Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes

Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed

Healing of simulated fault gouges aided by pressure solution: results from rock analogue experiments

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The role of deformation bands in dictating poromechanical properties of unconsolidated sand and sandstone

Cataclastic shear bands in sands and sandstones are typically stronger, stiffer, and exhibit lower permeability than the surrounding matrix, and therefore act as barriers to fluid flow. Previous work has quantified the reduction in permeability associated with these features; however, little is known about the role of shear band structure in controlling the way they impact permeability and elastic properties. Here, we report on a suite of laboratory measurements designed to measure the poromechanical properties for host material and natural shear bands, over effective stresses from 1–65 MPa. In order to investigate the role of host material properties in controlling poromechanical evolution with stress, we sampled shear bands from two well-studied sandstones representing structurally distinct end-members: a poorly cemented marine terrace sand from the footwall of the McKinleyville thrust fault in Humboldt County, California, and a strongly-cemented sandstone from the hanging wall of the Moab Fault in Moab, Utah. The permeability-porosity trends are similar for all samples, with permeability decreasing systematically with increasing effective stress and decreasing porosity. The permeability of the host material is consistently >1 order of magnitude greater than the shear bands for both localities. For the unconsolidated case, shear bands are less permeable and stiffer than the host material, whereas for the consolidated case, shear bands are slightly less permeable, and wave speeds are slower than in the host. We attribute the differences between the McKinleyville and Moab examples to changes in structure of the nearby host material that accompanied formation of the shear band

Impact of rural versus urban geographic location on length of stay after carotid endarterectomy

Abstract Background: Reducing the incidence of extended length of stay (ELOS) after carotid endarterectomy (CEA), defined as LOS \u3e I day, is an important quality improvement focus of the Vascular Quality Initiative (VQI). Rural patients with geographic barriers pose a particular challenge for discharge and may have higher incidences of ELOS as a result. The purpose of this study was to examine the impact of patients’ home geographic location on ELOS after CEA. Methods: The VQI national database for CEA comprised the sample for analyses (N = 66,900). Rural-Urban Commuting Area (RUCA) codes, a validated system used to classify the nation’s census tracts according to rural and urban status, was applied to the VQI database and used to indicate patients’ home geographic location. LOS was categorized into two groups: LOS ≤ 1 day (66%) and LOS \u3e 1 day (ELOS) (34%). Multivariable logistic regression was conducted to examine the effect of geographic location on ELOS after adjustment for age, gender, race, and comorbid conditions. Results: A total of 66,900 patients were analyzed and the mean age of the sample was 70.5 ± 9.3 years (40% female). After adjustment for covariates, the urban group had increased risk for ELOS (OR = 1.20, p \u3c 0.001). Other factors that significantly increased risk for ELOS were non-White race/Latinx/Hispanic ethnicity (OR = 1.44, p \u3c 0.001) and nonelective status (OR =3.31, p \u3c 0.001). In addition, patients treated at centers with a greater percentage of urban patients had greater risk for ELOS (OR = 1.008, p \u3c 0.001). Conclusions: These analyses found that geographic location did impact LOS, but not in the hypothesized direction. Even with adjustment for comorbidities and other factors, patients from urban areas and centers with more urban patients were more likely to have ELOS after CEA. These findings suggest that other mechanisms, such as racial disparities, barriers in access to care, and disparities in support after discharge for urban patients may have a significant impact on LOS

Permeability evolution in sorbing media. Analogies between organic-rich shale and coal

Shale gas reservoirs like coalbed methane (CBM) reservoirs are promising targets for geological sequestration of carbon dioxide (CO2). However, the evolution of permeability in shale reservoirs on injection of CO2 is poorly understood unlike CBM reservoirs. In this study, we report measurements of permeability evolution in shales infiltrated separately by nonsorbing (He) and sorbing (CO2) gases under varying gas pressures and confining stresses. Experiments are completed on Pennsylvanian shales containing both natural and artificial fractures under nonpropped and propped conditions. We use the models for permeability evolution in coal (Journal of Petroleum Science and Engineering, Under Revision) to codify the permeability evolution observed in the shale samples. It is observed that for a naturally fractured shale, the He permeability increases by approximately 15% as effective stress is reduced by increasing the gas pressure from 1 MPa to 6 MPa at constant confining stress of 10 MPa. Conversely, the CO2 permeability reduces by a factor of two under similar conditions. A second core is split with a fine saw to create a smooth artificial fracture and the permeabilities are measured for both nonpropped and propped fractures. The He permeability of a propped artificial fracture is approximately 2- to 3fold that of the nonpropped fracture. The He permeability increases with gas pressure under constant confining stress for both nonpropped and propped cases. However, the CO2 permeability of the propped fracture decreases by between one-half to one-third as the gas pressure increases from 1 to 4 MPa at constant confining stress. Interestingly, the CO2 permeability of nonpropped fracture increases with gas pressure at constant confining stress. The permeability evolution of nonpropped and propped artificial fractures in shale is found to be similar to those observed in coals but the extent of permeability reduction by swelling is much lower in shale due to its lower organic content. Optical profilometry is used to quantify the surface roughness. The changes in surface roughness indicate significant influence of proppant indentation on fracture surface in the shale sample. The trends of permeability evolution on injection of CO2 in coals and shales are found analogous; therefore, the permeability evolution models previously developed for coals are adopted to explain the permeability evolution in shales

