Thomas Mitchell

• Doctor of Philosophy
• Professor of Earthquake Geology and Rock Physics at University College London

Fluids in the crust may build pressure through a variety of mechanisms. This buildup can activate nearby faults, which then serve as conduits for fluid flow and as valves for pressure release. The rapid pressure drop promotes mineral precipitation and fault sealing, allowing the cycle to restart. While evidence of this cyclical interplay between fa...

3D printing technology offers the possibility of producing synthetic samples with accurately defined microstructures. As indicated by effective medium theory (EMT), the shapes, orientations, and sizes of voids significantly affect the overall elastic response of a solid body. By performing uniaxial compression tests on 20 types of 3D‐printed sample...

We have developed a new True Triaxial Apparatus (TTA) for rock deformation consisting of six servo-controlled loading rams that transmit maximum stresses of 220 MPa in the two horizontal axes and 400 MPa in the vertical axis to 50 mm side cubic rock samples. The sample and loading platens are introduced in a steel vessel where rock specimens can be...

We attempt to formalise the relationship between the poroelasticity theory and the effective medium theory of micromechanics. The assumptions of these two approaches vary, but both can be linked by considering the undrained response of a material; and that is the main focus of the paper. To analyse the linkage between poroelasticity and micromechan...

We propose the generalization of the anisotropic poroelasticity theory. At a large scale, a medium is viewed as quasi‐static, which is the original assumption of Biot. At a smaller scale, we distinguish different sets of pores or fractures that are characterized by various fluid pressures, which is the original poroelastic extension of Aifantis. In...

We measured poroelastic properties of cracked granite under triaxial conditions, at elevated confining pressure and a range of differential stresses. Skempton's coefficients and undrained Young's modulus and Poisson's ratio were determined directly by recording in situ fluid pressure during rapid cycles of axial and radial stress. Drained propertie...

The South Massif and Taurus‐Littrow valley represent a unique area for understanding recent geological processes on the Moon. The presence of two recent overlapping landslide deposits, and boulder falls, suggests that repetitive instability has affected the north‐east facing slope of the South Massif. The presence of the young Lee‐Lincoln lobate sc...

Anisotropy is an important property that is widely present in crustal rocks. Efforts have been devoted to providing a constitutive model that can describe both inherent and stress-induced anisotropy in rock. Different from classic models, that are based on stress invariants or strain tensors, we propose here an anisotropic damage microplane model t...

The Light Mantle landslide is a hypermobile landslide on the Moon. Apollo 17 astronauts collected a core sample of the top 60 cm of the Light Mantle deposit, which is currently being analyzed as part of the NASA's Apollo Next Generation Sample Analysis program. The origin of its hypermobility remains undetermined, as the proposed mechanisms are dif...

We attempt to formalise the relationship between the poroelasticity theory and the effective medium theory of micromechanics. The assumptions of these two approaches vary, but both can be linked by considering the undrained response of a material; and that is the main focus of the paper. To analyse the linkage between poroelasticity and micromechan...

We propose the generalisation of the anisotropic poroelasticity theory. At a larger scale, a medium is viewed as quasi-static, which is the original assumption of Biot. At a smaller scale, we distinguish different porosity clusters (sets of pores or fractures) that are described by various fluid pressures, which is the original poroelastic extensio...

Time-dependent cracking and brittle creep of rock is fundamental to understanding the long-term evolution and dynamic failure in underground rock engineering. In the present paper, we present a systematic laboratory investigation into the control of single open macrofractures of differing orientations on time-dependent cracking and brittle creep in...

We conduct friction experiments under vacuum to investigate the viability of dynamic friction weakening in lunar analogue anorthosite-bearing gouges. Our results show that localised dynamic friction weakening does not occur in these gouges.

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability o...

Plain Language Summary This study quantifies the surface deformation created by the 1959 Hebgen Lake earthquake. To measure the displacement field we use optical image correlation (OIC). OIC compares the pixel locations in the pre‐earthquake and post‐earthquake image and calculates any displacement from the pixel‐shift. The images cover a large are...

A new set-up to measure acoustic wave velocities through deforming rock samples at high pressures (<1 GPa), temperatures (<1000$^\circ$C) and loads (<1.5 GPa) has been developed in a recently refurbished gas medium triaxial deformation apparatus. The conditions span a wide range of geological environments, and allow us to accurately measure stress...

Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environments where stresses may evolve both spatially and temporally, such as volcanoes or active fault zones, these rocks may experience not only cyclic loading and unloading, but also rotation and/or reorientation of stresses. In such situations, any resulting crack di...

Pulverisation of rocks occurs through dynamic processes associated with earthquake ruptures. At the well-known pulverised rock outcrops along the San Andreas Fault (SAF), the intensity of pulverisation was shown to be highly heterogeneous on the outcrop (m-10s of m) scale, although detailed documentation of this heterogeneity is still lacking. Here...

