontal within the area enclosed by Riedel shears than in outside areas and that these compressional directions produce secondary lower-angle shears that are less helicoidal. It is shown that the formation of column-like structures of particles and their subsequent buckling play significant micromechanical roles in three-dimensionally wrenched shears. The results of the numerical analysis, such as shear intervals and striking angles, show a resemblance to observational results at sites where sediment contains coarse grains and is subjected to strike slip with a small dip component, although they are not exactly the same as those observed at locations with similar overburden thicknesses.

Silicification of pelitic schist in the Ryoke low‐pressure/temperature metamorphic belt, Southwest Japan: Origin of competent layers in the middle crust

AbstractWe propose that competent layers composed of silicified rocks in the Cretaceous Ryoke low‐pressure/temperature (low‐P/T) metamorphic belt in the Iwakuni–Yanai area, Southwest Japan, may be an exhumed equivalent of a seismically‐reflective bright‐layer in the middle crust. Silicified rocks crop out as layers or long lenticular bodies several to fifteen meters in thickness, and they are restricted to the area of greenschist facies conditions within a structural thickness of about one kilometer. The silicified rock layers exhibit a sharp contact with subjacent biotite schist but a gradational contact with overlying pelitic schist. Silicified rock is mainly composed of fine‐grained quartz and minor muscovite and biotite, and some colored minerals have been variably decolorized by alteration. This implies that silicification turned the color of pelitic schists to pale gray or milky white. Quartz veins cutting schistosity at high angles were preferentially developed in the silicified rocks, while schistosity‐parallel quartz veins, which underwent viscous flow, were observed in the underlying biotite schists. En echelon quartz veins and fishnet‐like quartz veins are characteristic of silicified rocks. The modes of occurrence of quartz veins indicate that silicified rocks were competent relative to underlying biotite schist. The combination of contact with high‐competence contrast adjacent to low‐permeability silicified rocks may be a good reflector of seismic waves. If the silicified rocks were distributed in the middle crust to a considerable extent, they may result in seismic bright‐layer reflections.

Development of a shear zone in brittle rock subjected to direct shear

Laboratory direct shear tests were carried out on a brittle synthetic rock at various normal loads. The observed fracture patterns changes as the normal load increased. A discrete element code was used to simulate the direct shear tests. The simulation results clearly showed that cracks that developed during the test were predominantly tension cracks. These cracks started to grow from the upper edge of the shear box due to the highly non-uniform stress distribution along the shear plane. The fracture development observed in discrete element simulation models showed that inclined fractures developed parallel to the maximum principal stress direction. En echelon type extensile fractures were also generated and assemblies of theses fractures formed shear bands similar to the process observed in conventional triaxial test using brittle rocks. The discrete element simulations showed that the macro-scale shear band in the direct shear tests resulted from progressive failure of tension-induced micro-cracks.

A kinematic model for the Rinconada fault system in central California based on structural analysis of en echelon folds and paleomagnetism

Deformation associated with the Rinconada fault, one of the major strands of the San Andreas fault system in central California, is accommodated by both discrete fault offset (∼18 km) and distributed off-fault deformation. En echelon folds adjacent to the Rinconada fault were studied in detail at two locations, near Williams Hill and Lake San Antonio, to characterize the magnitude and style of distributed deformation. The obliquity between fold hinges and the local strike of the fault was 27° and 14° at these two sites, respectively. Systematic outcrop-scale fault displacement measurements along roadcuts indicate that the maximum horizontal elongation occurs parallel to local fold hinges and ranges from 4 to 9%. We used the orientation and stretch of fold hinges to construct a transpressional kinematic model for distributed deformation. This modeling suggests a 20–50° angle of oblique convergence, 5 km of fault-parallel wrench deformation, and 2–4 km of fault-perpendicular shortening. Between 3° and 16° of clockwise rotation is also predicted by our model. This rotation is independently confirmed by a 14 ± 7° vertical axis rotation from regional paleomagnetic analyses. Integrating the regional discrete and distributed components of deformation suggests that the Rinconada fault system is 80% strike-slip partitioned.

