Fault Orientation Research Articles

Multiple Reactivations of the Rennick Graben Fault System (Northern Victoria Land, Antarctica): New Evidence From Paleostress Analysis

AbstractThe dense fault network that separates the tectonic units of northern Victoria Land to the E from the East Antarctic Craton to the W represents a regionally sized, NNW‐SSE trending deformation zone that is also known as the Rennick Graben Fault system (RGF). This long‐lived deformation zone is characterized by great structural complexity, with the superposition and polyphasic reactivation of regional faults; it is known to have been active since the Cambrian‐Ordovician, when it formed as a suture zone between different terranes, up to the recent. Its complete framework and geodynamic evolution are still debated and have only been partially investigated. In this research, we explore the Meso‐Cenozoic tectonic framework of the RGF by investigating the paleostress fields that characterized the last geodynamic events in the area and their associated brittle deformation. We analyzed faults and fractures data collected at 89 sites during several PNRA ItaliAntartide expeditions and combined fault‐slip data inversion with the azimuthal orientation of faults and the spatial distribution of fracture intensities across the RGF. The results from this multi methodological approach confirm the existence of two geotectonic provinces (Bowers Mountains province to the E and Usarp Mountains to the W) characterized by different spatial distributions of brittle deformation, which are significantly more intense in the Bower Mountains domain. Here, the repeated reactivation of the RGF led to the superposition of two recent (Meso‐Cenozoic) major tectonic events, with prevalent strike‐slip kinematics and characterized by fault reactivation with right‐lateral movement overprinting the previous left‐lateral event.

Mapping the geological structures in the Ras El Ush field (Gulf of Suez, Egypt), based on seismic interpretation and 3D modeling techniques

The Ras El Ush field (Gulf of Suez, Egypt) is characterized by various complex structural features, such as evaporite deposits and faulting. Such complex geologic setting results in difficulties in terms of high-resolution seismic interpretation. The seismic energy was masked by the Pre-Miocene salt formation and attenuated by evaporite deposits in the upper Miocene, complicating the seismic interpretation process. Proper geophysical seismic imaging and interpretation of Ras El Ush field's associated structures and sedimentation can provide insights into the geological controls on hydrocarbon systems in the Gulf of Suez rift zone. This study interpreted post-stacked reflection seismic and well data comprising the check shot velocity data to obtain and investigate detailed information about the structural fault orientation and dip of Ras El Ush field. Seismic attributes were employed for improving the fault attributes by eliminating noise and residue of non-faulting events. Additionally, depth maps and 3D structural models were constructed for Rudeis, Nukhul and Matulla formations, the main geological formations within the area of interest. This study presents results from seismic attributes analyses to enhance the detection of the Clysmic fault-offsetting aggregate as well as the minor and major cross faults that passed through the Ras El Ush field. The structural model of the Rudeis, Nukhul, and Matulla formations in the Ras El Ush field exhibits distinct NW-SE to NE-SW fault systems. The models helped in defining geological discontinuities and structural traps, which would help in the development of the field and nearby similar hydrocarbon fields in the Gulf of Suez (Egypt).

Potency density tensor inversion of complex body waveforms with time-adaptive smoothing constraint

SUMMARY Large earthquakes are often accompanied by complex fault rupture, but it has been difficult to reliably estimate such a complex rupture process with conventional waveform analysis tools due to modelling errors originating from limited accuracy of the fault geometry. Recently, a potency density tensor inversion method has been developed to solve this problem; allowing any types of faulting mechanism on an assumed model plane, the method replaces the modelling error of fault orientation with that of fault location, which is insensitive in teleseismic waveforms with low spatial resolution. The method has successfully unveiled earthquake source processes with geometrically complex fault rupture. However, the method imposes the same intensity of smoothing constraint on all the basis slip components irrespective of possible changes of slip direction during the rupture. This leads to excessive smoothing to a slip component with large amplitude, which results in obscuring the rupture process. In this study, we propose a time-adaptive smoothing constraint that dynamically adjusts the smoothness intensity inversely proportional to the amplitude for each basis slip function. Through a numerical experiment assigning an input model involving a drastic change in the focal mechanism (reverse, strike-slip and normal faulting) during the rupture, we find that the time-adaptive smoothing constraint solves the problem of excessive smoothing to the dominant slip component, and the spatiotemporally non-uniform rupture episodes with different focal mechanisms are successfully reproduced. To evaluate the feasibility and effectiveness of the time-adaptive smoothing constraint, we apply the method to the teleseismic body waves of the 2002 Denali fault and the 2008 Wenchuan earthquakes, which involve complex fault ruptures with changing focal mechanisms. We find that the developed method well captures the focal mechanism transition in space and time from reverse to strike-slip faulting during the ruptures of the 2002 Denali fault and the 2008 Wenchuan earthquakes. Even though these source models are built using only the teleseismic P waveforms with simple model fault geometry that is represented by a horizontal rectangular plane, they well explain the complex observed waveforms and agree with characteristics of source processes obtained in previous studies using seismic and geodetic data as well as field surveys. The potency density tensor inversion method with time-adaptive smoothing constraint is a powerful tool to analyse earthquake rupture processes with complex fault geometries involving different faulting types.

