Showing papers on "Fault (geology) published in 2020"

3D fault architecture controls the dynamism of earthquake swarms

Abstract: The vibrant evolutionary patterns made by earthquake swarms are incompatible with standard, effectively two-dimensional (2D) models for general fault architecture. We leverage advances in earthquake monitoring with a deep-learning algorithm to image a fault zone hosting a 4-year-long swarm in southern California. We infer that fluids are naturally injected into the fault zone from below and diffuse through strike-parallel channels while triggering earthquakes. A permeability barrier initially limits up-dip swarm migration but ultimately is circumvented. This enables fluid migration within a shallower section of the fault with fundamentally different mechanical properties. Our observations provide high-resolution constraints on the processes by which swarms initiate, grow, and arrest. These findings illustrate how swarm evolution is strongly controlled by 3D variations in fault architecture.

Slow slip source characterized by lithological and geometric heterogeneity

Abstract: Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

Complex Rupture of an Immature Fault Zone: A Simultaneous Kinematic Model of the 2019 Ridgecrest, CA Earthquakes

Abstract: The July 4, 2019 Mw6.4 and subsequent July 6, 2019 Mw7.1 Ridgecrest Sequence earthquakes ruptured orthogonal fault planes in the Little Lake Fault Zone, a low slip rate (1 mm/yr) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high-rate GNSS, strong motion, GNSS static offset, and InSAR data. We model the Coulomb stress change to evaluate how the first mainshock may have affected the second. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.

Fault valving and pore pressure evolution in simulations of earthquake sequences and aseismic slip.

Abstract: Fault-zone fluids control effective normal stress and fault strength. While most earthquake models assume a fixed pore fluid pressure distribution, geologists have documented fault valving behavior, that is, cyclic changes in pressure and unsteady fluid migration along faults. Here we quantify fault valving through 2-D antiplane shear simulations of earthquake sequences on a strike-slip fault with rate-and-state friction, upward Darcy flow along a permeable fault zone, and permeability evolution. Fluid overpressure develops during the interseismic period, when healing/sealing reduces fault permeability, and is released after earthquakes enhance permeability. Coupling between fluid flow, permeability and pressure evolution, and slip produces fluid-driven aseismic slip near the base of the seismogenic zone and earthquake swarms within the seismogenic zone, as ascending fluids pressurize and weaken the fault. This model might explain observations of late interseismic fault unlocking, slow slip and creep transients, swarm seismicity, and rapid pressure/stress transmission in induced seismicity sequences.

Dynamic Analysis of the Rock Burst Potential of a Longwall Panel Intersecting with a Fault

Abstract: Faults are one of the most common geological structures in underground mining. Affected by mining activities, fault-slip events will release large amounts of energy and trigger seismic waves, which could induce rock burst events and endanger mining operations. In this study, a longwall panel intersecting with a fault is introduced, as well as field microseismic (MS) monitoring. Static and dynamic numerical analyses are conducted to investigate the fault parameters’ effects on the behaviors of the fault. The results show that the friction angle (φf) significantly affects the shear displacement, magnitude and distribution of the seismic moment; the fault stiffness has a great effect on the magnitude of the seismic moment but smaller effects on the shear displacement and the distribution of the seismic moments. Based on the influence of the fault stiffness and φf on the seismic moment, reasonable fault parameters can be determined. By employing the calibrated parameters, the dynamic responses and the rock burst potential of the surrounding rocks were analyzed by means of the peak particle velocity (PPV) and stress distribution. The propagation of the seismic waves released by fault-slip events excites the particle velocity of the rock mass, and there is a strong correlation between the particle velocity and rock mass damage. As the working face advances toward the fault, the PPV and stress fluctuation of the peak abutment stress rise significantly, which result in a great increase in the rock burst potential. The rock burst potential changes with the mining activities; therefore, corresponding measures must be applied to prevent and control rock burst events. This study contributes to deepening our understanding of the fault parameters in numerical simulations and the dynamic responses and rock burst potential of the surrounding rocks due to mining activities and provides a back-analysis calibration method for the fault parameters.

