We observe the nucleation phase of in‐plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi‐static and an acceleration stage, followed by dynamic ...propagation. We propose an empirical model which describes the rupture length evolution: The quasi‐static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi‐static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.
Key Points
Nucleation evolves in 2 phases, 1:exponantial, 2:inverse power function of time
Transition occurs at critical length and velocity that scale inversely to stress
A typical time is scaled to earthquakes to get the duration of their nucleation
In this paper, we investigate the morphology of the events from the GWTC-1 catalog of compact binary coalescences as reconstructed by a method based on coherent excess power: we use an open-source ...version of the coherent WaveBurst (cWB) analysis pipeline, which does not make use of waveform models. The coherent response of the LIGO-Virgo network of detectors is estimated by using loose bounds on the duration and bandwidth of the signal. This pipeline version reproduces the same results that are reported for cWB in recent publications by the LIGO and Virgo collaborations. In particular, the sky localization and waveform reconstruction are in a good agreement with those produced by methods which exploit the detailed theoretical knowledge of the expected waveform for compact binary coalescences. However, in some cases cWB also detects features in excess in well-localized regions of the time-frequency plane. Here we focus on such deviations and present the methods devised to assess their significance. Out of the 11 events reported in the GWTC-1, in two cases-GW151012 and GW151226-cWB detects an excess of coherent energy after the coalescence (Δt≃0.2 and ≃0.1 s, respectively) with p-values that call for further investigations (0.004 and 0.03, respectively), though they are not sufficient to exclude noise fluctuations. We discuss the morphological properties and plausible interpretations of these features. In case they are genuine, we anticipate that several more such outliers will be uncovered by our methodology in the ongoing advanced LIGO-Virgo observation run (O3).
We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under ...triaxial compression. During each test, the effective pressure was first increased up to 190
MPa to investigate the effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to 20
MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5–2
km thick, and deviatoric stress was increased until failure of the specimens.
Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack densities:
ρ
i
the crack density of the pre-existing thermal cracks and
ρ
v
the crack density of the stress-induced cracks. In addition a link was established between elastic properties (elastic wave velocities
V
p
and
V
s
) and permeability using a statistical permeability model.
Our results show that the velocities increase with increasing hydrostatic pressure up to 190
MPa, due to the closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density
ρ
i
from ~
1 to 0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases of more than two orders of magnitude.
Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack density, from the location of the acoustic emission sources, and from microstructural observations. This competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on permeability change than pre-existing thermal cracks.
► Increasing pressure induces closure of the pre-existing cracks. ► Using the velocity data, crack density if found to decrease from ~1 to 0.2. ► With increasing pressure, the permeability (K) decrease of ~2 orders of magnitude. ► During deviatoric loading, two populations of cracks should be taken into account. ► Mechanical-induced cracks has lesser influence on K than pre-exiting cracks.
Stick-slip dynamic instability is a key mechanism governing frictional processes from microscale physics to earthquake faults and landslides; yet challenging questions are still open about its ...nucleation and propagation dynamics. Here we present novel observations on laboratory experimental faults where spontaneously nucleating fractures are produced, describing (1) an initial quasistatic, stable rupture front (propagating at about 5 per cent the shear wave velocity VS), accelerating to subshear and then to intersonic velocity; (2) the arisal of a higher degree of complexity when the friction to prestress ratio is increased on the sliding surface. The complex behaviour includes stop and go sequences, irregular propagation and rerupturing episodes within short-time intervals, implying rapid restrengthening of the surface and the formation of self-healing pulses, reproducing experimentally for the first time a behaviour observed on seismic faults.
Time-dependent brittle deformation is a fundamental and pervasive process operating in the Earth's upper crust. Its characterization is a pre-requisite to understanding and unraveling the ...complexities of crustal evolution and dynamics. The preferential chemical interaction between pore fluids and strained atomic bonds at crack tips, a mechanism known as stress corrosion, allows rock to fail under a constant stress that is well below its short-term strength over an extended period of time; a process known as brittle creep. Here we present the first experimental measurements of brittle creep in a basic igneous rock (a basalt from Mt. Etna volcano) under triaxial stress conditions. Results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress (and can be equally well fit by a power law or an exponential law); with a 20% increase in stress producing close to three orders of magnitude increase in creep strain rate. Results from stress-stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure. We show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure, with the remainder interpreted as due to a reduction in stress corrosion reactions; the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips. Overall, our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure, regardless of the level of applied stress and the time taken to reach this point. The experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law, and compared with the observed deformation history at Mt. Etna volcano.
► Basalt undergoes time-dependent brittle creep. ► Creep strain rates and times-to-failure are dependent on the level of stress. ► Increasing effective pressure decreases creep strain rates. ► No strain rate dependence during tertiary creep implies a single process. ► Results display temporal characteristics comparable to seismic activity at Mt. Etna.
