arXiv subjects

Nicolas Brantut

Publications and source records attributed to Nicolas Brantut.

25 records · Page 2Linked to original sources

arXiv2019

Stability of pulse-like earthquake ruptures

Pulse-like ruptures arise spontaneously in many elastodynamic rupture simulations and seem to be the dominant rupture mode along crustal faults. Pulse-like ruptures propagating under steady-state conditions can be efficiently analysed theoretically, but it remains unclear how they can arise and how they evolve if perturbed. Using thermal pressurisation as a representative constitutive law, we conduct elastodynamic simulations of pulse-like ruptures and determine the spatio-temporal evolution of slip, slip rate and pulse width perturbations induced by infinitesimal perturbations in background stress. These simulations indicate that steady-state pulses driven by thermal pressurisation are unstable. If the initial stress perturbation is negative, ruptures stop; conversely, if the perturbation is positive, ruptures grow and transition to either self-similar pulses (at low background stress) or expanding cracks (at elevated background stress). Based on a dynamic dislocation model, we develop an elastodynamic equation of motion for slip pulses, and demonstrate that steady-state slip pulses are unstable if their accrued slip $b$ is a decreasing function of the uniform background stress $τ_\mathrm{b}$. This condition is satisfied by slip pulses driven by thermal pressurisation. The equation of motion also predicts quantitatively the growth rate of perturbations, and provides a generic tool to analyse the propagation of slip pulses. The unstable character of steady-state slip pulses implies that this rupture mode is a key one determining the minimum stress conditions for sustainable ruptures along faults, i.e., their ``strength''. Furthermore, slip pulse instabilities can produce a remarkable complexity of rupture dynamics, even under uniform background stress conditions and material properties.

physics.geo-ph↗

arXiv2019

Micromechanics of high pressure compaction in granular quartz aggregates

The mechanical behaviour of porous sandstones is generally modelled using concepts from granular mechanics, often overlooking the effect of cementation. To probe the key differences between sand and sandstone mechanics, we performed triaxial deformation experiments on Ottawa quartz sand at $5-40$ MPa effective confining pressure. At $5$ MPa, the samples are able to dilate. At higher confinement, the aggregates show continuous compaction, displaying strain hardening. The stress-strain behaviour is non-linear and the exact onset of inelastic compaction could not be determined accurately. Measured P-wave velocities show the development of anisotropy. With increasing axial strain, the along-axis velocities tend to increase, while velocities perpendicular to the compression axis tend to decrease (at low pressure) or remain constant (at high pressure). In samples deformed under elevated pressure conditions, acoustic emission event locations are diffuse. Microstructural investigations show an increase in grain chipping and crushing with increasing confining pressure, but no evidence of localised compaction could be observed. The nature of the pore-fluid, either decane or water, does not significantly influence the mechanical behaviour at strain rates of $10^{-6}$ to $10^{-4}$ $s^{-1}$. Grain angularity and grain-size distribution also did not significantly change the mechanical behaviour. We infer that our observations indicate that the lack of cementation introduces additional degrees of freedom for grains to slide, rotate, and reorganise at the sample-scale, precluding the existence and sustainability of stress concentrations beyond the grain-scale. This results in progressive compaction and hardening, and lack of compaction localisation.

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↗

arXiv2019

Influence of fluids on $V_\mathrm{P}/V_\mathrm{S}$ ratio: Increase or decrease?

The evolution of $V_\mathrm{P}/V_\mathrm{S}$ with increasing fluid-saturated porosity is computed for isotropic rocks containing spheroidal pores. $V_\mathrm{P}/V_\mathrm{S}$ is shown to either decrease or increase with increasing porosity, depending on the aspect ratio $α$ of the pores, fluid to solid bulk modulus ratio $ζ$, and initial Poisson's ratio $ν_0$ of the solid. A critical initial Poisson's ratio $ν_\mathrm{0,crit}$ is computed, separating cases where $V_\mathrm{P}/V_\mathrm{S}$ increases (if $ν_0<ν_\mathrm{0,crit}$) or \emph{decreases} (if $ν_0>ν_\mathrm{0,crit}$) with increasing porosity. For thin cracks and highly compressible fluids, $ν_\mathrm{0,crit}$ is approximated by $0.157\,ζ/α$, whereas for spherical pores $ν_\mathrm{0,crit}$ is given by $0.2 + 0.8ζ$. If $ν_0$ is close to $ν_\mathrm{0,crit}$, the evolution of $V_\mathrm{P}/V_\mathrm{S}$ with increasing fluid-saturated porosity is near neutral and depends on subtle changes in pore shape and fluid properties. This regime is found to be relevant to partially dehydrated serpentinites in subduction zone conditions (porosity of aspect ratio near 0.1 and $ζ$ in the range 0.01--0.1), and makes detection of these rocks and possibly elevated fluid pressures difficult from $V_\mathrm{P}/V_\mathrm{S}$ only.

physics.geo-ph↗

arXiv2018

Low-Frequency Measurements of Seismic Velocity and Attenuation in Antigorite Serpentinite

Laboratory measurements of seismic velocity and attenuation in antigorite serpentinite at a confining pressure of $2$ kbar and temperatures up to $550^\circ$C (i.e., in the antigorite stability field) provide new results relevant to the interpretation of geophysical data in subduction zones. A polycrystalline antigorite specimen was tested via forced-oscillations at small strain amplitudes and seismic frequencies (mHZ--Hz). The shear modulus has a temperature sensitivity, $\partial G/ \partial T$, averaging $-0.017$ GPa K$^{-1}$. Increasing temperature above $500^\circ$C results in more intensive shear attenuation ($Q_G^{-1}$) and associated modulus dispersion, with $Q_G^{-1}$ increasing monotonically with increasing oscillation period and temperature. This "background" relaxation is adequately captured by a Burgers model for viscoelasticity and possibly results from intergranular mechanisms. Attenuation is higher in antigorite ($\log_{10} Q_G^{-1} \approx -1.5$ at $550^\circ$C and $0.01$ Hz than in olivine ($\log_{10} Q_G^{-1} \ll -2.0$ below $800^\circ$C), but such contrast does not appear to be strong enough to allow robust identification of antigorite from seismic models of attenuation only.

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↗