Transient Rheology Research Articles

Dominance of transient rheology in far-field postseismic deformation following the 2012 Mw 7.8 Haida Gwaii and 2013 Mw 7.5 Craig earthquakes

The Mw 7.8 2012 Haida Gwaii earthquake and the Mw 7.5 2013 Craig earthquake occurred along the Queen Charlotte transpressive margin, two months and 340 km apart. On and near Vancouver Island, hundreds of kilometres southeast of the two events, many continuous Global Navigation Satellite Systems sites underwent no discernable coseismic motion but up to 1 cm southeast postseismic motion (away from epicenters) which lasted for only 2 – 3 years. To resolve the postseismic signal of the two earthquakes, we used a minimum of four years of measurements before the Haida Gwaii earthquake to remove the longer-term motion of these sites that reflect mainly the locking of the Cascadia megathrust. To study the geodynamic process that governed the observed far-field postseismic motion, in particular its short duration, we employ a model of an elastic lithosphere with thickness 20 km and rigidity (μE) 32 GPa overlying Burgers-viscoelastic mantle layers in a self-gravitating spherical Earth which captures the regionally integrated effect of Earth rheology despite the complex and controversial tectonics of the study area. We find that the short-lived far-field postseismic motion was controlled entirely by the transient rheology. The observations can be explained by a steady-state (Maxwell) viscosity ηM of the upper mantle no lower than 1×1019 Pa s with a transient (Kelvin) viscosity ηK of about 6×1017 Pa s (Kelvin relaxation time τK ∼1.2 years) extending to large depths such as > 400 km. The far-field postseismic deformation is the result of the elastic plate serving as a stress guide but resisted by the asthenospheric mantle undergoing transient creep. The lithosphere-asthenosphere coupling is quantified by a “plate Kelvin response time” τKE=ηK/μE. Rheological parameters can be scaled to other values, but as long as τK and τKE stay unchanged, the far-field deformation results remain the same. Our observations presented a unique opportunity to quantify the effect of the transient rheology separately from that of the steady-state rheology.

Surface loading on a self-gravitating, linear viscoelastic Earth: moving beyond Maxwell

SUMMARY Constitutive laws are a necessary ingredient in calculations of glacial isostatic adjustment (GIA) or other surface loading problems (e.g. loading by ocean tides). An idealized constitutive law governed by the Maxwell viscoelastic model is widely used but increasing attention is being directed towards more intricate constitutive laws that, in particular, include transient rheology. In this context, transient rheology collectively refers to dissipative mechanisms activated in addition to creep modelled by the Maxwell viscoelastic model. Consideration of such viscoelastic models in GIA is in its infancy and to encourage their wider use, we present constitutive laws for several experimentally derived transient rheologies and outline a flexible method in which to incorporate them into geophysical problems, such as the viscoelastic deformation of the Earth induced by surface loading. To further motivate this need, we demonstrate, via the Love number collocation method, how predictions of crustal displacement depart significantly between Earth models that adopt only Maxwell viscoelasticity and those with transient rheology. Throughout this paper, we highlight the differences in terminology and emphases between the rock mechanics, seismology and GIA communities, which have perhaps contributed towards the relative scarcity in integrating this broader and more realistic class of constitutive laws within GIA. We focus on transient rheology since the associated deformation has been demonstrated to operate on timescales that range from hours to decades. With ice mass loss enhanced at similar timescales as a consequence of anthropogenically caused climate change, the ability to model GIA with more accurate constitutive laws is an important tool to investigate such problems.

Timescales of glacial isostatic adjustment in Greenland: is transient rheology required?

