Experimental and theoretical constraints on amino acid formation from PAHs in asteroidal settings

Laboratory astrophysics and astrochemistr

Microphysically based modelling of fault friction and earthquake rupture

Earthquakes are among the most destructive of natural hazards known, and lead to considerable loss of life and property world-wide every year. In spite of the large impact that earthquakes have on society, little is known about the mechanisms through which earthquakes nucleate and dynamic ruptures propagate. This hampers earthquake hazard assessments and forecasting attempts, which are at present largely based on statistical inferences rather than physical principles. Laboratory studies of fault friction may contribute to the accuracy of earthquake hazard assessments by detailed investigation of the mechanisms involved in fault rock deformation. However, the spatial- and temporal scales of a typical laboratory experiment are dwarfed by those of natural faults and earthquakes, and so upscaling of the laboratory results is required. This is often performed through a mathematical framework called rate-and-state friction. This framework is empirical in nature, and so predictions made through the rate-and-state friction framework must be interpreted with great care. In this thesis, alternative means of upscaling are explored that are based on physical principles and have a micro-mechanical origin, as opposed to being empirical. This is done by conducting laboratory friction experiments, and interpreting the results in terms of micro-scale processes that constitute deformation, such as frictional grain sliding (granular flow) and pressure solution creep. Constitutive relations describing these micro-scale processes are subsequently implemented into numerical models, and the model predictions are tested against laboratory results. Finally, the microphysically based numerical models are used to extrapolate the laboratory observations to natural scales and conditions, and to make predictions pertaining to the natural seismic cycle. The outcomes of this thesis demonstrate that microphysically based models can explain a wide variety of laboratory and natural observations, and are a suitable means for the extrapolation of laboratory results to nature. This offers new opportunities for future studies of earthquake hazard and risk by providing a physical basis for making long-term predictions

Time‐dependent compaction as a mechanism for regular stick‐slips

Owing to their destructive potential, earthquakes receive considerable attention from laboratory studies. In friction experiments, stick‐slips are studied as the laboratory equivalent of natural earthquakes, and numerous attempts have been made to simulate stick‐slips numerically using the Discrete Element Method (DEM). However, while laboratory stick‐slips commonly exhibit regular stress drops and recurrence times, stick‐slips generated in DEM simulations are highly irregular. This discrepancy highlights a gap in our understanding of stick‐slip mechanics, which propagates into our understanding of earthquakes. In this work, we show that regular stick‐slips emerge in DEM when time‐dependent compaction by pressure solution is considered. We further show that the stress drop and recurrence time of stick‐slips is directly controlled by the kinetics of pressure solution. Since compaction is known to operate in faults, this mechanism for frictional instabilities directly relates to natural seismicity

Time‐dependent compaction as a mechanism for regular stick‐slips

Owing to their destructive potential, earthquakes receive considerable attention from laboratory studies. In friction experiments, stick‐slips are studied as the laboratory equivalent of natural earthquakes, and numerous attempts have been made to simulate stick‐slips numerically using the Discrete Element Method (DEM). However, while laboratory stick‐slips commonly exhibit regular stress drops and recurrence times, stick‐slips generated in DEM simulations are highly irregular. This discrepancy highlights a gap in our understanding of stick‐slip mechanics, which propagates into our understanding of earthquakes. In this work, we show that regular stick‐slips emerge in DEM when time‐dependent compaction by pressure solution is considered. We further show that the stress drop and recurrence time of stick‐slips is directly controlled by the kinetics of pressure solution. Since compaction is known to operate in faults, this mechanism for frictional instabilities directly relates to natural seismicity

Investigating compaction by intergranular pressure solution using the Discrete Element Method

Intergranular pressure solution creep is an important deformation mechanism in the Earth's crust. The phenomenon has been frequently studied and several analytical models have been proposed that describe its constitutive behavior. These models require assumptions regarding the geometry of the aggregate and the grain size distribution in order to solve for the contact stresses, and often neglect shear tractions. Furthermore, analytical models tend to overestimate experimental compaction rates at low porosities, an observation for which the underlying mechanisms remain to be elucidated. Here we present a conceptually simple, 3D Discrete Element Method (DEM) approach for simulating intergranular pressure solution creep that explicitly models individual grains, relaxing many of the assumptions that are required by analytical models. The DEM model is validated against experiments by direct comparison of macroscopic sample compaction rates. Furthermore, the sensitivity of the overall DEM compaction rate to the grain size and applied stress is tested. The effects of the interparticle friction and of a distributed grain size on macroscopic strain rates are subsequently investigated. Overall, we find that the DEM model is capable of reproducing realistic compaction behavior, and that the strain rates produced by the model are in good agreement with uniaxial compaction experiments. Characteristic features, such as the dependence of the strain rate on grain size and applied stress, as predicted by analytical models, are also observed in the simulations. DEM results show that interparticle friction and a distributed grain size affect the compaction rates by less than half an order of magnitude