Determination of the stress field in the Kopeh Dagh and Eastern Iran regions

The land of Iran is situated in a compression zone with a high seismicity rate due to the convergence of Arabian and Eurasian plates. In this study, the stress field of two provinces of Kopeh Dagh and Eastern Iran, which are located in the east and northeast of Iran, has been studied and compared. These two provinces are different in terms of seismicity patterns so that Kopeh Dagh is much more active than Eastern Iran. To achieve this goal, the iterative joint inversion method for stress and fault orientations developed by Vavryčuk (2014) has been used. The average direction of the maximum principal stress (σ1) and the maximum horizontal stress (SHmax) in the study area are estimated to be N30.1°E and N12.1°E, respectively. These directions are in agreement with the convergence of the Arabian plate and the Eurasian plate. The value of the optimal friction coefficient was determined for both regions and it was found that this coefficient is lower in Kopeh Dagh than in Eastern Iran. This indicates a potential relationship of the fault friction with the activity pattern and seismicity of in the region.

Statistical Analysis of Small Faults in Rotated Blocks of Crust Near the Húsavík‐Flatey Transform Fault, Northern Iceland

AbstractThe dextral Húsavík‐Flatey fault, in northern Iceland, provides a unique opportunity to observe an oceanic transform fault on land. Structural and paleomagnetic measurements from lavas and dikes on Flateyjarskagi peninsula indicate that rocks adjacent to the fault have rotated clockwise more than 100°. We examine the effect of this rotation on the orientations of small faults that developed in rocks adjacent to the transform fault. We also use patterns in small‐scale data to build on models of the style of deformation surrounding the fault. We apply statistical tools including spatial regressions to structural data to estimate the amount of distributed shearing around the transform fault, and to predict the orientations of rotated structures. Some small faults are consistent with predicted rotations, while others are consistent with the modern kinematics, suggesting that they postdate block rotation. This protracted temporal history for small faults complicates paleostress interpretations from rocks adjacent to the Húsavík‐Flatey fault. Previous studies assumed the small faults formed in their modern orientations and attributed their variability to stress variations, whereas we identify populations of faults that have likely rotated to their present‐day attitudes.

A novel framework on intelligent detection for module defects of PV plant combining the visible and infrared images

Solar Photovoltaic (PV) industry has achieved rapid development in recent years. However, it is difficult and costly to detect the micro fault area in a large PV power plant due to environmental factors and missing data. Most faults can be detected by the infrared temperature measurement method, but the infrared camera characteristics constrain it. This paper proposed a novel framework, consisting of image acquirement, image segmentation, fault orientation and defect warning, to remedy the limitations for PV module defects. The visible and infrared PV array images are taken under the same conditions by a dual infrared camera at low altitudes. The deep learning methods, including the fifth version of You Only Look Once (YOLOv5) algorithm and Deep Residual Network (ResNet) algorithm, are introduced to this framework. Hence, this framework has strong capability to suit almost all brightness conditions, by the combination of image segmentation from visible images and fault location on infrared images. The results show that this framework dramatically improves the separation speed of photovoltaic array to 36 Fps and the accuracy of fault detection to 95% by infrared image marked with the segmented area.