Evolution of elastic and mechanical properties during fault shear. The roles of clay content, fabric development, and porosity

Phyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite-quartz fault gouges (10–100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite-poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300–1,800 m/s, Vs ~ 900–1,100 m/s, K ~ 1–4 GPa, and G ~ 1–2 GPa, and low smectite gouges have values of Vp ~ 2,300–2,500 m/s, Vs ~ 1,200–1,300 m/s, K ~ 5–8 GPa, and G ~ 2.5–3 GPa. We find that, even in smectite-poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine-grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces

Deterministic and stochastic chaos characterize laboratory earthquakes

We analyze frictional motion for a laboratory fault as it passes through the stability transition from stable sliding to unstable motion. We study frictional stick-slip events, which are the lab equivalent of earthquakes, via dynamical system tools in order to retrieve information on the underlying dynamics and to assess whether there are dynamical changes associated with the transition from stable to unstable motion. We find that the seismic cycle exhibits characteristics of a low-dimensional system with average dimension similar to that of natural slow earthquakes (<5). We also investigate local properties of the attractor and find maximum instantaneous dimension ≳10, indicating that some regions of the phase space require a high number of degrees of freedom (dofs). Our analysis does not preclude deterministic chaos, but the lab seismic cycle is best explained by a random attractor based on rate- and state-dependent friction whose dynamics is stochastically perturbed. We find that minimal variations of 0.05% of the shear and normal stresses applied to the experimental fault influence the large-scale dynamics and the recurrence time of labquakes. While complicated motion including period doubling is observed near the stability transition, even in the fully unstable regime we do not observe truly periodic behavior. Friction's nonlinear nature amplifies small scale perturbations, reducing the predictability of the otherwise periodic macroscopic dynamics. As applied to tectonic faults, our results imply that even small stress field fluctuations (≲150 kPa) can induce coefficient of variations in earthquake repeat time of a few percent. Moreover, these perturbations can drive an otherwise fast-slipping fault, close to the critical stability condition, into a mixed behavior involving slow and fast ruptures

Breakdown pressure and fracture surface morphology of hydraulic fracturing in shale with H2O, CO2 and N2

Slick-water fracturing is the most routine form of well stimulation in shales; however N2, LPG and CO2 have all been used as “exotic” stimulants in various hydrocarbon reservoirs. We explore the use of these gases as stimulants on Green River shale to compare the form and behavior of fractures in shale driven by different gas compositions and states and indexed by breakdown pressure and the resulting morphology of the fracture networks. Fracturing is completed on cylindrical samples containing a single blind axial borehole under simple triaxial conditions with confining pressure ranging from 10 to 25 MPa and axial stress ranging from 0 to 35 MPa (σ1 > σ2 = σ3). Results show that: (1) under the same stress conditions, CO2 returns the highest breakdown pressure, followed by N2, and with H2O exhibiting the lowest breakdown pressure; (2) CO2 fracturing, compared to other fracturing fluids, creates nominally the most complex fracturing patterns as well as the roughest fracture surface and with the greatest apparent local damage followed by H2O and then N2; (3) under conditions of constant injection rate, the CO2 pressure build-up record exhibits condensation between ~5 and 7 MPa and transits from gas to liquid through a mixed-phase region rather than directly to liquid as for H2O and N2 which do not; (4) there is a positive correlation between minimum principal stress and breakdown pressure for failure both by transverse fracturing (σ3axial) and by longitudinal fracturing (σ3radial) for each fracturing fluid with CO2 having the highest correlation coefficient/slope and lowest for H2O. We explain these results in terms of a mechanistic understanding of breakdown, and through correlations with the specific properties of the stimulating fluids

Frictional and lithological controls on shallow slow slip at the Northern Hikurangi Margin

Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide-hold-slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 × 10−9 m/s) to ∼1 mm/s (10−3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a-b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of ∼0.3 µm/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay-rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1–10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs

Using a physics-informed neural network and fault zone acoustic monitoring to predict lab earthquakes

Predicting failure in solids has broad applications including earthquake prediction which remains an unattainable goal. However, recent machine learning work shows that laboratory earthquakes can be predicted using micro-failure events and temporal evolution of fault zone elastic properties. Remarkably, these results come from purely data-driven models trained with large datasets. Such data are equivalent to centuries of fault motion rendering application to tectonic faulting unclear. In addition, the underlying physics of such predictions is poorly understood. Here, we address scalability using a novel Physics-Informed Neural Network (PINN). Our model encodes fault physics in the deep learning loss function using time-lapse ultrasonic data. PINN models outperform data-driven models and significantly improve transfer learning for small training datasets and conditions outside those used in training. Our work suggests that PINN offers a promising path for machine learning-based failure prediction and, ultimately for improving our understanding of earthquake physics and prediction