SUMMARY The possibility of a transient rheological response to ice age loading, first discussed in the literature of the 1980s, has received renewed attention. Transient behaviour across centennial to millennial timescales has been invoked to reconcile apparently contradictory inferences of steady-state (Maxwell) viscosity based on two distinct data sets from Greenland: Holocene sea-level curves and Global Navigation Satellite System (GNSS) derived modern crustal uplift data. To revisit this issue, we first compute depth-dependent Fréchet kernels using 1-D Maxwell viscoelastic Earth models and demonstrate that the mantle resolving power of the two Greenland data sets is highly distinct, reflecting the differing spatial scale of the associated surface loading: the sea-level records are sensitive to viscosity structure across the entire upper mantle while uplift rates associated with post-1000 CE fluctuations of the Greenland Ice Sheet have a dominant sensitivity to shallow asthenosphere viscosity. Guided by these results, we present forward models which demonstrate that a moderate low viscosity zone beneath the lithosphere in Maxwell Earth models provides a simple route to simultaneously reconciling both data sets by significantly increasing predictions of present-day uplift rates in Greenland whilst having negligible impact on predictions of Holocene relative sea-level curves from the region. Our analysis does not rule out the possibility of transient deformation, but it suggests that it is not required to simultaneously explain these two data sets. A definitive demonstration of transient behaviour requires that one account for the resolving power of the data sets in modelling the glacial isostatic adjustment process.

Function Model Based on Nonlinear Transient Rheology of Rocks: An Analysis of Decadal GNSS Time Series After the 2011 Tohoku‐oki Earthquake

AbstractThe postseismic Global Navigation Satellite System (GNSS) time series after the 2011 Tohoku‐oki earthquake is often empirically explained by one or more logarithmic and exponential decay functions. However, a function model with these decay functions struggles to illuminate various deformation mechanisms. Here, we propose a new function model incorporating laboratory‐derived constitutive laws (power‐law flow and velocity‐strengthening friction) utilized in mechanical postseismic models of the last decade. We demonstrated that our function model accurately predicts the decade‐long time series of inland GNSS stations, including coastal areas where significant postseismic uplift continues now. Moreover, it decomposes them into displacements due to viscoelastic relaxation and afterslip, similar to results produced by previous stress‐dependent postseismic models of the 2011 Tohoku‐oki earthquake. This physics‐based function model is an effective tool to ease the process of forecasting long‐term GNSS time series by dividing them into two dominant mechanisms at the plate interface and the surrounding viscoelastic mantle.

A Microphysical Model of Rock Friction and the Brittle‐Ductile Transition Controlled by Dislocation Glide and Backstress Evolution

AbstractRate‐ and state‐friction (RSF) is an empirical framework that describes the complex velocity‐, time‐, and slip‐dependent phenomena observed during frictional sliding of rocks and gouge in the laboratory. Despite its widespread use in earthquake nucleation and recurrence models, our understanding of RSF, particularly its time‐ and/or slip‐dependence, is still largely empirical, limiting our confidence in extrapolating laboratory behavior to the seismogenic zone. While many microphysical models have been proposed over the past few decades, none have explicitly incorporated the effects of strain hardening, anelasticity, or transient elastoplastic rheology. Here we present a new model of rock friction that incorporates these phenomena directly from the microphysical behavior of lattice dislocations. This model of rock friction exhibits the same logarithmic dependence on sliding velocity (strain rate) as RSF and displays a dependence on the internal backstress caused by long‐range interactions among geometrically necessary dislocations (GNDs). Changes in the backstress (internal stress) evolve exponentially with plastic strain of asperities and are dependent on both the current backstress and previous deformation, which give rise to phenomena consistent with interpretations of the “critical slip distance,” “memory effect,” and “evolution effect” of RSF. The rate dependence of friction in this model is primarily controlled by the evolution of backstress and temperature. We provide several analytical predictions for RSF‐like behavior and the “brittle‐ductile” transition based on microphysical mechanisms and measurable parameters such as the GND density and strain‐dependent hardening modulus.

Unveiling the restricted mobility of carbon nanotubes inside a long chain branched polymer matrix via probing the shear flow effects on the rheological and electrical properties of the filled systems.