The emplacement mechanisms of long runout landslides across the Solar System and the formation mechanisms of longitudinal ridges associated with their deposits remain subjects of debate. The similarity of longitudinal ridges in martian long runout landslides and terrestrial landslides emplaced on ice suggests that an icy surface could explain both...

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformation...

The formation mechanism of longitudinal ridges in long runout landslides has been proposed to require ice and/or clay minerals, as low friction materials would allow the spreading of the deposit, causing the development of longitudinal ridges by tensile deformation of the slide. The necessity of ice in the formation of longitudinal ridges has been...

Fault zone structure is one of the controlling factors of earthquake nucleation and arrest, seismic sequence evolution (i.e., foreshock and aftershock), rupture speed and length, and ground motion and seismic radiation pattern. Here we describe the internal structure of the Bolfin Fault Zone (BFZ), a >40-km-long seismogenic, splay fault of the sini...

In the present paper, we investigate how water acts to weaken rock in two complementary ways: mechanically through the generalized effective stress principle, and chemically through time-dependent rock-fluid reactions that allow subcritical crack growth. These processes, together with capillary suction and stress corrosion, were incorporated into a...

How major crustal‐scale seismogenic faults nucleate and evolve in crystalline basements represents a long‐standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio‐temporal evolution of the Bolfin Fault Zone (BFZ), a >40‐km‐long exhumed seismogenic splay fault of the 1000‐km‐long strike‐slip Ataca...

Tectonic pseudotachylytes are thought to be unique to certain water–deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid–rich faults hosted in the continental crust. Pseudotachylytes were found...

The Gonghe Basin on the Qinghai-Tibet Plateau has a cold, arid climate and has suffered severe land degradation. Climate change as well as anthropogenic activities including overgrazing have resulted in widespread blowout development and the formation of some of Earth’s largest blowouts. The blowouts are part of an aeolian dominated landscape that...

Shale gas exploitation has been the game-changer in energy development of the past decade. However, the existing methods of estimating gas in place in deep formations sufer from large uncertainties. Here, we demonstrate, by using novel high-pressure experimental techniques, that the gas in place within deep shale gas reservoirs can be up to five ti...

The Husmuli zone of the SW-Iceland Hellisheidi geothermal field is currently being used for re-injection of geothermal fluids and geothermal CO2 for its permanent storage in the form of carbonate minerals. A fully coupled hydro-thermo-mechanical numerical model was employed to investigate the coupled impacts of these complex processes on the calibr...

Seismological observations indicate strong variations in wave velocities around faults both co‐seismically during earthquakes, and post‐seismically. Recovery is commonly associated with a reduction in crack damage. Here, we explore the recovery associated with time‐dependent mechanical closure of cracks. We report results from laboratory experiment...

The transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where...

How major crustal-scale seismogenic faults nucleate and evolve in the crystalline basement represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip At...

Important fault zone processes can be discerned from the characterization of fracture damage and chemical transformations associated with active seismic sources. To characterize the 2010 M7.2 El Mayor-Cucapah rupture zone, continuous samples of fault core and 23 samples of damaged rock were collected perpendicular to strike of the Borrego fault. Sa...

The physics of water imbibition into initially unsaturated sandstone is critical to the understanding of displacement processes and fluid transport in the vadose zone. The distribution of water within rock is important due to its significant influence on rock mechanical behavior. Here, therefore, we used nuclear magnetic resonance (NMR) technology...

Understanding how major crustal-scale seismogenic faults nucleate and evolve in crystalline basement from the viscous to the brittle realm represents a long-standing, but still poorly constrained, target in structural geology and fault mechanics. Here, we addressed the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long seismoge...

In natural friction melts, or pseudotachylites, clast textures and glass compositions can influence the frictional behavior of faults hosting pseudotachylites, and are, in turn, sensitive to the processes involved in pseudotachylite formation. Quantification of these parameters in situations where the host rock composition and formation conditions...

Rock heterogeneities often control microcracking on the grain scale and therefore the mechanical behavior of the rock on the macroscale. We therefore studied the fracture patterns, mechanical behavior and strength of granular rocks using the three-dimensional numerical modeling code 3DEC. The numerical rock specimens, built using software package N...

In active volcanic arcs such as the Andean volcanic mountain belt, magmatically sourced fluids are channeled through the brittle crust by faults and fracture networks. In the Andes, volcanoes, geothermal springs, and major mineral deposits have a spatial and genetic relationship with NNE trending, margin‐parallel faults and margin‐oblique, NW trend...

Efforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipit...

The presence of pressurized fluids influences the mechanical behavior of faults. To test the roles of normal stress and fluid pressure on shear strength and localization behavior of calcite gouges, we conducted a series of rotary‐shear experiments with pore fluid pressures up to 10.5 MPa and difference between normal stress and fluid pressure up to...