Geometry and texture of quartz veins in Wadi Atalla area, Central Eastern Desert, Egypt

Several quartz vein sets with varying orientation, geometry and internal structure were recognized in the Atalla area. The veins were associated with the deformation phases affecting the area. En echelon and extensional veins are the main geometrical types. Syn-kinematic veins associated with the major northeast-over-southwest thrust faults were later boudinaged, folded and re-folded. En echelon veins, fibrous veins, and extensional veins are associated with the NNW–SSE faults. Other veins are associated with the NW–SE, N–S, NE–SW and E–W faults. Veins are concentrated at the intersection zones between faults. The internal structure of the veins comprises syntaxial, antitaxial, and composite types and reflects a change from a compressive stress regime to an extensional one. Chocolate-tablet structures and synchronous and co-genetic vein networks indicate later multi-directional extension of the area. Interaction between cracking and sealing of fractures is a common feature in the study area indicating that it was easy for the pore pressure to open pre-existing fractures instead of creating new ones. The reopening of pre-existing fractures rather than creating new ones is also indicated by the scattering of vein data around σ 3. There is an alteration and change in characteristics of the wall rock due to increase in fluid flow rate. Fault-valving probably is also a cause of the complex geometry of some veins.

Development of en echelon magmatic segments along oblique spreading ridges

En echelon magmatic segments commonly develop along obliquely spreading oceanic ridges. To clarify some of the dynamic aspects of this plate boundary, we performed a series of thermo-mechanical numerical tests. When extension of oceanic lithosphere becomes oblique, deformation within the axial region localizes into distinct upwelling centers. Temperatures are elevated in the upwelling cells, which are shallow mantle features that form the new plate boundary. The predicted features are similar to the axial volcanic ridges documented at Mohns and Reykjanes Ridges, and we conclude that they become the new loci of extensional deforma tion, upwelling, and magmatic activity. These ridges, suborthogonal to the plate spreading direction, only develop when the axis rift zone is weak. The subsegment length and spacing depend primarily on obliquity and axial width. Predicted crustal thickness along the subsegmented axis varies discernibly; this might explain the morphology and satellite gravity of the flanks of oblique spreading ridges.

Oblique rift geometry of the West Siberian Basin: tectonic setting for the Siberian flood basalts

We use magnetic intensity data to determine the geometries of basalt-filled rifts of the West Siberian Basin. En echelon graben arrays suggest a component of right-lateral, north–south shear during east–west extension (present co-ordinates). Several major exposed faults at the basin margins, mainly within the Altaid orogenic belt, underwent right-lateral strike-slip in the Late Permian–Early Triassic interval. The combined datasets show that the Siberian flood basalts were erupted during right-lateral oblique extension between the Urals and the Siberian craton, centred on a triple junction in the NE of the West Siberian Basin.

