Sismologie | EPFL Graph Search

thumb|Station sismologique Basse-Terre au sommet du morne Mazeau en Guadeloupe. La sismologie ou séismologie (ce dernier est un anglicisme de seismology) est une discipline scientifique qui étudie les séismes (tremblements de terre) et la propagation des ondes élastiques (dites ondes sismiques) à l'intérieur de la Terre. La sismologie moderne utilise les concepts de la mécanique newtonienne appliqués à la connaissance de la Terre. Les principales disciplines sont la sismogenèse, la sismotectonique, la sismologie de l'ingénieur, la sismologie globale, la sismique active et la sismologie spatiale. Cette liste n'est pas exhaustive. Il existe aussi des disciplines associées où la sismologie est importante comme la paléosismologie, la mécanique des roches ou l'héliosismologie. Histoire Pré-scientifique Les tremblements de terre ont longtemps été considérés comme des messages divins. Pour les Chinois, c'était un signe que le Ciel (les dieux) désavouait la légitim

Energy budget of laboratory earthquakes: a comparison between linear elastic fracture mechanics approach and experimental approach

Federica Paglialunga, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

Earthquakes correspond to a sudden release of elastic energy stored during inter-seismic period by tectonic loading around fault. The earthquake energy budget consists of four non-independent terms: the energy release rate (by unit crack length), the fracture energy, the heat term and finally the radiated energy. These terms depend on the rupture and sliding velocities, the amount of slip and the stress drop. Because of the impossibility to access to stress and strain conditions at depth, the earthquake energy budget cannot be fully constrained from seismological data, limiting our understanding of its influence on rupture propagation. To address this issue, we conducted stick-slip experiments with large samples in a biaxial configuration apparatus. By imposing constant normal load and increasing shear load, seismic events were produced on a 20 cm long fault, for which the energy budget was estimated using different methods. Fracture energy was estimated by recording the strain field around the crack tip through high frequency (2 MHz) strain gage rosettes array and comparing it to the theoretical LEFM strain field predictions obtained for same conditions (i.e. rupture velocity, distance from the fault). Fracture energies were then inverted and found to range in between 1 and 10 J/m2. At the same time the energy partitioning was estimated through stress-slip evolution during rupture. The fracture energies obtained from this method are almost one order of magnitude larger than the ones inverted from LEFM and range in between 1 and 90 J/m2. Moreover, the energy partitioning shows the radiated energy ranging between 80 and 300 J/m2 and finally the heat/thermal energy as the largest fraction of the energy partitioning with values ranging from 200 to 2500 J/m2. Our preliminary results highlight the importance of understanding the contribution of heat energy in frictional processes, since this term cannot be estimated from seismological data.

2020

On the Scale Dependence in the Dynamics of Frictional Rupture

Mathias Alexandre David Lebihain, Federica Paglialunga, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

When an earthquake nucleates in the earth crust, the potential energy accumulated during the inter-seismic period is released into breakdown work, heat energy and radiated energy. Often the breakdown work is considered a seismological equivalent of the fracture energy. However, discrepancies related to the definition of the two are not yet fully solved. To this end, we reproduced frictional ruptures in the laboratory to study the relationship between these two energies. A dual strength weakening is observed, reflected in a scale dependent evolution of breakdown work with fault’ slip, contrarily to fracture energy which is, by definition, scale independent. This behavior shows to be probably caused by thermal weakening (i.e. flash heating) activated during slip and to be well described by the recently developed unconventional theory of frictional ruptures (i.e. rupture driven by a non-square root singularity). Importantly, these results highlight, from an experimental point of view, the presumable unconventional nature of earthquakes, solving the discrepancies between breakdown work and fracture energy. Moreover, it suggests that an analysis of the propagating rupture in the framework of linear elastic fracture mechanics could prove to be not always sufficiently exhaustive when frictional weakenings occur, as it is expected along crustal faults.

2021

On the scale dependence in the dynamics of frictional rupture: Constant fracture energy versus size-dependent breakdown work

Fabian Barras, Mathias Alexandre David Lebihain, Federica Paglialunga, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

Potential energy stored during the inter-seismic period by tectonic loading around faults is released during earthquakes as radiated energy, frictional dissipation and fracture energy. The latter is of first importance since it is expected to control the nucleation, the propagation and the arrest of the seismic rupture. On one side, the seismological fracture energy estimated for natural earthquakes (commonly called breakdown work) ranges between 1 J/m2and tens of MJ/m2for the largest events, and shows a clear slip dependence. On the other side, recent experimental studies highlighted that, concerning rupture experiments, fracture energy is a material property (energy required to break the fault interface) independently of the size of the event, i.e. of the seismic slip. To reconcile these contradictory observations and definitions, we performed stick-slip experiments, as analog for earthquakes, in a bi-axial shear configuration. We estimated fracture energy through both Linear Elastic Fracture Mechanics (LEFM) and a Cohesive Zone Model (CZM) and through the integration of the near-fault stress-slip evolution. We show that, at the scale of our experiments, fault weakening is divided into a near-tip weakening, corresponding to an energy of few J/m2, consistent with the one estimated through LEFM and CZM, and a long-tailed weakening corresponding to a larger energy not localized at the rupture tip, increasing with slip. Through numerical simulations, we demonstrate that only near-tip weakening controls the rupture initiation and that long-tailed weakening can enhance slip during rupture propagation and allow the rupture to overcome stress heterogeneity along the fault. We conclude that the origin of the seismological estimates of breakdown work could be related to the energy dissipated in the long-tailed weakening rather than to the one dissipated near the tip.

2022