A Worldwide and Unified Database of Surface Ruptures (SURE) for Fault Displacement Hazard Analyses

Abstract: Fault displacement hazard assessment is based on empirical relationshipsthat are established using historic earthquake fault ruptures.These relationships evaluate the likelihood of coseismicsurface slip considering on-fault and off-fault ruptures, for givenearthquake magnitude and distance to fault. Moreover, theyallow predicting the amount of fault slip at and close to theactive fault of concern. Applications of this approach includeland use planning, structural design of infrastructure, and criticalfacilities located on or close to an active fault.To date, the current equations are based on sparsely populateddatasets, including a limited number of pre-2000 events. In2015, an international effort started to constitute a worldwideand unified fault displacement database (SUrface Ruptures dueto Earthquakes [SURE]) to improve further hazard estimations.After two workshops, it was decided to unify the existingdatasets (field-based slip measurements) to incorporate recentand future cases, and to include new parameters relevant toproperly describe the rupture.This contribution presents the status of the SURE databaseand delineates some perspectives to improve the surface-faultingassessment. Original data have been compiled and adaptedto the structure. The database encompasses 45 earthquakesfrom magnitude 5–7.9, with more than 15,000 coseismic surfacedeformation observations (including slip measurements)and 56,000 of rupture segments. Twenty earthquake cases arefrom Japan, 15 from United States, two from Mexico, Italy,and New Zealand, one from Kyrgystan, Ecuador, Turkey,and Argentina. Twenty-four earthquakes are strike-slip faultingevents, 11 are normal or normal oblique, and 10 are reversefaulting.To pursue the momentum, the initial and common implementationeffort needs to be continued and coordinated, and themaintenance and longevity of the database must be guaranteed.This effort must remain based on a large and open communityof earthquake geologists to create a free and open accessdatabase.

Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications

Abstract: Southeastern Tibet, which has complex topography and strong tectonic activity, is an important area for studying the subsurface deformation of the Tibetan Plateau Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau, which incorporates ambient noise data, we obtain the interstation phase velocity dispersion data in the period range of 5–150 s Then, we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method We find two low-velocity belts in the mid-lower crust One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block, whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part, the Yunnan-Guizhou Plateau The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening Moreover, the significant positive radial anisotropy (VSH>VSV) around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSV model This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam, which may be closely related to heat sources in the upper mantle The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system, which is exactly in the inner and intermediate zones of the Emeishan large igneous province (ELIP) We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP, which may represent maficultramafic material that remained in the crust when the ELIP formed In the upper mantle, there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases Through our velocity model, we think that southeastern Tibet is undergoing three different tectonic modes at the same time: (1) the upper crust is rigid, and as a result, the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults; (2) the viscoplastic materials in the middle-lower crust, separated by rigid materials related to the ELIP, migrate plastically southward under the control of the regional stress field and fault systems; and (3) the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma

Seismic and Aseismic Preparatory Processes Before Large Stick–Slip Failure

Abstract: Natural earthquakes often have very few observable foreshocks which significantly complicates tracking potential preparatory processes. To better characterize expected preparatory processes before failures, we study stick-slip events in a series of triaxial compression tests on faulted Westerly granite samples. We focus on the influence of fault roughness on the duration and magnitude of recordable precursors before large stick–slip failure. Rupture preparation in the experiments is detectable over long time scales and involves acoustic emission (AE) and aseismic deformation events. Preparatory fault slip is found to be accelerating during the entire pre-failure loading period, and is accompanied by increasing AE rates punctuated by distinct activity spikes associated with large slip events. Damage evolution across the fault zones and surrounding wall rocks is manifested by precursory decrease of seismic b-values and spatial correlation dimensions. Peaks in spatial event correlation suggest that large slip initiation occurs by failure of multiple asperities. Shear strain estimated from AE data represents only a small fraction (< 1%) of total shear strain accumulated during the preparation phase, implying that most precursory deformation is aseismic. The relative contribution of aseismic deformation is amplified by larger fault roughness. Similarly, seismic coupling is larger for smooth saw-cut faults compared to rough faults. The laboratory observations point towards a long-lasting and continuous preparation process leading to failure and large seismic events. The strain partitioning between aseismic and observable seismic signatures depends on fault structure and instrument resolution.