The present work has aimed at gaining a deeper understanding of the effects of shear flow on the behaviors of nano filler evolution inside linear and long chain branched polymer matrices. Accordingly, measurements consisting of transient start-up shear rheology coupled with small amplitude oscillatory sweep (SAOS) and dielectric tests were designed. Linear polypropylene (PPC) and polypropylene (PPH) with long chain branching (LCB) were chosen as the polymer matrices and carbon nanotubes (CNTs) as the nanofillers. The percolation threshold of the LCB PPH nanocomposites was found to be higher than for linear PPC, due to the high viscosity and elasticity of LCB PPH. A transient shear with different shear rates was imposed on the composites after which SAOS and electrical conductivity measurements were conducted. The liquid-solid transitions of the nanocomposites were found to be different and to depend on the shear flow conditions (shear rate). For the linear PPC, higher shear rates caused the filler network to break down while lower shear rates helped the nanofillers to agglomerate. Interestingly, for LCB PPH, both higher and lower pre-shear rates resulted in the breakup of the filler networks, which was due to the restricted mobility of the CNTs by the LCB. The confinement of the polymer chains to the CNTs and their aggregates made it difficult for the fillers to move thus causing the formed network to be easily destroyed even under slow and slight shears. Similarly, the trend was also found after shear flows as reflected by the increase and decrease of electrical conductivities.

Three-Dimensional Sag Tracking in Falling Liquid Films

Coating defects often arise during application in the flash stage, which constitutes the ∼10 min interval immediately following film application when the solvent evaporates. Understanding the transient rheology and kinematics of a coating system is necessary to avoid defects such as sag, which results in undesirable appearance. A new technique named variable angle inspection microscopy (VAIM) aimed at measuring these phenomena was developed and is summarized herein. The essence of this new, non-invasive, rheological technique is the measurement of a flow field in response to a known gravitational stress. VAIM was used to measure the flow profile through a volume of a liquid thin film at an arbitrary orientation. Flow kinematics of the falling thin film was inferred from particle tracking measurements. Initial benchmarking measurements in the absence of drying tracked the velocity of silica probe particles in ∼140 μm thick films of known viscosity, much greater than water, at incline angles of 5° and 10°. Probe particles were tracked through the entire thickness of the film and at speeds as high as ∼100 μm/s. The sag flow field was well resolved in ∼10 μm thick cross sections, and in general the VAIM measurements were highly reproducible. Complementary profilometer measurements of film thinning were utilized to predict sag velocities with a known model. The model predictions showed good agreement with measurements, which validated the effectiveness of this new method in relating material properties and flow kinematics.

Correction to: Simulated orientational morphology from the measured transient rheology of polycarbonate–carbon fiber composites

Correction to: Simulated orientational morphology from the measured transient rheology of polycarbonate–carbon fiber composites

Rate and State Friction as a Spatially Regularized Transient Viscous Flow Law

AbstractThe theory of rate and state friction unifies field, laboratory, and theoretical analysis of the evolution of slip on natural faults. While the observational study of earthquakes and aseismic fault slip is hampered by its strong multi‐scale character in space and time, numerical simulations are well‐positioned to link the laboratory study of grain‐scale processes to the scale at which rock masses move. However, challenges remain in accurately representing the complex and permanently evolving sub‐surface fault networks that exist in nature. Additionally, the common representation of faults as interfaces may miss important physical aspects governing volumetric fault system behavior. In response, we propose a transient viscous rheology that produces shear bands that closely mimic the rate‐ and state‐dependent sliding behavior of equivalent fault interfaces. Critically, we show that the expected tendency of the continuum rheology for runaway localization and mesh dependence can be halted by including an artificial diffusion‐type regularization of anelastic strain rate in the softening law. We demonstrate analytically and numerically using a simplified fault transect that important aspects of the frictional behavior are not significantly affected by the introduced regularization. Any discrepancies with respect to the interfacial description of fault behavior are critically evaluated using one dimensional numerical velocity stepping and spring‐slider experiments. Since no new physical parameters are introduced, our model may be straightforwardly used to extend the existing modeling techniques. The model predicts the emergence of complex patterns of shear localization and delocalization that may inform the interpretation of complex damage distributions observed around faults in nature.

