Evolution of asperity contacts during shear failure on frictional interfaces: implications for the initiation of crustal earthquakes

Fault zones and associated slip play a key role in the development of structures within the Earth's upper crust. Fault activity controls crustal-scale fluid flow and fault movement is crucial in the accommodation of strain. Although fault slip is broadly classified as brittle-frictional deformation, there is much we do not understand about the physical mechanisms controlling frictional strength, especially under the extreme conditions accompanying earthquake rupture. This thesis presents the results of five experimental studies. These complementary studies have sought to answer fundamental questions about dynamic frictional processes, including 1) how the structure and properties of materials on the fault interface changes with the application of high strain rates and normal stresses during fault slip; and 2) how slip-induced changes in interface properties modify the strength and behaviour of faults. Experimental results are underpinned by the parallel development of a new interferometry-based displacement sensor and the synchronous acquisition of fault strain data. Experiments have been undertaken on a Paterson triaxial apparatus, focusing on SiO2, both in its crystalline (quartz) and amorphous (fused quartz) forms. Results show that over time scales of less than one millisecond and displacements of tens of microns, significant changes occur in the structure of materials on fault interfaces. At high normal stresses and slip velocities less than ~ 0.05 ms-1, the crystalline structure of the quartz partially lost though the process of mechanical amorphization. At higher slip velocities > 0.05 ms-1, enough heat is generated to melt quartz. However, the onset of amorphization or melting does not necessarily drive fault weakening; the amorphous layer must reach temperatures sufficient to cross the kinematic threshold referred to as the 'glass transition' to allow strain to be accommodated through viscous shearing. When melted regions quench at the end of slip, they can weld the fault surfaces together, instantaneously increasing the cohesive strength and changing conditions necessary to reactivate the fault. The molecular structure of these melt-welded regions is shown to be altered to a densified form, resulting from both the rapid cooling rates and exposure to very high pressures. Synchronised data acquisition allows measurement of both the onset of fault rupture and sample shortening resulting from fault slip. Contrary to assumptions made in many previous studies, experimental fault slip differs from natural earthquake slip, with the former occurring only after the entire surface has ruptured. The synchronised sensors also detect the passage of elastic waves propagating from the rupture and reflecting withing the loading assembly. As the waves pass through the sample, they momentarily change the stress conditions on the fault, potentially enhancing slip or contributing to arrest. These results highlight how technological advances have given us a new understanding of the link between mechanical and microstructural processes, while underscoring the underlying link between sample and apparatus behaviour. In the future, these developments will allow us to explore the fundamental physics of fault slip, which in turn, can be applied to macroscopic understanding of the earthquake cycle and the evolution of fault rupture

The strength and mechanical behaviour of quartz slip interfaces: an experimental investigation

An experimental study has been undertaken to explore the strength, mechanical behaviour and microstructural evolution of bare interfaces in quartz sandstone during slip. These experiments were designed to simulate fault processes with increasing depth in the continental crust. Two main aspects have been explored: (1) the effect of temperature and confining pressure on the behaviour and stability of favourably-oriented faults, and (2) the influence of reactivation angle on the mechanical behaviour and associated microstructural evolution of a fault zone. Experiments were conducted on Fontainebleau sandstone using a triaxial deformation apparatus, at normal stresses comparable to that in the continental seismogenic regime and over small slip displacements. The first suite of experiments was conducted at temperatures of 400-927°C and confining pressures of 50-200MPa. Experiments reveal complex transitions in fault behaviour between stick-slip and stable sliding regimes. Mechanical results are coupled with microstructural analysis using multiple techniques (including high resolution FE-SEM, and FIB-TEM) that provide insights into fault surface processes down to the nano-scale. Significant findings include the identification of a partially amorphous layer formed during aseismic creep and the generation of pure-silica frictional melt (pseudotachylyte) during high temperature seismic slip events. The pseudotachylyte is recognisable by the formation drawn-out glass filaments and fractured glass patches on the fault surfaces, forming a discontinuous layer up to 2µm thick and covering 10-60% of the fault surface. At normal stresses > 200MPa, frictional melt develops within the first 50µm of rapid slip, correlating with changes in slip acceleration and velocity. High temperature hydrothermal treatment of melt-covered fault surfaces indicates that the pseudotachylyte has a short lifespan (<1 hour) in the presence of high temperature, reactive fluids. The second suite of experiments explores reactivation of fault surfaces inclined between 25º and 70º to the maximum shortening direction, representing faults that vary from optimally-oriented to severely-misoriented for failure. These faults have been reactivated in both dry and fluid-saturated conditions, using two different loading mechanisms. ‘Stress-driven failure’ involves increasing the axial load at constant rate until failure, whereas ‘fluid-driven failure’ is achieved by maintaining a constant axial load and increasing pore fluid pressure until slip occurs. While the initial reactivation of faults obeys frictional theory, continued reactivation is strongly influenced by the microstructural evolution of the fault surface, most notably through the development of frictional melt. Rapid-slip events form a locally-continuous layer of frictional melt in both the dry and water-saturated samples. The presence of pseudotachylyte increases fault cohesive strength through a process termed ‘melt-welding’. Melt-welded regions serve as a nucleation point for the development of off-fault damage and on the most unfavourably-oriented faults, cause lock-up and the failure of a new, more favourably-oriented fault. This work provides new insights into the behaviour and microstructural development of fault surfaces during the early stages of seismic instability. These results have implications for the interpretation of slip processes in natural fault zones, and also more generally for understanding slip mechanics, weakening distances and coseismic fault strength within the continental seismogenic regime