Preparation zones for large crustal earthquakes consequent on fault-valve action

Abstract: A combination of geological evidence (in the form of hydrothermal vein systems in exhumed fault systems) and geophysical information around active faults supports the localized invasion of near-lithostatically overpressured aqueous fluids into lower portions of the crustal seismogenic zone which commonly extends to depths between 10 and 20 km. This is especially the case for compressional–transpressional tectonic regimes which, beside leading to crustal thickening and dewatering through prograde metamorphism, are also better at containing overpressure and are ‘load-strengthening’ (mean stress rising with increasing shear stress), the most extreme examples being associated with areas undergoing active compressional inversion where existing faults are poorly oriented for reactivation. In these circumstances, ‘fault-valve’ action from ascending overpressured fluids is likely to be widespread with fault failure dual-driven by a combination of rising fluid pressure in the lower seismogenic zone lowering fault frictional strength, as well as rising shear stress. Localized fluid overpressuring nucleates ruptures at particular sites, but ruptures on large existing faults may extend well beyond the regions of intense overpressure. Postfailure, enhanced fracture along fault rupture zones promotes fluid discharge through the aftershock period, increasing fault frictional strength before hydrothermal sealing occurs and overpressures begin to reaccumulate. The association of rupture nucleation sites with local concentrations of fluid overpressure is consistent with selective invasion of overpressured fluid into the roots of major fault zones and with observed non-uniform spacing of major hydrothermal vein systems along exhumed brittle–ductile shear zones. A range of seismological observations in compressional–transpressional settings are compatible with this hypothesis. There is a tendency for large crustal earthquakes to be associated with extensive (L ~ 100–200 km) low-velocity zones in the lower seismogenic crust, with more local Vp/Vs anomalies (L ~ 10–30 km) associated with rupture nucleation sites. In some instances, these low-velocity zones also exhibit high electrical conductivity. Systematic, rigorous evaluation is needed to test how widespread these associations are in different tectonic settings, and to see whether they exhibit time-dependent behaviour before and after major earthquake ruptures.

Surface rupture and shallow fault reactivation during the 2019 Mw 4.9 Le Teil earthquake, France

Abstract: The Rhone River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric syntheticaperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouviere fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates.

Imbricated Aseismic Slip and Fluid Diffusion Drive a Seismic Swarm in the Corinth Gulf, Greece

Abstract: The primary processes driving seismic swarms are still under debate. Here, we study the temporal evolution of a seismic swarm that occurred over a 10-day period in October 2015 in the extensional rift of the Corinth Gulf (Greece) using high-resolution earthquakes relocations. The seismicity radially migrates on a normal fault at a fluid diffusion velocity (~125 m/day). However, this migration occurs intermittently, with periods of fast expansion (2-to-10 km/day) during short seismic bursts alternating with quiescent periods. Moreover, the growing phases of the swarm illuminates a high number of repeaters. The swarm migration is likely the results of a combination of multiple driving processes. Fluid up flow in the fault may induce aseismic slip episodes, separated by phases of fluid pressure build-up. The stress perturbation due to aseismic slip may activate small asperities that produce bursts of seismicity during the most intense phase of the swarm.

Back-propagating supershear rupture in the 2016 M w 7.1 Romanche transform fault earthquake

Abstract: How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments. In one earthquake, an oceanic transform fault ruptured in one direction and then backwards at a speed exceeding that of shear-wave propagation, according to an analysis of data recorded by nearby seafloor and global seismometers.