The effects of couple stress lubricants and surface roughness on squeeze EHL motion between porous medium layer and elastic ball

This paper investigated the effects of porous media layer (PML) and couple stress (CS) lubricants at pure squeeze action in an isothermal elastohydrodynamic lubrication (EHL) point contact with surface roughness (SRN) under constant load condition. The modified transient stochastic Reynolds equation was derived in polar coordinates by means of the Christensen’s stochastic theory and the Stokes’s CS fluid theory. The Gauss-Seidel method (GSM) and the finite difference method (FDM) were both used to solve simultaneously modified transient stochastic Reynolds, force equilibrium, elasticity deformation, and rheology equations. The simulation results revealed that the greater permeability ( K) and porous layer thickness (Φ) are, the smaller central pressures ( Pc) are, and the smaller film thicknesses ( H) are. The times required for achieving maximum central pressures (Pcmax) decrease with increasing K and Φ. Due to squeeze and elastic deformation effects, the slope values of central velocity ( Vc) change from negative to positive. The magnitude of Vc and Vmin decreased with decreasing K and Φ. The Hcmin of circular type RN are greater than that of radial type RN. The Pcmax of radial type RN are slightly greater than that of circular type RN.

Simulated orientational morphology from the measured transient rheology of polycarbonate–carbon fiber composites

Simulated orientational morphology from the measured transient rheology of polycarbonate–carbon fiber composites

Interactions between the protein-epigallocatechin gallate complex and nanocrystalline cellulose: A systematic study

In this work, the noncovalent interactions between the protein-epigallocatechin gallate (protein-EGCG) complex and nanocrystalline cellulose (NCC) were systematically investigated. Rheological properties, including large amplitude oscillation shear (LAOS), transient rheology, and conventional rheology of the mixture were also evaluated. As obtained, flexible myofibrillar protein-epigallocatechin gallate (MP-EGCG) and rigid ovalbumin-epigallocatechin gallate (Ova-EGCG) follow different rules in the stability of the regulatory system because of the difference in noncovalent interactions, triggering different rheological responses of the complexes. Additionally, MP-EGCG/NCC showed an obvious strain overshoot during LAOS flow, which could not be obtained in Ova-EGCG/NCC. Notably, the fibrous MP-EGCG network was the factor that dominated the formation of a more stable suspension system with strong hydrogen bonding, dipole–dipole interactions and/or van der Waals forces. This study can provide some ideas for the future study of the interaction between protein–polyphenol complexes and polysaccharides.

Transient and steady shear rheology of particle-laden viscoelastic suspensions

We study the transient and steady shear rheology of rigid particle suspensions in Boger fluids via complete 3D numerical simulations and experiments. We calculate the transient per-particle extra viscosity and primary stress coefficients for suspensions at different particle volume fractions ϕ for a range of Weissenberg numbers (Wi). The per-particle viscosity ( ηp) and the primary normal coefficient ( ψ1p) increase monotonically to steady state in body-fitted (BF) simulations (for dilute suspensions) and immersed boundary (IB) simulations (for nondilute suspensions). We also present experimental measurements including small amplitude oscillatory shear, steady shear, and transient shear measurements at different particle volume fraction suspensions in a Boger fluid. The simulations and experiments suggest that longer strains are needed to achieve steady state at higher ϕ and Wi. We also show the comparison of the BF and the IB simulations with experimental data for the per-particle viscosity and find excellent quantitative agreement between simulations and experiments at Wi=3 but the IB simulations underpredict the steady values at higher Wi=6. Nevertheless, the IB simulations show an increase in the per-particle viscosity with ϕ as witnessed in the experiments. To understand this behavior, we examine the particle-induced fluid stress (PIFS) and the stresslet contributions using a novel method developed for the IB simulations in this work. We find that the PIFS is independent of ϕ but the stresslet values increase with ϕ. Thus, the particle-particle hydrodynamic interactions in nondilute suspensions affect the stresslet and, in turn, the per-particle viscosity at a given Wi.