Plain Language Summary We present results from a series of laboratory thermal stressing experiments using three igneous rocks of different composition, grain size, and origin: a Granophyre (SGP) from the Slaufrudalur pluton in Iceland, an Andesite from Santorini, Greece (SA), and a Basalt from the Seljadalur region of Iceland (SB), in which acousti...

Main points: • The majority of thermal crack damage is produced during heating in the coarse-grained granophyre, but during cooling in the finer-grained andesite and basalt. • A temperature-memory effect is exhibited during thermal cycling in the quartz-rich granophyre but not in the quartz-poor andesite or basalt. • Interpretation of pre-failure s...

Earthquake instability occurs as a result of strength loss during sliding on a fault. It has been known for over 50 years that fault compaction or dilatancy may cause significant weakening or strengthening by dramatically changing the fluid pressure trapped in faults. Despite this fundamental importance, we have no real understanding of the exact c...

Elastic strain energy released during shear failure in rock is partially spent as fracture energy Γ to propagate the rupture further. Γ is dissipated within the rupture tip process zone, and includes energy dissipated as off‐fault damage, Γoff. Quantifying off‐fault damage formed during rupture is crucial to understand its effect on rupture dynamic...

We extend the isotropic nonlinear damage rheology model with a scalar damage parameter to a more complex formulation that accounts for anisotropic damage growth under true triaxial loading. The model takes account of both the anisotropy of elastic properties (associated with textural rock structure) and the stress- and damage-induced anisotropy (as...

In this work, we apply digital rock physics (DRP) to characterize the pore networks of the Brae Formation sandstones from two different wells in the Miller field area (North Sea, UK). Using X-ray micro-CT scans, we calculate the porosity and permeability and generate pore network models to assess pore shape characteristics. The porous samples are m...

Elastic strain energy released during shear failure in rock is partially spent as fracture energy $\Gamma$ to propagate the rupture further. $\Gamma$ is dissipated within the rupture tip process zone, and includes energy dissipated as off-fault damage, $\Gamma_\mathrm{off}$. Quantifying off-fault damage formed during rupture is crucial to understan...

A three-dimensional discrete element grain-based stress corrosion model incorporating the theories of subcritical crack growth and chemical reaction rate was built to explore the time-dependent behavior of damage evolution and fracture patterns of brittle rocks on a mesoscopic scale. The model was first validated and the model accurately captured t...

High strain rate loading causes pervasive dynamic microfracturing in crystalline materials, with dynamic pulverization being the extreme end‐member. Hydraulic properties (permeability, porosity, and storage capacity) are primarily controlled by fracture damage and will therefore change significantly by intense dynamic fracturing—by how much is curr...

The complex process of tip-propagation and growth of natural faults remains poorly understood. We analyse field structural data of strike-slip faults from the Atacama Fault System using fracture mechanics theory to depict the mechanical controls of fault growth in crystalline rocks. We calculate the displacement-length relationship of faults develo...

The moment magnitude 7.2 El Mayor−Cucapah (EMC) earthquake of 2010 in northern Baja California, Mexico produced a cascading rupture that propagated through a geometrically diverse network of intersecting faults. These faults have been exhumed from depths of 6−10 km since the late Miocene based on low-temperature thermochronology, synkinematic alter...

Complex fracture damage around large faults is often simplified to fit exponential or power law decay in fracture density with distance from the fault. Noise in these datasets is attributed to large subsidiary faults or random natural variation. Through a field study of the Borrego Fault (Baja California) damage zone, combining mm-resolution struct...

The spatial resolution of 3D imaging techniques is often balanced by the achievable field of view. Since pore size in shales spans more than two orders of magnitude, a compromise between representativeness and accuracy of the 3D reconstructed shale microstructure is needed. In this study, we characterise the effect of imaging resolution on the micr...

Pseudotachylytes (i.e., solidified frictional melts) remain the only unambiguous signature of seismic slip in the rock record so far. However, pseudotachylytes are considered to be rare in the geological record and found discontinuously along faults. Here we describe, to our knowledge, the first pseudotachylytes ever found in South America. The ps...

In an active volcanic arc, magmatically sourced fluids are channeled through the brittle crust by structural features. This interaction is observed in the Andean volcanic mountain belt, where volcanoes, geothermal springs and the locations of major mineral deposits coincide with NNE-striking, convergent margin-parallel faults and margin-oblique, NW...

The presence of longitudinal ridges documented in long runout landslides across our solar system is commonly associated with the existence of a basal layer of ice. However, their development, the link between their occurrence and the emplacement mechanisms of long runout landslides, and the necessity of a basal ice layer remain poorly understood. H...