Lateral propagation-induced subduction initiation at passive continental margins controlled by preexisting lithospheric weakness

Abstract: Understanding the conditions for forming new subduction zones at passive continental margins is important for understanding plate tectonics and the Wilson cycle. Previous models of subduction initiation (SI) at passive margins generally ignore effects due to the lateral transition from oceanic to continental lithosphere. Here, we use three-dimensional numerical models to study the possibility of propagating convergent plate margins from preexisting intraoceanic subduction zones along passive margins [subduction propagation (SP)]. Three possible regimes are achieved: (i) subducting slab tearing along a STEP fault, (ii) lateral propagation-induced SI at passive margin, and (iii) aborted SI with slab break-off. Passive margin SP requires a significant preexisting lithospheric weakness and a strong slab pull from neighboring subduction zones. The Atlantic passive margin to the north of Lesser Antilles could experience SP if it has a notable lithospheric weakness. In contrast, the Scotia subduction zone in the Southern Atlantic will most likely not propagate laterally.

In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults.

Abstract: Establishing temporal constraints of faulting is of importance for tectonic and seismicity reconstructions and predictions. Conventional fault dating techniques commonly use bulk samples of syn-kinematic illite and other K-bearing minerals in fault gouges, which results in mixed ages of repeatedly reactivated faults as well as grain-size dependent age variations. Here we present a new approach to resolve fault reactivation histories by applying high-spatial resolution Rb-Sr dating to fine-grained mineral slickenfibres in faults occurring in Paleoproterozoic crystalline rocks. Slickenfibre illite and/or K-feldspar together with co-genetic calcite and/or albite were targeted with 50 µm laser ablation triple quadrupole inductively coupled plasma mass spectrometry analyses (LA-ICP-MS/MS). The ages obtained disclose slickenfibre growth at several occasions spanning over 1 billion years, from at least 1527 Ma to 349 ± 9 Ma. The timing of these growth phases and the associated structural orientation information of the kinematic indicators on the fracture surfaces are linked to far-field tectonic events, including the Caledonian orogeny. Our approach links faulting to individual regional deformation events by minimizing age mixing through micro-scale analysis of individual grains and narrow crystal zones in common fault mineral assemblages.

The 2020 Mw 6.8 Elazığ (Turkey) Earthquake Reveals Rupture Behavior of the East Anatolian Fault

Abstract: The 2020 Mw 6.8 Elazig earthquake was the largest along the Eastern Anatolian Fault (EAF) in over a century and so provides valuable insights into its rupture behavior. Because the EAF is of low-to...

The Zagreb (Croatia) M5.5 Earthquake on 22 March 2020

Abstract: On 22 March 2020, Zagreb was struck by an M55 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, ie, seismic motion amplification Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M55 earthquake shows that ground motions of approximately 016–019 g were amplified at least twice The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2 Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area

Development of 3-D Rift Heterogeneity Through Fault Network Evolution

Abstract: Observations of rift and rifted margin architecture suggest that significant spatial and temporal structural heterogeneity develops during the multiphase evolution of continental rifting. Inheritance is often invoked to explain this heterogeneity, such as preexisting anisotropies in rock composition, rheology, and deformation. Here, we use high-resolution 3-D thermal-mechanical numerical models of continental extension to demonstrate that rift-parallel heterogeneity may develop solely through fault network evolution during the transition from distributed to localized deformation. In our models, the initial phase of distributed normal faulting is seeded through randomized initial strength perturbations in an otherwise laterally homogeneous lithosphere extending at a constant rate. Continued extension localizes deformation onto lithosphere-scale faults, which are laterally offset by tens of km and discontinuous along-strike. These results demonstrate that rift- and margin-parallel heterogeneity of large-scale fault patterns may in-part be a natural byproduct of fault network coalescence.