Hydrodynamic response time of magnetorheological fluid in valve mode: an initial study

The transient behaviour of magnetorheological (MR) damper is a very important parameter affecting the performance of this technology in modern semi-actively controlled suspension systems. Currently, the transient behaviour of the MR damper is limited by dynamics of the MR fluid (MRF) itself. The significant part of MRF response time is a hydrodynamic response time which is connected with transient rheology and development of velocity profile in the slit gap. In this paper, the method for measuring the hydrodynamic response time of MRF operating in valve mode is presented. The hydrodynamic response time of MRF-132DG achieved value of τ90 = 0.78 ms for H = 17.5 kA/m a value of τ90 = 0.65 ms for H = 34 kA/m for given geometry of gap. The difference between model and experiment is lower in higher yield stresses of MRF.

Constitutive modeling of dilute wormlike micelle solutions: Shear-induced structure and transient dynamics

We present a reformulation of the 'reactive rod model' (RRM) of Dutta and Graham [Dutta, Sarit and Graham, Michael D., JNNFM 251 (2018)], a constitutive model for describing the behavior of dilute wormlike micelle solutions. The RRM treats wormlike micelle solutions as dilute suspensions of rigid Brownian rods undergoing reversible scission and growth in flow. Evolution equations for micelle orientation and stress contribution are coupled to a kinetic reaction equation for a collective micelle length, producing dynamic variations in the length and rotational diffusivity of the rods. This model has previously shown success in capturing many critical steady-state rheological features of dilute wormlike micelle solutions, particularly shear-thickening and -thinning, non-zero normal stress differences, and a reentrant shear stress-shear rate curve, and could fit a variety of steady state experimental data. The present work improves on this framework, which showed difficulty in capturing transient dynamics and high-shear behavior, by reformulating the kinetic equation for micelle growth on a more microstructural (though still highly idealized) basis. In particular, we allow for micelle growth associated with strong alignment of rods and breakage due to tensile stresses along the micelles. This new formulation captures both steady and transient shear rheology in good agreement with experiments. We also find good agreement with available steady state extensional rheology.

Modeling Short-Range Interactions in Concentrated Newtonian Fiber Bundle Suspensions

Abstract Sheet Molding Compounds (SMC) offer a cost efficient way to enhance mechanical properties of a polymer with long discontinuous fibers, while maintaining formability to integrate functions, such as ribs, beads or other structural reinforcements. During SMC manufacturing, fibers remain often in a bundled configuration and the resulting fiber architecture determines part properties. Accurate prediction of this architecture by simulation of flow under consideration of the transient rheology and transient fiber orientations can speed up the development process. In particular, the interaction of bundles is of significance to predict molding pressures correctly in a direct simulation approach, which resolves individual fiber bundles. Thus, this work investigates the tangential short-range lubrication forces between fiber bundles with analytical and numerical techniques. A relation between the effective sheared gap between bundles and the bundle separation distance at the contact point is found and compared to experimental results from literature. The result is implemented in an ABAQUS contact subroutine to incorporate short-range interactions in a direct bundle simulation framework.