Coeval development of extensional and contractional features along transform margins: insights from the Diaz Marginal Ridge

Abstract The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian–Hauterivian-aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery. Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east–west-orientated maximum stress and north–south-orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localized magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

Coseismic Deformation Field Extraction and Fault Slip Inversion of the 2021 Yangbi MW 6.1 Earthquake, Yunnan Province, Based on Time-Series InSAR

An earthquake of moderate magnitude (MW 6.1) occurred in Yangbi County, Dali, Yunnan Province, China, on 21 May 2021. Compared to strong earthquakes, the measurement of the deformation fields of moderate earthquakes is more susceptible to errors associated with atmospheric, orbital, and topographic features. We adopted a new time-series InSAR method to process preseismic and postseismic Sentinel-1A SAR time-series images and separated the coseismic deformation signals from various error signals. This method uses preseismic time-series interferograms to estimate the spatially correlated look angle error induced by the digital elevation model and the atmospheric and orbital errors in the master image. The preseismic and postseismic time-series interferograms were then segmented for spatio-temporal filtering to provide a precise estimate of the atmospheric and orbital errors in slave images. Such time-series processing accurately separates various errors from the coseismic deformation signal and prevents the coseismic deformation signal from being included as noise in the error estimation during filtering. Based on this approach, we effectively eliminated the masking of the deformation signal by the errors and extracted coseismic deformation field of the Yangbi MW 6.1 earthquake with high precision. The maximum LOS displacement in the ascending and descending tracks were determined to be −74 and −62 mm, respectively. Subsequently, we used the Geodetic Bayesian Inversion Software to invert the fault geometric parameters of this earthquake, and based on this inverted the rupture slip distribution using the least-squares method. The results showed that the fault orientation is 133.43°, dip angle is 76.98°, source depth is 5.5 km, fault sliding mode is right-lateral strike-slip. The moment magnitude (MW) was calculated to be 6.07.

Source characteristics of the Mw 6 Mutatá earthquake, Murindo seismic cluster, northwestern Colombia

The Mutatá earthquake is a Mw 6 earthquake which occurred in northwestern Colombia on September 14, 2016. This region is located at the junction between three tectonic plates, namely the South American, Nazca and Caribbean plates, and the Chocó-Panamá and Northern Andes Blocks. This event took place in the Murindo seismic zone, a zone characterized by a high seismic activity involving the Uramita fault zone, which defines the contact between the two blocks. In this study, we relocate the mainshock – aftershocks sequence and analyze the source characteristics of the Mutatá earthquake. Using data from the Colombian Seismological National Network, and after re-picking the 411 events, we obtain absolute locations exhibiting a NW-SE oriented cloud, with the mainshock being located 8 km away from its original location and at a depth of 17.6 km. The event cloud is situated at the intersection of three faults with different orientations, the NNW-SSE Uramita Fault, a NW-SE fault, and a NNE-SSW inferred fault. Using data coming from 8 broad-band seismographs within 300 km of the mainshock, we perform moment tensor and kinematic slip distribution inversions. The moment tensor inversion points to an event centroid at 20 km depth, with a predominantly double-couple mechanism. The fault orientations in the area, NW-SE orientation of the event cloud, and hypocenter – centroid technique, indicate that the NW-SE nodal plane likely corresponds to the fault plane giving a right-lateral strike-slip mechanism on a SW dipping plane. The rupture model estimated on this plane shows different slip patches, one being close to the mainshock centroid, and few other patches distributed around the mainshock except to the southeast where most of the aftershocks are located. The maximum slip for this model is approximately 0.16 m. The source characteristics of the 2016 Mutatá earthquake suggest then that secondary faults within the Murindo seismic zone can generate large earthquakes, potentially consisting in an important source of seismic hazard in this region.

Explaining Structural Difference Between the Eastern and Western Zones of the Qiongdongnan Basin, Northern South China Sea: Insights From Scaled Physical Models

AbstractThe western and eastern zones of the Qiongdongnan Basin (China) show clear contrasts in sub‐basin distribution, fault orientation, and structural styles. Generally, pre‐existing structures are considered to control such differences, that is, NE‐striking main faults in the western zone underwent orthogonal stretching, while E–W‐ and NE‐striking faults in the eastern zone underwent oblique stretching. In addition to the influence of the pre‐existing structures, we suggest that depth‐dependent stretching controlled by the strength of the lower crust together with a change in extension direction may also affect the structural differences of the western and eastern Qiongdongnan Basin. In this study, we use physical analog modeling to explain the structural differences between the eastern and western zones of the Qiongdongnan Basin with the main variables being the strength of the lower crust and a change in extension direction. The model with weak lower crust is successful in accounting for the observed first‐order structural characteristics of this basin. This finding indicates that the rift formation is depth‐dependent and controlled by the orientation of the velocity discontinuities (VDs) and thus the extension direction. Experiment with a change of the extensional direction and weak lower crust reveal that the pre‐existing VDs mainly control the structural belts’ location and fault strikes; some minor structures (late features) respond to the extension change, whereas the main faults are inherited and developed from the previous stage of deformation.