Education vs TFP: Empirical evidence from the Sub-Saharan Countries

This single-case, mixed-method study explored the feasibility of self-administered, home-based SMART (sensorimotor active rehabilitation training) Arm training for a 57-yr-old man with severe upper-limb disability after a right frontoparietal hemorrhagic stroke 9 mo earlier. Over 4 wk of self-administered, home-based SMART Arm training, the participant completed 2,100 repetitions unassisted. His wife provided support for equipment set-up and training progressions. Clinically meaningful improvements in arm impairment (strength), activity (arm and hand tasks), and participation (use of arm in everyday tasks) occurred after training (at 4 wk) and at follow-up (at 16 wk). Areas for refinement of SMART Arm training derived from thematic analysis of the participant's and researchers' journals focused on enabling independence, ensuring home and user friendliness, maintaining the motivation to persevere, progressing toward everyday tasks, and integrating practice into daily routine. These findings suggest that further investigation of self-administered, home-based SMART Arm training is warranted for people with stroke who have severe upper-limb disability

Melt Welding and Its Role in Fault Reactivation and Localization of Fracture Damage in Seismically Active Faults

Low displacement fracture damage plays an important role in influencing the behavior and mechanical evolution of faults. Fracture damage zones influence slip behavior through changing near-field stress orientations, altering fluid pathways and modifying fault structure. Here we use small displacement triaxial experiments to explore the development of fault zone damage, frictional lock-up, and the generation of new faults using samples with preground faults, oriented in 5° increments between 25° and 65° relative to the shortening direction. With increasing reactivation angle, faults support higher peak normal stresses (104–845 MPa) and behavior transitions from stable sliding to stick slip. Frictional melting occurs on surfaces where stick slip is initiated, forming micron-thick layers that locally weld asperity contacts. The extent of melt welding is correlated with normal stress and melt-welded zones increase fault cohesion. Distribution of fracture damage adjacent to the fault is spatially correlated with melt-welded zones and the corresponding concentrations of stress and elastic strain. In a process referred to as “adhesive wear,” fractures bypassing welded zones transfer melt-adhered material from one side of the fault to the other, forming new geometric asperities. On faults with high reactivation angles (55°–60°) the increase in cohesive strength resulting from melt-welded contacts drives fault lock-up after an initial slip event; subsequent slip localizes on new, favorably oriented faults. Given their size, melt-welded zones are likely to be short-lived in nature but may play a significant and previously unrecognized role in the development of fault-related damage.This study was supported by Australian Research Council grant DP130102687 and an Australian National University Major Equipment Grant (16MEC12). K. Hayward gratefully acknowledges scholarship support from the Australian Government Research Training Program

Screening large-scale association study data: exploiting interactions using random forests