Pegylated Carboxyhemoglobin (SANGUINATE®) Restores Blood Flow and Tissue Oxygenation in a Novel Rat Model of Acute Vaso-Occlusive Crisis

INTRODUCTION: Sickle Cell Disease (SCD) hemoglobin S (HbS) promotes hemoglobin polymerization and red blood cell (RBC) sickling during hypoxia. PEGylated carboxyhemoglobin bovine (PEG-COHb; SANGUINATE®) has been previously shown to reverse sickling of SCD RBC in vitro by directed gas transfer to hypoxic RBC. A rat model was used to evaluate the capability of SANGUINATE® to restore cardiovascular rheology and tissue oxygenation (PISFO2) at sites distal to acute Vaso-Occlusions (VOC).OBJECTIVES: Evaluate the in vivo capability of PEG-COHb to improve rheology in rats placed in active VOC through transfusion of human SCD RBC while monitoring treatment effects on PISFO2 and systemic functions.METHODS: Human homozygous (HbSS) whole blood was obtained from consented SCD volunteers. The SCD RBC were isolated by centrifugation, triple-washed and resuspended in lactated Ringer's saline (LRS). Male Sprague Dawley rats were anesthetized, cannulated for fluid transfer, and surgically prepared for microscopic study of the spinotrapezius microcirculation. Isovolemic exchange transfusion of rat blood with suspended SC RBCs (27.5 ± 3.5% total blood volume) was conducted until PISFO2 at the distal portion of the spinotrapezius muscle was < 1 mmHg. VOC was visually confirmed as wide-spread precapillary arteriolar flow-arrest. Animals in VOC were subsequently treated 15 minutes later with PEG-COHb or LRS administered as a bolus of 8.0 ml/kg over 5 minutes. Animals were observed for up to 4 hours post-treatment and then euthanized. Measurements included systemic cardiovascular parameters, survival, blood chemistry, and PISFO2 via phosphorescence quenching microscopy and were obtained across treatment groups and the no treatment [Sham] control group.RESULTS: Exchange transfusion successfully produced a reduction in PISFO2 in the distal portion of the spinotrapezius muscle for all animals to <1 mmHg presumably through Vaso-Occlusions. VOC was visually apparent as flow arrest was initially observed in capillaries and most small venules and arterioles, and then progressed proximally into sites with larger feed vessels and higher intravascular pressures with subsequent declines in local PISFO2. Intervention with PEG-COHb and LRS both showed a visual improvement in microvascular flow, but yielded differential effects on PISFO2 in all sites (average return to 96 ± 6% and 64 ± 20% of baseline, respectively). PEG-COHb -treated animals survived the entire 4 h observation phase with a final global spinotrapezius PISFO2 level within 10% of baseline values. In contrast, global spinotrapezius PISFO2 declined to 0 mmHg for LRS treated animals by 3 h with rapid decline in mean arterial pressure and death occurring prior to 4 h. Serum lactate levels were monitored to provide an assessment of systemic ischemia from time 0 after exchange transfusion to the last collection point near the end of the experiment. Lactate increased for untreated Sham (34 ± 4.5 to 124 ± 43.0 mg/dl) and LRS groups (48 ± 15.0 to 110 ± 43.0 mg/dl), but remained stable for SANGUINATE®-treated animals (42 ± 1.5 to 40 ± 4.5 mg/dl). Total microcirculatory collapse occurred in Sham animals by 3h along with a null-reading of PISFO2, which was followed by a rapid decline in mean arterial pressure and death at 3.5 h similar to the LRS group.SUMMARY: This study was conducted in a novel rat model providing the ability to monitor real-time microcirculatory events and systemic parameters, which is difficult in murine models due to their smaller stature. PEG-COHb treatment promoted VOC resolution as evidenced through improved blood flow and concomitant effects on PISFO2. Importantly, the gas transfer properties of SANGUINATE® not only acutely resolved VOC, but provided the added benefit of restoring PISFO2 to near baseline values. In contrast, LRS animals showed transient rheology and minimal PISFO2 improvements, but levels declined over time and outcomes were similar to sham controls. Restoring blood flow and PISFO2 are key events to limiting VOC pain and complications and the treatment with PEG-COHb demonstrated the ability to achieve these critical functions suggesting it may provide improvements in treating acute VOC in human SCD individuals. DisclosuresSong:Prolong Pharmaceuticals: Consultancy, Research Funding. Nugent:Prolong Pharmaceuticals: Consultancy, Research Funding.