Induced and triggered seismicity below the city of Strasbourg, France from November 2019 to January 2021

Between November 2019 and January 2021, a series of seismic events were felt by the population of the city of Strasbourg, France. The first main event (MLv 3.0) that occurred on November 12, 2019, was part of a seismic swarm (the southern cluster) that has been initiated a few days before, lasted four months, and was located by the BCSF-Rénass (EOST), below La Robertsau area at a depth of 5 km. Its location in the vicinity of the deep geothermal wells (Geoven), the temporal correlation with the injection activity on site, the similarity of the depth between the bottom of the wells and the hypocenter of the event, the lack of local seismicity before the event occurrence, the known geological structures including crustal faults in the area, all strongly support the possible triggering of the events by the deep geothermal activities despite the relatively large distance (4–5 km). From template matching and double-difference relocations, a complex fault zone is evidenced in this southern cluster area that extends over 800 m. Focal mechanisms of the two largest events of the cluster are consistent with the known orientation of the main fault zone in the area. The regional stress field in combination with the fault orientation and a Coulomb failure criterion shows that the seismic cluster location is in an unstable domain, if the cohesion of the fault is weak, particularly sensitive to stress perturbations. In October 2020, after a new series of hydraulic tests, second cluster of seismic events with more felt earthquakes (the northern cluster) developed closer to the geothermal wells (<1 km) below the La Wantzenau area. It includes the largest event (MLv 3.6) that was induced on December 4, 2020, and caused the definitive arrest of the project. On January 22, 2021, three weeks after the shut-in of the wells, an MLv 3.3 event happened with the same location and focal mechanism. We propose here an extended seismotectonic analysis of both seismic clusters.

Pre-evaluation of fault stability for underground mining based on geomechanical fault-slip analysis

In underground near-fault mining, fault reactivation is related to many kinds of mine disasters such as mine water inrush, mine earthquakes, gas outburst, rock burst, roof collapse. pre-evaluation of fault stability in the pre-mining phase is of great significance for predicting possible mine fault-related disasters. In this study, based on the geomechanical analysis of fault slip tendency, a technique to the pre-mining evaluation of fault stability under Andersonian stress system was introduced by extending the original Morris’s theory of fault slip-tendency, effects of fault orientation, in-situ stress and pore fluid pressure on fault stability were analyzed. Taking Shilin coal mine as the research background, a pre-evaluation on the stability of all 15 faults distributed in the whole coalfield at the mining depths of 369 and 514 m was conducted, the analysis results demonstrated that all faults in the Shilin coalfield are quite stable at the two mining depths. The work presented in this study is practical and can provide a quantitative geomechanical way in the analysis of pre-mining fault stability and in the risk pre-assessment of fault-related mine disasters.

Corrigendum: Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

Figure 13. Regional fault orientations and plate boundary parameters used for kinematic analysis and interpretation. Solid orange arrows are maximum and minimum horizontal stress orientations, ! and " , respectively (Hardebeck and Hauksson, 2001), large black and white outlined arrows are plate motion vectors, and the average orientations for Group-1, Group-2, and La Nacion fault zones are solid black, toothed lines. Large black hatched lines are regional faults. G1, Group 1; G2, Group 2. Background fault traces (thin red lines) are from USGS Quaternary fault database (USGS, 2019). Background topography and bathymetry are from the National Centers for Environmental Information Southern California Coastal Relief Model (v2) (Calsbeek et al., 2013).

