Etude du cycle sismique sur une expérience analogique de zone de faille : caractérisation de la déformation par suivi micro-sismique

Abstract : The deformation observed along a seismic fault can be described as the succession of phases for which the fault accumulate stress imposed by the steady deformation of the surrounding regions, and phases of sudden sliding during which the stress is relaxed: the earthquakes. After the rupture, strengthening mechanisms are required to make possible the new accumulation of elastic stress. Therefore, the seismic cycle results in the steady competition between strengthening and damage. The aim of this study is to explore the role of cohesion-healing on the fault deformation dynamic, as well as to characterize the effect of slip rate on the seismicity. The experimental set-up designed by Weiss et al (2016) has been extended in this study to carry out a micro-seismic monitoring of the deformation. This experiment consists in the shear deformation of a fault created in a thin ice plate overlying a water column. Cohesion-healing mechanisms are achieved through freezing of the water along the fault. The damage mechanisms and the spatial and temporal distribution of the deformation can be characterized thanks to the detectable elastic waves emitted by the fracturing. Because of the plate geometry and underlying water column, we observed guided waves similar to the Lambs symmetric and antisymmetric modes.The largest fractures distribute according to a power law of the form $10^{-bm}$ that is similar to the one observed in seismology. At a constant sliding rate, we observe a large $b$ value, $simeq 3$, which is much larger than the value observed in the Earth's crust ($b=1$). This large $b$ value indicates that the deformation is mainly accommodated aseismically or by small, undetected, fractures. During Slide-Hold-Slide experiment that corresponds to a case for which the cohesion-healing is enhanced compared to the damage, we observe a decrease in the $b$-value likely due to a decrease in fault heterogeneity and an increase of the fault ability to store more elastic stress before the rupture, allowing the fractures to grow larger. An important part of the fractures are multiplets, swarms of fractures, which seem to be passive by-products of the imposed deformation. This behaviour is similar to the one observed for swarm seismicity triggered by slip transient: high $b$-value, no identified mainshock, and very little triggering. For small driving rate $Omega$, we observe an increase in torque drop amplitude with magnitude, $Delta Gamma sim M_0 sim 10^{1.2m}$, similar to the relation observed in seismology, $M_0 sim 10^{1.5m}$. Thus, the latter could be extended to small magnitudes observed in this study. A decrease of the seismic coupling is observed through the decrease in the number of fractures per unit of slip, and because in average a fracture behaves similarly at the different $Omega$ tested. Finally, for a given magnitude interval, we observe a decrease in torque drop amplitude with the increase in $Omega$. This could be explained by the observed decrease in seismic coupling or by a decrease in strengthening rate with $Omega$ that is not observed.
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Cédric Lachaud. Etude du cycle sismique sur une expérience analogique de zone de faille : caractérisation de la déformation par suivi micro-sismique. Sciences de la Terre. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAU002⟩. ⟨tel-02148192⟩

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