BACKGROUND: Genome-wide association studies for complex diseases will produce genotypes on hundreds of thousands of single nucleotide polymorphisms (SNPs). A logical first approach to dealing with massive numbers of SNPs is to use some test to screen the SNPs, retaining only those that meet some criterion for futher study. For example, SNPs can be ranked by p-value, and those with the lowest p-values retained. When SNPs have large interaction effects but small marginal effects in a population, they are unlikely to be retained when univariate tests are used for screening. However, model-based screens that pre-specify interactions are impractical for data sets with thousands of SNPs. Random forest analysis is an alternative method that produces a single measure of importance for each predictor variable that takes into account interactions among variables without requiring model specification. Interactions increase the importance for the individual interacting variables, making them more likely to be given high importance relative to other variables. We test the performance of random forests as a screening procedure to identify small numbers of risk-associated SNPs from among large numbers of unassociated SNPs using complex disease models with up to 32 loci, incorporating both genetic heterogeneity and multi-locus interaction. RESULTS: Keeping other factors constant, if risk SNPs interact, the random forest importance measure significantly outperforms the Fisher Exact test as a screening tool. As the number of interacting SNPs increases, the improvement in performance of random forest analysis relative to Fisher Exact test for screening also increases. Random forests perform similarly to the univariate Fisher Exact test as a screening tool when SNPs in the analysis do not interact. CONCLUSIONS: In the context of large-scale genetic association studies where unknown interactions exist among true risk-associated SNPs or SNPs and environmental covariates, screening SNPs using random forest analyses can significantly reduce the number of SNPs that need to be retained for further study compared to standard univariate screening methods

Perseverance with home-based upper limb practice after stroke: perspectives of stroke survivors and their significant others

Purpose: The aim of this study was to explore factors that influence stroke survivors’ ability to persevere with home-based upper limb practice. Methods: A qualitative descriptive study embedded within a theoretical framework was conducted. Data were collected through semi-structured focus group, dyadic, and individual interviews. The Theoretical Domains Framework and Capability, Opportunity, Motivation – Behaviour (COM-B) model guided data collection and directed content analysis. Findings: Participants were 31 adult stroke survivors with upper limb impairment, with 13 significant other/s, who were living at home in Queensland, Australia. Three central tenants aligned with the COM-B and six themes were identified. Stroke survivors’ capability to persevere was influenced by being physically able to practice and being able to understand, monitor and modify practice, their opportunity to persevere was influenced by accessing therapy and equipment required for practice and fitting practice into everyday life, and their motivation to persevere was influenced by having goals and experiencing meaningful outcomes and having support and being accountable. Conclusion: Persevering with practice is multifaceted for stroke survivors. All facets need to be addressed in the design of strategies to enhance stroke survivors’ ability to persevere and in turn, enhance their potential for continued upper limb recovery

Perseverance with technology-facilitated home-based upper limb practice after stroke: a systematic mixed studies review

Background: Technology is being increasingly investigated as an option to allow stroke survivors to exploit their full potential for recovery by facilitating home-based upper limb practice. This review seeks to explore the factors that influence perseverance with technology-facilitated home-based upper limb practice after stroke. Methods: A systematic mixed studies review with sequential exploratory synthesis was undertaken. Studies investigating adult stroke survivors with upper limb disability undertaking technology-facilitated home-based upper limb practice administered ≥ 3 times/week over a period of ≥ 4 weeks were included. Qualitative outcomes were stroke survivors’ and family members’ perceptions of their experience utilising technology to facilitate home-based upper limb practice. Quantitative outcomes were adherence and dropouts, as surrogate measures of perseverance. The Mixed Methods Appraisal Tool was used to assess quality of included studies. Results: Forty-two studies were included. Six studies were qualitative and of high quality; 28 studies were quantitative and eight were mixed methods studies, all moderate to low quality. A conceptual framework of perseverance with three stages was formed: (1) getting in the game; (2) sticking with it, and; (3) continuing or moving on. Conditions perceived to influence perseverance, and factors mediating these conditions were identified at each stage. Adherence with prescribed dose ranged from 13 to 140%. Participants were found to be less likely to adhere when prescribed sessions were more frequent (6–7 days/week) or of longer duration (≥ 12 weeks). Conclusion: From the mixed methods findings, we propose a framework for perseverance with technology-facilitated home-based upper limb practice. The framework offers opportunities for clinicians and researchers to design strategies targeting factors that influence perseverance with practice, in both the clinical prescription of practice and technology design. To confirm the clinical utility of this framework, further research is required to explore perseverance and the factors influencing perseverance

Increasing access to CBT for psychosis patients: a feasibility, randomised controlled trial evaluating brief, targeted CBT for distressing voices delivered by assistant psychologists (GiVE2)