The initiation of the Mesoproterozoic Bylot basins (Nunavut, Arctic Canada) as recorded in the Nyeboe Formation, Fury and Hecla Group

ABSTRACT Reconstructing Precambrian sedimentary environments over broad cratonic regions often relies on a combination of facies, structural, and provenance analyses. The Mesoproterozoic (ca. 1270–1090 Ma) Fury and Hecla Group, exposed on the Melville Peninsula and northern Baffin Island (Nunavut, Canada), is considered broadly correlative with strata of the Borden, Hunting–Aston, and Thule basins (together referred to as the Bylot basins). We present the results of updated mapping and the first high-resolution sedimentologic and stratigraphic analysis of the lowermost unit in the Fury and Hecla Group, the Nyeboe Formation. The Nyeboe Formation comprises five distinct facies associations: alluvial to fluvial, eolian-backshore, marine-intertidal, marine foreshore to shoreface, and marine-offshore. Thin mafic units are interbedded within the marine shoreface facies and are interpreted to represent volcanic flows. Lateral relationships between facies associations are complex, but generally, facies associations transition from a terrestrial environment at the base to a nearshore marine environment at the top, indicating a transgression. Considering both the along-strike and -dip thickness trends, the presence of mafic volcanic rock units, and possible syndepositional fault orientations crosscutting the deposits, we infer that the Fury and Hecla Group was deposited in a regime of crustal thinning in a half-graben setting. Our results from the Nyeboe Formation suggest a lithostratigraphic correlation to the Nauyat and Adams Sound formations of the Borden Basin. Therefore, this study establishes a geodynamic link between the opening of the Fury and Hecla Basin to the other Bylot basins and contributes to the understanding of a large late Mesoproterozoic intracontinental-basin system.

What comes first: The fault or the ductile shear zone?

Crustal scale fault zones extend below the brittle-ductile transition as ductile shear zones. Here we address the question of which regime, brittle or ductile, initiates and controls the overall system of shear localization. Observations of crustal scale but low displacement conjugate strike-slip faults show that they are typically nearly orthogonal, as expected from plastic shear criteria. Sub-crustal scale conjugate strike-slip faults, however, have acute dihedral angles in accordance with the Coulomb fracture criterion as Anderson's theory predicts. We modeled the crustal scale system with strain weakening rheologies that follow the Coulomb and von Mises criteria respectively, within the brittle and ductile regimes. We find that when the strain weakening rate in the ductile regime exceeds a critical value the entire system shears in the von Mises mode with orthogonal conjugate shears forming at all depths, in accordance with the observations. There is, as observed, no deflection of fault orientation at the brittle-ductile transition. Anderson's theory of faulting thus breaks down for crustal scale faults. This difference in behavior was already evident in data presented in Anderson's 1951 book, but its significance was not understood at that time.

The Effect of Fore‐Arc Deformation on Shallow Earthquake Rupture Behavior in the Cascadia Subduction Zone

AbstractWithin the fore‐arc of the Cascadia Subduction Zone, there are significant along‐strike differences in the orientation of splay faults, sediment consolidation, and fault roughness. Here, we use dynamic rupture simulations of megathrust earthquakes on different realizations of a fault system that incorporate fore‐arc properties representative of offshore Oregon and Washington to estimate how splay faults may behave in future megathrust earthquakes in Cascadia. While splay faults were activated in all of our simulations, splay orientation is a primary control on slip amplitude. Seaward vergent faults accommodate significant amounts of slip resulting in large seafloor uplift and significantly larger tsunami amplitudes. For example, our median tsunami heights including splay faults are about a factor of two larger than those that did not include splay fault deformation. We suggest that there is an urgent need to revisit existing approaches to tsunami hazard assessment in Cascadia to include the influence of splay faults.

Strike-slip faulting on Titan: Modeling tidal stresses and shear failure conditions due to pore fluid interactions

Expressions of strike-slip faulting are well documented on multiple ocean worlds (e.g., Europa, Enceladus, and Ganymede) and the motion along these faults is thought to be driven by variations in diurnal tidal stresses. Titan exhibits a complex and dynamic geology with a varied surface morphology developed from fluvial, aeolian, and possible cryovolcanic and tectonic activity. However, observational data are currently limited and no clear strike-slip indicators have been identified so far. Nevertheless, the inferred presence of a porous ice layer saturated with liquid hydrocarbons in Titan's shallow subsurface provides a unique environment for studying zones of frictional weakness, shear heating, and the promotion of cryovolcanism. This study examines Titan's ability to host shear deformation mechanisms, including considerations for how the presence of near-surface liquid hydrocarbons and the crustal porosity of ice significantly reduce the resistance to shear failure of strike-slip faults in flexed areas under maximum diurnal tidal stresses. We conduct a sensitivity analysis of Titan's shear failure tendencies, given optimal failure conditions that may exist due to pore fluid interactions, and we examine the possibility for strike-slip tectonics guided by Coulomb failure laws and tidal stress mechanisms. Using the SatStress tidal stress model for a Titan-appropriate rheology, we compute the diurnal tidal stress tensor and resolve shear and normal stresses onto a suite of fault orientations. We adopt plausible coefficients of friction and include the effect of a hydrostatic pore fluid pressure gradient for Titan. At shallow fault depths (< 1 km) and for optimally oriented faults, this exploratory model suggests shear failure is achievable under diurnal tidal stress conditions subject to such pore fluid pressures, especially in the polar regions. Our results suggest that under these conditions, shear failure is not only possible, but may be an active deformation mechanism on the surface and in the subsurface of Titan, and could potentially serve as a pathway for the exchange of surface and subsurface materials through shear heating mechanisms. Moreover, the inference of conditions supporting strike-slip tectonism on Titan could have important implications for future Dragonfly observations and for future studies directed at better understanding the structural development of the surfaces of icy moons.

