arXiv subjects

Thomas M. Mitchell

Publications and source records attributed to Thomas M. Mitchell.

9 recordsLinked to original sources

arXiv2026

The heterogeneous near-surface velocity structure of a carbonate-hosted seismogenic fault zone and its dependence on the investigated length scale

Field geological studies highlighted the heterogeneous structure of fault zones from the meter- to millimeter scale, but such internal variability is not generally resolved by seismological techniques due to spatial resolution limits. The near-surface velocity structure of the Vado di Corno seismogenic fault zone was quantified at different length scales, from laboratory measurements of ultrasonic velocities (few centimeters rock samples, 1 MHz source) to high-resolution first-arrival seismic tomography (spatial resolution to a few meters). The fault zone juxtaposed structural units with contrasting ultrasonic velocities. The fault core cataclastic units were slower compared to damage zone units. A negative correlation between ultrasonic velocity and porosity was observed, with dispersion in fault core units related to varying degree of textural maturity and pore space sealing by calcite. Low-velocity outliers in the damage zone were instead linked to microfracture networks with local cataclasis and partial calcite sealing. P-wave high-resolution seismic tomography imaged distinct fault-bounded rock bodies, matching the geometry and size of field-mapped structural units. At this length scale, relatively fast fault core units and low-strain damage zones contrasted with a very slow intensely fractured high-strain damage zone. The discrepancy between higher ultrasonic velocities and lower tomography-derived ones was reconciled through an effective medium approach considering the effect of meso-scale fractures in each unit. This revealed a heterogeneous fault zone velocity structure with different scaling among structural units. Lastly, the persistence of a thick compliant high-strain damage zone at shallow depth may significantly affect fault zone mechanics and the distribution of near-surface deformations.

physics.geo-ph↗

arXiv2025

A new True Triaxial Apparatus with pore fluid system for rock deformation under representative crustal stress conditions

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 subjected to up to 60 MPa of confining pressure, and pore fluids line connected to two pump intensifiers allow for highly accurate permeability measurements along the three loading axes. We present a suite of Finite Element Method (FEM) models implemented to determine the conditions and loading configuration that minimise the loading boundary effects during true triaxial loading. These observations are generic and we expect they will contribute to the development of true triaxial loading systems generally. Finally, we validate our experimental configuration by presenting results on permeability measurements along the three axes on cubic samples of three types of well studied rocks: Darley Dale sandstone, Crab Orchard sandstone, and Etna basalt.

physics.geo-ph↗

arXiv2023

Uniaxial compression of 3D printed samples with voids: laboratory measurements compared with predictions from Effective Medium Theory

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 twenty types of 3D-printed samples containing voids of different geometries, we examine whether the measured effective elasticities are accurately predicted by EMT. To manufacture the sample, we selected printers that use different technologies; fused deposition modelling (FDM), and stereolithography (SLA). We show how printer settings (FDM case) or sample cure time (SLA case) affect the measured properties. We also examine the reproducibility of elasticity tests on identically designed samples. To obtain the range of theoretical predictions, we assume either uniform strain or uniform stress. Our study of over two hundred samples shows that measured effective elastic moduli can fit EMT predictions with an error of less than 5% using both FDM and SLA methods if certain printing specifications and sample design considerations are taken into account. Notably, we find that the pore volume fraction of the designed samples should be above 1% to induce a measurable softening effect, but below 5% to produce accurate EMT estimations that fit the measured elastic properties of the samples. Our results highlight both the strengths of EMT for predicting the effective properties of solids with low pore fraction volume microstructural configurations, and the limitations for high porosity microstructures.

physics.geo-ph↗

arXiv2023

Multi-porous extension of anisotropic poroelasticity: consolidation and related coefficients

We propose the generalisation 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 porosity clusters (sets of pores or fractures) that are characterized by various fluid pressures, which is the original poroelastic extension of Aifantis. In consequence, both instantaneous and time-dependent deformation lead to fluid content variations that are different in each cluster. We present the equations for such phenomena, where the anisotropic properties of both the solid matrix and pore sets are assumed. Novel poroelastic coefficients that relate solid and fluid phases in our extension are proposed, and their physical meaning is determined. To demonstrate the utility of our equations and emphasize the meaning of new coefficients, we perform numerical simulations of a triple-porosity consolidation. These simulations reveal positive pore pressure transients in the drained behaviour of weakly connected pore sets, and these may result in mechanical weakening of the material.