Shear localization in a mature mylonitic rock analogue during fast slip

Highly localized slip zones, seen within ductile shear zones developed in nature, such as pseudotachylite bands occurring within mylonites, are widely recognized as evidence for earthquake nucleation and/or propagation within the ductile regime. To understand brittle/frictional localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation, we performed large velocity step-change tests on a brine-saturated, 80:20 (wt.%) mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional-viscous flow. The direct effect a on shear strength that occurs in response to an instantaneous upward velocity-step is an important parameter in determining the potential for, and nature of, seismic rupture nucleation and propagation. We obtained reproducible results regarding low velocity mechanical behavior compared with previous work, but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution, at velocities above a critical velocity Vc (~20 μm/s). We found that once a ductile, mylonitic structure has developed in a shear zone, subsequent cataclastic deformation is consistently localized in a narrow zone. This switch to localized deformation is controlled by the imposed velocity, and becomes most apparent at velocities above Vc. In addition, the direct effect drops rapidly when the velocity exceeds Vc. This implies that slip can accelerate towards seismic velocities almost instantly and without much loss of energy, once Vc is exceeded. Obtaining a measure for Vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle-ductile transitional regime

A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip

Rate-and-state friction (RSF) is commonly used for the characterisation of laboratory friction experiments, such as velocity-step tests. However, the RSF framework provides little physical basis for the extrapolation of these results to the scales and conditions of natural fault systems, and so open questions remain regarding the applicability of the experimentally obtained RSF parameters for predicting seismic cycle transients. As an alternative to classical RSF, microphysics-based models offer means for interpreting laboratory and field observations, but are generally over-simplified with respect to heterogeneous natural systems. In order to bridge the temporal and spatial gap between the laboratory and nature, we have implemented existing microphysical model formulations into an earthquake cycle simulator. Through this numerical framework, we make a direct comparison between simulations exhibiting RSF-controlled fault rheology, and simulations in which the fault rheology is dictated by the microphysical model. Even though the input parameters for the RSF simulation are directly derived from the microphysical model, the microphysics-based simulations produce significantly smaller seismic event sizes than the RSF-based simulation, and suggest a more stable fault slip behaviour. Our results reveal fundamental limitations in using classical rate-and-state friction for the extrapolation of laboratory results. The microphysics-based approach offers a more complete framework in this respect, and may be used for a more detailed study of the seismic cycle in relation to material properties and fault zone pressure-temperature conditions

A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip

Rate-and-state friction (RSF) is commonly used for the characterisation of laboratory friction experiments, such as velocity-step tests. However, the RSF framework provides little physical basis for the extrapolation of these results to the scales and conditions of natural fault systems, and so open questions remain regarding the applicability of the experimentally obtained RSF parameters for predicting seismic cycle transients. As an alternative to classical RSF, microphysics-based models offer means for interpreting laboratory and field observations, but are generally over-simplified with respect to heterogeneous natural systems. In order to bridge the temporal and spatial gap between the laboratory and nature, we have implemented existing microphysical model formulations into an earthquake cycle simulator. Through this numerical framework, we make a direct comparison between simulations exhibiting RSF-controlled fault rheology, and simulations in which the fault rheology is dictated by the microphysical model. Even though the input parameters for the RSF simulation are directly derived from the microphysical model, the microphysics-based simulations produce significantly smaller seismic event sizes than the RSF-based simulation, and suggest a more stable fault slip behaviour. Our results reveal fundamental limitations in using classical rate-and-state friction for the extrapolation of laboratory results. The microphysics-based approach offers a more complete framework in this respect, and may be used for a more detailed study of the seismic cycle in relation to material properties and fault zone pressure-temperature conditions

In-situ nano-scale investigation of step retreat on fluoranthene crystal surfaces

Fluoranthene, a polycyclic aromatic hydrocarbon, has been detected on Earth as well as in asteroids and meteorites and may have played a role in the formation of life. Increasing the ionic strength of aqueous solutions has been observed to lower the fluoranthene solubility, but it is unclear how solution composition controls the release rate of fluoranthene to an aqueous solution. To elucidate this, we performed in situ atomic force microscopy experiments in which we characterized the sublimation and dissolution behavior of fluoranthene crystal surfaces. From this, we quantify the step retreat rate upon exposure to air, deionized water, and a 0.4 M NaCl or 0.1 M MgSO4 solution. Surface roughness is the main factor that determines the dissolution or sublimation rate. The results imply that during fluoranthene remediation or breakdown in meteorites and asteroids, ionic strength will be more important than chemical composition for controlling fluoranthene release into solution