Transient interfacial rheology and polar component dynamics at oil-brine interfaces

Interfacial deformation processes have diverse impacts on numerous industrial applications, including the petroleum industry. This study aimed to investigate the interfacial rheology and polar component dynamics in crude oil and brine systems using a wide range of salinity. The oscillating pendant-drop technique was utilized to evaluate the transient and equilibrium oil-brine interfacial rheological behavior and the associated diffusion, adsorption, and rearrangement processes of the bulk-dispersed and interfacially adsorbed polar components. A new logistic equation is proposed to model the transitory rheology of oil-brine interfaces, enabling the quantitative comparison of their behaviors. The conducted modeling approach indicated that the diffusion/adsorption processes are the dominant relaxation mechanisms over reorientation/rearrangement processes (negligible) when the interfaces are perturbed at different aging times. Furthermore, the results show that (a) the rate at which apparent interfacial elasticity increases is frequency-independent, (b) there are two local maximums for apparent interfacial viscoelasticity with brine salinity variation, (c) polar component dynamics in the studied crude oil are mainly controlled by diffusion, and (d) absence of significant lateral interactions of adsorbed species on the interface. Transient interfacial rheology is demonstrated to be of great importance when defining final apparent interfacial properties, and the potential consequences of these new findings on enhanced oil recovery applications are discussed.

Rheology and structure of suspensions of spherocylinders via Brownian dynamics simulations

Brownian dynamics simulations were employed to investigate the rheological properties and structure of suspensions of rigid spherocylinders, as a model for rodlike colloids. The spherocylinders interacted only through a soft repulsive force that mimicked a hard spherocylinder interaction. The translational and rotational diffusivities of hard spherocylinder suspensions were reproduced. Liquid crystalline phases, including isotropic, nematic, and smectic and solid phases were identified using orientational and hexatic order parameters and pair distribution functions. Typical flow curves observed experimentally for rodlike colloidal suspensions were reproduced in the simulations, with two shear thinning regions that bracketed a viscosity plateau at intermediate Péclet numbers. The transient rheology and structure of suspensions that were nematic at rest exhibited a variety of behaviors that depended on the Péclet number and concentration. Systemwide domains that aligned in and kayaked about the vorticity direction, domains that rotated coherently locally, and layered domains were observed. Oscillations in the order parameter, viscosity, and the first and second normal stress differences were associated with changes in the structure.

Thin crème brûlée rheological structure for the Eastern California Shear Zone

AbstractSince the occurrence of the 1992 CE Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes in the Mojave Desert (California, USA), postseismic deformation following both earthquakes has been intensively studied, and models with a strong crust overlying a low-viscosity mantle asthenosphere have been favored. However, we recently found that the near-field postseismic transients after the two earthquakes have lasted longer than previously thought, which requires a revision of the postseismic modeling. Our new modeling results based on the revised postseismic transients show that: (1) the effective viscosity of the lower crust beneath the Mojave region at the decadal time scale is ∼2 × 1020 Pa·s (transient viscosity ∼2 × 1019 Pa·s), i.e., only ∼5 times that of the underlying mantle asthenosphere, and (2) the transient viscosity of the upper mantle exhibits a time-dependent increase, providing fresh geodetic evidence for frequency-dependent rheology (e.g., Andrade or extended Burgers rheology). The inferred transient rheology for the first year agrees well with that obtained for the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquakes ∼180 km north of the two Mojave events. Our modeling results support a thin crème brûlée model for the Eastern California Shear Zone (part of the Pacific-North America plate boundary) in which both the lower crust and the upper mantle exhibit ductility at decadal time scales.