The Bend Justifies the Leans: Interpreting Recumbent Ichnofabrics

Abstract En echelon, bent burrow fabrics are commonly interpreted as an ethological response to high population densities or for filter feeding in conditions influenced by a dominant current direction. However, systematically bent burrows are more likely the result of simple shear on inclined depositional surfaces. A modern example of bent burrows is drawn from tidally influenced point bars situated near the mouth of the Shepody River Estuary, Canada. There the deposits are composed of interbedded silty mud and rare sandy mud beds that dip channelwards. The dipping bedsets are referred to as inclined heterolithic stratification (IHS). Throughout much of the area, the upper 5 to 20 cm of the substrate is unconsolidated and is to locally fluid. The fine sediment resists resuspension by the tidally generated currents, but it flows (plastically) down depositional slopes. Tension gashes, minor mud flows, and deformed biogenic sedimentary structures attest to consistent downslope shearing of the sedimentary substrate. Below the zone of mobile mud, the sediment is comparatively cohesive (firm). The change in cohesiveness is abrupt. Particularly at the to transition, sediment tends to creep downslope, straining the sedimentary fabric. Burrows that descend through the soft sediment into the comparatively firm substrate are noticeably bent at that contact. Burrows are predictably deformed due to down-slope sediment creep—i.e., they are deformed such that the basal part of the burrows point up dip, indicating downslope movement of the upper mobile layer. Burrow fabrics within the older, firm sediment (i.e., in the historical layers) are generally highly strained. Scrutiny of the modern and historical biogenic fabrics shows that: (1) active infauna display a preference to construct dwellings that are oriented perpendicular to bedding planes; (2) the presence of pristine, undeformed burrows in the active layers contrasts with pervasive deformation observed in the historical layers; (3) the temporal nature of the sediment creep occurs at a slower rate than the animal's ability to maintain the burrow structure, thus the strained burrows observed in the historical deposits were deformed after burrow abandonment; (4) the magnitude of strain varies between 1.5 and 2.1 to 1; (5) strains that exceed 2 to 1 essentially eradicate the biogenic fabric and represent a potential upper limit for the preservation of ichnofossils under similar conditions; (6) the recumbent-appearing ichnofossil assemblage in the historical layers does not specifically indicate a response to population dynamics or a preferred current direction.

Differences between veins and joints using the example of the Jurassic limestones of Somerset

Abstract Although veins and joints are both extension fractures, they should be treated separately, especially in field analyses. For example, they commonly form at different times and under different conditions. Excellent exposures of the Liassic limestones at Kilve, Somerset, UK, are used as a case study. These rocks show calcite veins related to the Mesozoic extension and Alpine contraction of the Bristol Channel Basin, and joints that appear to have been caused by late- or post-Alpine stress release. The veins at Kilve show the following characteristics that distinguish them from joints. (1) They are mineralized. (2) Veins have measurable widths (up to tens of millimetres wide) and represent measurable strain. En echelon veins are common at Kilve, and indicate shear strains. (3) Vein widths may obey a power-law scaling relationship. (4) The veins were open for long enough and under the correct conditions for calcite to be deposited, indicating high fluid pressures. They are also typically en echelon. (5) Veins form sets with a narrow range of strikes, whereas the joints typically form complex networks. (6) The veins at Kilve are commonly clustered around faults, and are precursors to, or synchronous with, the faults.

Weak Zone Related Seismic Cycles in Progressive Failure Leading to Collapse in Brittle Crust

Until quite recently our understanding of the basic mechanical process responsible for earthquakes and faulting was not well known. It can be argued that this was partly a consequence of the complex nature of fracture in crust and in part because evidence of brittle phenomena in the natural laboratory of the earth is often obliterated or obscured by other geological processes. While it is well understood that the spatial and temporal complexity of earthquakes and the fault structures emerge from geometrical and material built-in heterogeneities, one important open question is how the shearing becomes localized into a band of intense fractures. Here we address these questions through a numerical approach of a tectonic plate by considering rockmass heterogeneity both in microscopic scale and in mesoscopic scale. Numerical simulations of the progressive failure leading to collapse under long-range slow driving forces in the far-field show earthquake-like rupture behavior. En Echelon crack-arrays are reproduced in the numerical simulation. It is demonstrated that the underlying fracturing induced acoustic emissions (or seismic events) display self-organized criticality—from disorder to order. The seismic cycles and the geometric structures of the fracture faces, which are found greatly depending on the material heterogeneity (especially on the macroscopic scale), agree with that observed experimentally in real brittle materials. It is concluded that in order to predict a main shock, one must have extremely detailed knowledge on very minor features of the earth's crust far from the place where the earthquake originated. If correct, the model proposed here seemingly provides an explanation as to why earthquakes to date are not predicted so successfully. The reason is not that we do not understand earthquake mechanisms very well but that we still know little about our earth's crust.