Fluid‐induced stress perturbations in the crust at seismogenic depths caused by sources such as tidal or seasonal loading may trigger earthquakes. We investigate the role of small periodic pore ...pressure (Pp) perturbation in rupture nucleation by performing laboratory triaxial creep experiments on Fontainebleau sandstone, saturated in water, under sinusoidal Pp variations. Results show that recorded acoustic emissions (AEs) correlate with Pp as the rock approaches failure. More interestingly, AEs occur significantly more when Pp is decreasing, that is, when strain rate is maximum with a progressive increase of Pp‐AEs correlation in time as the rock approaches failure. This suggests that the correlation of small stress perturbations and AEs not only depends on Pp amplitude but also on the criticality of the rock. Observations at the laboratory scale support field observations where tidal loading may have modulated seismic rates during the nucleation phase of the 2004 Sumatra‐Andaman and 2011 Tohoku‐Oki earthquakes.
Key Points
We investigate the role of periodic pore fluid pressure on AEs triggering during creep deformation experiments
AEs correlation with pore pressure oscillations increases as the rock approaches failure
AEs occur significantly more when pore pressure decreases, that is, when deformation is maximum
Volcanic edifices, such as Mt. Etna (Italy), are commonly subject to repeated cycles of stress over time due to the combination of magma emplacement from deep reservoirs to shallow depths and ...superimposed tectonic stresses. Such repeated stress cycles lead to anisotropic deformation and an increase in the level of crack damage within the rocks of the edifice and hence changes to their elastic moduli, which are a key parameter for reliable modelling of deformation sources. We therefore report results of changes in elastic moduli measured during increasing amplitude cyclic stressing experiments on dry and water-saturated samples of Etna basalt. In all experiments, the Young's modulus decreased by approximately 30% over the total sequence of loading cycles, and the Poisson's ratio increased by a factor of approximately 3
±
0.5. Microseismicity, in terms of acoustic emission (AE) output, was also recorded throughout each experiment. Our results demonstrate that AE output only re-commences during any loading cycle when the level of stress where AE ceased during the unloading portion of the previous cycle is exceeded; a manifestation of the Kaiser stress-memory effect. In cycles where no AE output was generated, we also observed no change in elastic moduli. This result holds for both mechanical and thermal stressing. Our results are interpreted in relation to measurements of volcano-tectonic seismicity and deformation at Mt. Etna volcano.
To improve our understanding of the complex coupling between circulating fluids and the development of crack damage, we performed flow‐through tests on samples of Etna basalt and Westerly granite ...that were cyclically loaded by deviatoric stresses. The basalt was naturally microfractured, while the relatively crack‐free Westerly granite was thermally pretreated to 500°C and 800°C to generate microcrack damage. Samples were repeatedly loaded and then unloaded under deviatoric stress paths and ultimately to failure. Permeability and water volume content were measured throughout the loading history together with the differential stress. Permeability decreases at low differential stresses and increases at intermediate differential stresses up to a steady value at failure. We use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density. We use crack models to represent the evolution of permeability as a function of loading state and are able to independently link it to the observed evolution of deformability, used as an independent measure of crack density.
Key Points
Identification of 3 regimes of permeability approaching failure
Identification of the role of cooling rates on crack geometry and connectivity
Prediction of the coupled permeability and microcrack network evolution
The elastic moduli of rock in areas susceptible to crack damage, such as within fault zones or volcanic edifices, can be subject to large modifications. Knowledge of how elastic moduli may vary in ...such situations is important for both the reliable modelling of volcano deformation and stability and for linear and non-linear elastic crack models for earthquake rupture. Furthermore, it has previously been shown that changes in elastic moduli can induce changes in the stress field surrounding faults. Here we report both uniaxial experimental measurements of changes in elastic moduli during increasing-amplitude cyclic stressing experiments on a range of different rock types (basalts, sandstones and granite) and the results of modelled stress modifications. The trend in elastic moduli evolution with increasing damage was remarkably similar for each rock type, with the exception of essentially crack-free intrusive basalt that exhibited very minor changes. In general, Young's modulus decreased by between 11 and 32 per cent and Poisson's ratio increased by between 72 and 600 per cent over the total sequence of loading cycles. These changes are attributed to an increasing level of anisotropic crack damage within the samples. Our results also show that acoustic emission (AE) output during any loading cycle only commenced when new crack damage was generated. This corresponded to the level of stress where AE ceased during the unloading portion of the previous cycle. Using the multilayer elastic model of Faulkner et al. we demonstrate that the damage-induced changes in elastic moduli also result in significant decreases in differential stress, increases in mean stress and rotation of the applied greatest principal stress relative to the orientation of the mechanical layering. The similar trend in the evolution of the elastic moduli of all the rocks tested suggests that stress modification in the damage zone of faults might take the same form, regardless of the lithology through which the fault runs. These observations are discussed in terms of their applicability to both fault zones and deformation at volcanoes.
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