Impoundment‐Associated Hydro‐Mechanical Changes and Regional Seismicity Near the Xiluodu Reservoir, Southwestern China

AbstractFour large hydropower stations have recently been built downstream the Jinsha River in Southwestern China with a strong regional tectonic activity background. There is widely felt seismicity since the impoundment of the Xiluodu and Xiangjiaba reservoirs, increasing the public concern in this region. We begin with a criticality analysis of the faults near these reservoirs to quantify their susceptibility to triggered seismicity. Then we focus on the Xiluodu reservoir to investigate the correlation between the impoundment and seismicity nearby. We analyze the spatio‐temporal distribution of seismicity near the Xiluodu reservoir, and identify the plausible rapid and delayed seismic response due to the impoundment. According to the impoundment record, we explicitly model the hydro‐mechanical changes due to diffusion and reservoir water load, that is, pore pressure, elastic stress, and the resulting Coulomb stress. Our results show that the pore pressure changes can reach a level that may trigger fault reactivation and consequently, seismicity nearby. The water load can also induce the positive Coulomb stress changes on faults, depending on the fault orientation, which is especially important for understanding the earthquakes that occurred shortly after the impoundment and at more than 10 km distance from the reservoir. The combination of these two effects can induce positive Coulomb stress change over a larger area, which overlaps the majority of the events after the impoundment, including two M5+ events. While the causal relationship between the impoundment and seismicity warrants further analysis, we hope to inform the regional seismic impact of impoundment with this timely study.

Novel Bathymetry of Lake Afdera Reveals Fault Structures and Volcano-Tectonic Features of an Incipient Transform Zone (Afar, Ethiopia)

Lake Afdera is a hypersaline endorheic lake situated at 112 m below sea-level in the Danakil Depression. The Danakil Depression is located in the northern part of the Ethiopian Afar and features an advanced stage of continental rifting. The remoteness and inhospitable environment explain the limited scientific research and knowledge about this lake. Bathymetric data were acquired during 2 weeks expeditions in January/February 2016 and 2017 using an easily deployable echosounder system mounted on an inflatable motorized boat. This study presents the first complete bathymetric map of the lake Afdera. Bathymetric results show that the lake has an average depth of 20.9 m and a total volume of 2.4 km3. The maximum measured depth is 80 m, making Lake Afdera the deepest known lake in Afar and the lowest elevation of the Danakil Depression. Comparison with historical reports shows that the lake level did not fluctuate significantly during the last 50 years. Two distinct tectonic basins to the north and the south are recognized. Faults of different orientations control the morphology of the northern basin. In contrast, the southern basin is affected by volcano-tectonic processes, unveiling a large submerged caldera. Comparison between the orientation of faults throughout the lake with the regional fault pattern indicates that the lake is part of two transfer zones: the major Alayta–Afdera Transfer Zone and the smaller Erta Ale–Tat’Ali Transfer Zone. The interaction between these Transfer Zones and the rift axis forms the equivalent of a developing nodal basin which explains the lake’s position as the deepest point of the depression. This study provides evidence for the development of an incipient transform fault on the floor of the Afar depression.

Estimation of Absolute Stress in the Hypocentral Region of the 2019 Ridgecrest, California, Earthquakes

AbstractStrength of the upper brittle part of the Earth's lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the most debated quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. I show that the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi‐planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a recent sequence of strong earthquakes suggests in situ coefficient of friction of 0.4–0.6, and depth‐averaged shear stress on the order of 25–40 MPa, intermediate between predictions of the “strong” and “weak” fault theories.