Background: The National Institute for Health and Care Excellence (NICE) recommends that Cognitive Behaviour Therapy for psychosis (CBTp) is offered to all patients with a psychosis diagnosis. However, only a minority of psychosis patients in England and Wales are offered CBTp. This is attributable, in part, to the resource-intensive nature of CBTp. One response to this problem has been the development of CBTp in brief formats that are targeted at a single symptom and the mechanisms that maintain distress. We have developed a brief form of CBTp for distressing voices and reported preliminary evidence for its effectiveness when delivered by highly trained therapists (clinical psychologists). This study will investigate the delivery of this intervention by a cost-effective workforce of assistant psychologists following a brief training and evaluate the acceptability and feasibility of conducting a future, definitive, randomised controlled trial (RCT). Methods: This is a feasibility study for a pragmatic, three-arm, parallel-group, superiority 1:1:1 RCT comparing a Guided self-help CBT intervention for voices and treatment as usual (GiVE) to Supportive Counselling and treatment as usual (SC) to treatment as usual alone (TAU), recruiting across two sites, with blinded post-treatment and follow-up assessments. A process evaluation will quantitatively and qualitatively explore stakeholder experience. Discussion: Expected outcomes will include an assessment of the feasibility of conducting a definitive RCT, and data to inform the calculation of its sample size. If evidence from a subsequent, fully powered RCT suggests that GiVE is clinically and cost-effective when delivered by briefly trained assistant psychologists, CBTp offered in these less resource-intensive forms has the potential to generate benefits for individual patients (reduced distress, enhanced recovery and enhanced quality of life), service-level patient benefit (increased access to evidence-based psychological therapies) and economic benefits to the NHS (in terms of the reduced use of mental health inpatient services). Trial registration: Current Controlled Trials, ISRCTN registration number: 16166070. Registered on 5 February 2019

SMART arm with outcome-triggered electrical stimulation: a pilot randomized clinical trial

Background: The SMART (SensoriMotor Active Rehabilitation Training) Arm is a nonrobotic device designed to allow stroke survivors with severe paresis to practice reaching. It can be used with or without outcome-triggered electrical stimulation (OT-stim) to augment movement. The aim of this study was to evaluate the efficacy of SMART Arm training when used with or without OT-stim, in addition to usual care, as compared with usual care alone during inpatient rehabilitation. Methods: Eight stroke survivors received 20 hours of SMART Arm training over 4 weeks; they were randomly assigned to either (1) SMART Arm training with OT-stim or (2) SMART Arm training alone. Usual therapy was also provided. A historical cohort of 20 stroke survivors formed the control group and received only usual therapy. The primary outcome was Motor Assessment Scale Item 6, Upper Arm Function. Results: Findings for all participants were comparable at baseline. SMART Arm training, with or without OT-stim, led to a significantly greater improvement in upper arm function than usual therapy alone (P=.024). There was no difference in improvement between training with or without OT-stim. Initial motor severity and presence of OT-stim influenced the number of repetitions performed and the progression of SMART Arm training practice conditions. Conclusion: Usual therapy in combination with SMART Arm training, with or without OT-stim, appears to be more effective than usual therapy alone for stroke survivors with severe paresis. These findings warrant further investigation into the benefits of SMART Arm training for stroke survivors with severe paresis undergoing inpatient rehabilitation during the subacute phase of recovery

Identification of influential probe types in epigenetic predictions of human traits: implications for microarray design

BACKGROUND: CpG methylation levels can help to explain inter-individual differences in phenotypic traits. Few studies have explored whether identifying probe subsets based on their biological and statistical properties can maximise predictions whilst minimising array content. Variance component analyses and penalised regression (epigenetic predictors) were used to test the influence of (i) the number of probes considered, (ii) mean probe variability and (iii) methylation QTL status on the variance captured in eighteen traits by blood DNA methylation. Training and test samples comprised ≤ 4450 and ≤ 2578 unrelated individuals from Generation Scotland, respectively. RESULTS: As the number of probes under consideration decreased, so too did the estimates from variance components and prediction analyses. Methylation QTL status and mean probe variability did not influence variance components. However, relative effect sizes were 15% larger for epigenetic predictors based on probes with known or reported methylation QTLs compared to probes without reported methylation QTLs. Relative effect sizes were 45% larger for predictors based on probes with mean Beta-values between 10 and 90% compared to those based on hypo- or hypermethylated probes (Beta-value ≤ 10% or ≥ 90%). CONCLUSIONS: Arrays with fewer probes could reduce costs, leading to increased sample sizes for analyses. Our results show that reducing array content can restrict prediction metrics and careful attention must be given to the biological and distribution properties of CpG probes in array content selection. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13148-022-01320-9