physics.geo-ph↗

arXiv2023

Multi-porous extension of anisotropic poroelasticity: linkage with micromechanics

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 micromechanics, we do not limit ourselves to the original theory of Biot. Instead, we consider a multi-porous extension of anisotropic poroelasticity, where pore fluid pressure may vary within the bulk medium of interest. As a consequence, any inhomogeneities in the material are not necessarily interconnected; instead, they may form isolated pore sets that are described by different poroelastic parameters and fluid pressures. We attempt to incorporate the effective methods inside Biot-like theory and investigate the poroelastic response of various microstructures. We show the cases where such implementation is valid and the others that appear to be questionable. During micromechanical analysis, we derive a particular case of cylindrical transverse isotropy -- commonly assumed in conventional laboratory triaxial tests -- where the symmetry is induced by sets of aligned cracks.

physics.geo-ph↗

arXiv2020

Off-fault damage characterisation during and after experimental quasi-static and dynamic rupture in crustal rock from laboratory P-wave tomography and microstructures

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, $Γ_\mathrm{off}$. Quantifying off-fault damage formed during rupture is crucial to understand its effect on rupture dynamics and slip-weakening processes behind the rupture tip, and its contribution to seismic radiation. Here, we quantify $Γ_\mathrm{off}$ and associated change in off-fault mechanical properties during and after quasi-static and dynamic rupture. We do so by performing dynamic and quasi-static shear failure experiments on intact Lanhélin granite under triaxial conditions. We quantify the change in elastic moduli around the fault from time-resolved 3D $P$-wave velocity tomography obtained during and after failure. We measure the off-fault microfracture damage after failure. From the tomography, we observe a localised maximum 25\% drop in $P$-wave velocity around the shear failure interface for both quasi-static and dynamic failure. Microfracture density data reveals a damage zone width of around 10 mm after quasi-static failure, and 20 mm after dynamic failure. Microfracture densities obtained from $P$-wave velocity tomography models using an effective medium approach are in good agreement with the measured off-fault microfracture damage. $Γ_\mathrm{off}$ obtained from off-fault microfracture measurements is around 3 kJm$^{2}$ for quasi-static rupture, and 5.5 kJm$^{2}$ for dynamic rupture. We argue that rupture velocity determines damage zone width for slip up to a few mm, and that shear fracture energy $Γ$ increases with increasing rupture velocity.

physics.geo-ph↗

arXiv2019

Absence of Stress-induced Anisotropy during Brittle Deformation in Antigorite Serpentinite

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 confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress-induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behaviour is elastic up to high stress ($\gtrsim80$\% of rock strength) and non-dilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at $90-95$\% of their compressive strength. Microcrack damage is extremely localised near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by "mode II" shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite is comparable to that in granite. The mechanical behaviour and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.

physics.geo-ph↗

arXiv2019

Assessing the efficiency of thermal pressurisation using natural pseudotachylyte-bearing rocks

The efficiency of thermal pressurisation 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 pressurisation by comparing predictions of temperature rise to field estimates based on pseudotachylyte-bearing rocks. We measure hydraulic and transport properties of a suite of fault rocks (a healed cataclasite, an unhealed breccia and the intact parent rock) from the pseudotachylyte-bearing Gole Larghe fault in the Adamello batholith (Italy), and use them as inputs in numerical simulations of thermal pressurisation. We find that the melting temperature can be reached only if damaged, unhealed rock properties are used. A tenfold increase in permeability, or a fourfold increase in pore compressibility of the intact rock is required to achieve melting. Our results emphasise the importance of damage processes that strongly modify fault rock properties and dynamic weakening processes during earthquake propagation.

physics.geo-ph↗

arXiv2018

Fault reactivation by fluid injection: Controls from stress state and injection rate

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 borehole leading to reactivation depends on injection rate. The higher the injection rate, the higher the peak fluid pressure allowing fault reactivation. Elastic wave velocity measurements along fault strike highlight that high injection rates induce significant fluid pressure heterogeneities, which explains that the onset of fault reactivation is not determined by a conventional Coulomb law and effective stress principle, but rather by a nonlocal rupture initiation criterion. Our results demonstrate that increasing the injection rate enhances the transition from drained to undrained conditions, where local but intense fluid pressures perturbations can reactivate large faults.

physics.geo-ph↗