Nucleation of Laboratory Earthquakes: Quantitative Analysis and Scalings

In this study we use the precursory acoustic emission (AE) activity during the nucleation of stick-slip instability as a proxy to investigate foreshock occurrence prior to natural earthquakes. We report on three stick-slip experiments performed on cylindrical samples of Indian metagabbro under upper crustal stress conditions (30–60 MPa). AEs were continuously recorded by eight calibrated acoustic sensors during the experiments. Seismological parameters (moment magnitude, corner frequency and stress-drop) of the detected AEs (−8.8 ≤ Mw ≤ −7) follow the scaling law between moment magnitude and corner frequency that characterizes natural earthquakes. AE activity always increases toward failure and is driven by along fault slip velocity. The stacked AE foreshock sequences follow an inverse Omori type law, with a characteristic Omori time c inversely proportional to normal stress. AEs moment magnitudes increase toward failure, as manifested by a decrease in b-value from ∼1 to ∼0.5 at the end of the nucleation process. During nucleation, foreshocks migrate toward the mainshock epicenter location, and stabilize at a distance from the latter compatible with the predicted Rate-and-State nucleation size. Importantly, the nucleation characteristic timescale also scales inversely with applied normal stress and the expected nucleation size. Finally, we infer that foreshocks are the byproducts of the nucleation phase which is an almost fully aseismic process. Nevertheless, the seismic/aseismic energy release ratio continuously increases during nucleation, highlighting that, the nucleation process starts as a fully aseismic process, and evolves toward a cascading process at the onset of dynamic rupture

Fracture and damage localization in volcanic edifice rocks from El Hierro, Stromboli and Tenerife

© 2018 The Author(s). We present elastic wave velocity and strength data from a suite of three volcanic rocks taken from the volcanic edifices of El Hierro and Tenerife (Canary Islands, Spain), and Stromboli (Aeolian Islands, Italy). These rocks span a range of porosity and are taken from volcanoes that suffer from edifice instability. We measure elastic wave velocities at known incident angles to the generated through-going fault as a function of imposed strain, and examine the effect of the damage zone on P-wave velocity. Such data are important as field measurements of elastic wave tomography are key tools for understanding volcanic regions, yet hidden fractures are likely to have a significant effect on elastic wave velocity. We then use elastic wave velocity evolution to calculate concomitant crack density evolution which ranges from 0 to 0.17: highest values were correlated to the damage zone in rocks with the highest initial porosity

Acoustic-Friction Networks and the Evolution of Precursory Rupture Fronts in Laboratory Earthquakes

We show that the mesoscopic and transport characteristics of networks follow the same trends for the same type of the shear ruptures in terms of rupture speed while also comparing the results of three different friction experiments.The classified fronts obtained from a saw cut Westerly granite fault regarding friction network parameters show a clear separation into two groups indicating two different rupture fronts. With respect to the scaling of local ruptures durations with the networks parameters we show that the gap is related to the possibility of a separation between slow and regular fronts

Experimental characterisation of textile compaction response: A benchmark exercise

This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions. All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38% to 58%. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress. Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter

Mathematical models of supersonic and intersonic crack propagation in linear elastodynamics

This paper presents mathematical models of supersonic and intersonic crack propagation exhibiting Mach type of shock wave patterns that closely resemble the growing body of experimental and computational evidence reported in recent years. The models are developed in the form of weak discontinuous solutions of the equations of motion for isotropic linear elasticity in two dimensions. Instead of the classical second order elastodynamics equations in terms of the displacement field, equivalent first order equations in terms of the evolution of velocity and displacement gradient fields are used together with their associated jump conditions across solution discontinuities. The paper postulates supersonic and intersonic steady-state crack propagation solutions consisting of regions of constant deformation and velocity separated by pressure and shear shock waves converging at the crack tip and obtains the necessary requirements for their existence. It shows that such mathematical solutions exist for significant ranges of material properties both in plane stress and plane strain. Both mode I and mode II fracture configurations are considered. In line with the linear elasticity theory used, the solutions obtained satisfy exact energy conservation, which implies that strain energy in the unfractured material is converted in its entirety into kinetic energy as the crack propagates. This neglects dissipation phenomena both in the material and in the creation of the new crack surface. This leads to the conclusion that fast crack propagation beyond the classical limit of the Rayleigh wave speed is a phenomenon dominated by the transfer of strain energy into kinetic energy rather than by the transfer into surface energy, which is the basis of Griffiths theory

From Sub-Rayleigh to Supershear Ruptures During Stick-Slip Experiments on Crustal Rocks

International audienceSupershear earthquake ruptures propagate faster than the shear wave velocity. Although there is evidence that this occurs in nature, it has not been experimentally demonstrated with the use of crustal rocks. We performed stick-slip experiments with Westerly granite under controlled upper-crustal stress conditions. Supershear ruptures systematically occur when the normal stress exceeds 43 megapascals (MPa) with resulting stress drops on the order of 3 to 25 MPa, comparable to the stress drops inferred by seismology for crustal earthquakes. In our experiments, the sub-Rayleigh-to-supershear transition length is a few centimeters at most, suggesting that the rupture of asperities along a fault may propagate locally at supershear velocities. In turn, these sudden accelerations and decelerations could play an important role in the generation of high-frequency radiation and the overall rupture-energy budget

Paleocene of Menat Formation, France, reveals an extraordinary diversity of orthopterans and the last known survivor of a Mesozoic Elcanidae

The orthopteran fauna of the Paleocene of Menat Formation (France) is revised. It comprises at least 12 species in the following clades: Grylloidea (an undescribed species, Menatgryllus longixiphus gen. et sp. nov.); Tettigoniidae (Prophasgonura lineatocollis); Elcanidae (Cenoelcanus menatensis gen. et sp. nov.); two Eumastacoidea (Paleochina duvergeri gen. et sp. nov. and Paleochina minuta sp. nov., tentatively placed in the extant family Chorotypidae). These two last taxa are compared to the other described fossil Eumastacoidea. As all these Eumastacoidea are represented by tegmina or hindwings, their previous attributions to the Eumastacidae sensu stricto are questionable. All previously described fossil Caelifera from Menat are considered of uncertain position. Those that were previously considered as Acridoidea are excluded from this clade. In consequence, the oldest described Acridoidea are Oligocene, at the time of diversification of the grasses on which these insects predominantly live, in accordance with the most recent molecular dating of the Acrididae. Cenoelcanus menatensis is the youngest and first Cenozoic representative of the Mesozoic Elcanidae, showing that this family survived the Cretaceous–Paleocene extinction and became extinct during the Paleogene