The so-called “brittle-ductile transition” is thought to be the strongest part of the lithosphere, and defines the lower limit of the seismogenic zone. It is characterized not only by a transition from localized to distributed (ductile) deformation, but also by a gradual change in microscale deformation mechanism, from microcracking to crystal plas...

The energy released during earthquake rupture is partly radiated as seismic waves and mostly dissipated by frictional heating on the fault interface and by off‐fault fracturing of surrounding host rock. Quantification of these individual components is crucial to understand the physics of rupture. We use a quasi‐static rock fracture experiment combi...

During the past decade, enhanced oil recovery (EOR) by CO2 in shale oils has received substantial attention. In shale oil reservoirs, CO2 diffusion into the resident oil has been considered as the dominant interaction between the CO2 in fractures and the oil in the matrices. CO2 diffusion will lead to oil swelling and improvement in oil viscosity....

The relationship between hydrothermal fluids and fault development is studied through a petrographic analysis of faulted veins in an open-pit copper mine, Radomiro Tomic (RT), northern Chile. Brittle deformation in RT was initiated with the formation of veins, disrupting low-strain crystal-plastic deformation. Following cooling, shear fractures pro...

We performed friction experiments using the rotary shear apparatus SHIVA (INGV, Rome) in order to determine the viability of thermally activated dynamic weakening processes in anorthosites to explain the exceptional runout of the Apollo 17 landslide. We performed a series of experiments in high vacuum conditions (1.6e-4 mbar) so to simulate the abs...

It is unknown how the formation and subsequent evolution of calderas are influenced by the physical properties of the faults that form calderas and their related basement faults. This is despite the fact that there are many hazardous caldera volcanoes around the world. Caldera fault zones have very different physical properties from that of their s...

The geometry of fault zones (e.g. fault surface roughness, fault rock distribution, network of secondary faults and fractures) is one of the main factors controlling earthquake nucleation, rupture speed and length of main shocks, foreshock and aftershock evolution, ground motion and seismic radiation pattern. Despite the pivotal role that fault geo...

Cracks and pores in crustal rocks are often fluid-saturated and subjected to stresses arising from both the overburden and regional tectonics. These stresses produce changes both in the shape and volume of the rock, and in the shape and volume of the voids i.e. the cracks and pores. Patterns of parallel fluid-filled cracks around major earthquake p...

Fluid transport within low porosity rocks in the Earths crust is primarily controlled by fracture permeability developed by brittle deformation processes. Field and numerical studies consistently show that the maximum per-meability correlates with the direction of greatest fracture connectivity, generally corresponding to the orientation at which f...

During the past decade, enhanced oil recovery (EOR) by CO2 in shale oils has received substantial attention. In shale oil reservoirs, CO2 diffusion into the resident oil has been considered to be the dominant interaction between the CO2 in fractures and the oil in the matrices. CO2 diffusion will lead to oil swelling and improvement in oil viscosit...

The evolution of crack damage in volcanic and geothermal systems is largely controlled by the mechanical and thermal stresses acting on them. It is therefore important to understand the response of volcanic rocks to thermo-mechanical loading. Here, we present results from a series of thermal stressing experiments in which acoustic emission (AE) was...

During earthquake propagation, geologic faults lose their strength, then strengthen as slip slows and stops. Many slip-weakening mechanisms are active in the upper-mid crust, but healing is not always well-explained. Here we show that the distinct structure and rate-dependent properties of amorphous nanopowder (not silica gel) formed by grinding of...

We studied the influence of stress state and fluid injection rate on the reactivation of faults. We conducted experiments on a saw cut Westerly granite sample under triaxial stress conditions. Fault reactivation was triggered by injecting fluids through a borehole directly connected to the fault. Our results show that the peak fluid pressure at the...

Knowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confin...

La Cordillera Principal del alto río Teno constituye un dominio transicional de la zona volcánica sur de los Andes Centrales de Chile, y corresponde a un área en la que se ha documentado una estrecha interacción entre volcanes, zonas de alteración hidrotermal y sistemas de falla durante el Cenozoico. En los valles del río Tinguiririca y río Teno, s...

Pseudotachylytes are generated by the cooling and solidification of frictional melt produced along a fault surface during seismic slip. Pseudotachylytes can, therefore, provide important constraints on thermal histories of faults during coseismic slip: survivor clast mineralogies and quenched crystallite morphologies have previously been used to co...

We studied the influence of stress state and fluid injection rate on the reactivation of faults. We conducted experiments on a saw-cut Westerly granite sample under triaxial stress conditions. Fault reactivation was triggered by injecting fluids through a borehole directly connected to the fault. Our results show that the peak fluid pressure at the...

The efficiency of thermal pressurization as a dynamic weakening mechanism relies on the thermal and hydraulic properties of the rocks forming the fault core. Here we assess the effectiveness of thermal pressurization by comparing predictions of temperature rise to field estimates based on pseudotachylyte-bearing rocks. We measure hydraulic and tran...