The relationship between synsedimentary fault activity and reservoir quality — A case study of the Ek1 formation in the Wang Guantun area, China

Abstract: The activity of synsedimentary faults plays an important role in controlling the distribution of sand bodies in basins and furthermore the porosity and permeability of reservoirs. We have u...

Porosity zoning characteristics of fault floor under fluid-solid coupling

Abstract: During exploitation approaching to faults structure, due to the influence of mining stress, the rock mass inside the fault zone and between the fault and the coal pillar is prone to damage and activate, so it is the key research and protection position. The dynamic response of faults is closely related to the failure process of rocks, and the influence of the change in mining-induced stress will also result in the differential distribution of the mechanical properties of floor, and ultimately affects the failure of floor and the outburst process of the confined water. In order to quantitatively analyze the influence of mining pressure on the porosity change of floor and water pressure distribution in coal seam mining process, the change characteristics of rock material mechanics parameters such as Young’s modulus, porosity, and permeability under the action of mining disturbance and confined water pressure were studied. A similar material simulation study on the water pressure and stress distribution of floor at different depths under different mining distance conditions was carried out. According to the results, the energy dissipation of coal floor rock mass is closely related to the change of porosity. Meanwhile, the changes in the porosity of rock mass under the action of uneven water pressure and the partition and distribution model of pore water pressure of the floor were established. When mining is carried out near the faults, the spatiotemporal position when and where the upper and lower boundaries of the fault zone in the lower water-resisting layer release energy at the same time is critical to the occurrence of water inrush.

Structurally controlled rock slope deformation in northern Norway

Abstract: Gravitational forcing of oversteepened rock mass leads to progressive failure, including rupture, creeping, sliding and eventual avalanching of the unstable mass. As the point of rupture initiation typically follows pre-existing structural discontinuities within the rock mass, understanding the structural setting of slopes is necessary for an accurate characterisation of the hazards and estimation of the risk to life and infrastructure. Northern Norway is an alpine region with a high frequency of large rock slope deformations. Inherited structures in the metamorphic bedrock create a recurring pattern of anisotropy, that, given certain valley orientations, causes mass instability. We review the geomorphology, structural mechanics and kinematics of nine deforming rock slopes in Troms County, with the aim of linking styles of deformation. The limits of the unstable rock mass follow either foliation planes, joint planes or inherited faults, depending on the valley aspect, slope angle, foliation dip and proximity to fault structures. We present an updated geotechnical model of the different failure mechanisms, based on the interpretations at each site of the review.

Complexity of Fault Rupture and Fluid Leakage in Shale: Insights From a Controlled Fault Activation Experiment

Abstract: We observed rupture growth caused by controlled fluid injections at 340-m depth within a fault zone in the low-permeability Opalinus Clay in the Mont Terri Underground Research Laboratory (Switzerland). The rupture mechanisms were evaluated using measurements of the three-component borehole wall displacements and fluid pressure in two sections of the fault zone and located horizontally 3 m apart from each other. One section was set across a secondary segment of the fault and used for stepwise fluid injection intended to trigger rupture growth. The other section was set across the principal shear zone of the fault for monitoring. After stepwise pressure increase up to 5.95 MPa at injection, rupture initiated as slip activation, followed by an overall opening of the fault planes connected to the injection. After 19 s of continued injection, displacements arrived at the monitoring point on the principal shear zone. These displacements are about 2.4 times larger than in the secondary fault segment. Overall, the displacements corresponded to a normal fault activation. About 9 s after the displacement front arrived, a strong pressure increase of 4.17 MPa was measured at the monitoring point, indicating a hydraulic connection had formed along the initially very low permeability fault planes between the injection and the monitoring points. Our analyses highlight that the fault activation is consistent with the state of stress but that injection pressure must be close to the normal stress acting on the fault for permeability to be generated and for fluid leakage to occur.