Dynamic Overshoot Near Trench Caused by Large Asperity Break at Depth

International audienceIn an attempt to explain the large shallow slip that occurred near the trench during the 2011 Tohoku-Oki earthquake, numerical simulations of earthquake dynamic rupture were carried out using a fault model with a subduction interface containing a bump-shaped asperity, which might result from subduction of an old submarine volcano or seamount. It was assumed that during the interseismic period, slip only occurs outside the bump area and that stress accumulates inside the bump, creating a seismogenic asperity. We roughly evaluated the amount of slip outside the bump during the interseismic period, assuming a constant long-term subduction rate. Then we could estimate the accumulated stress inside the bump. We constructed the initial stress distribution based on the stress change caused by the slip-deficit distribution. A constitutive relation was constructed based on a slip-weakening friction law and was used to compute spontaneous ruptures. The results indicate that a large slip can occur between the trench and the bump, even though a very small amount of stress is accumulated there before the rupture. This is due to an interaction between the free surface and the fault that causes slip overshoot. On the region of the fault below the bump, such overshoot cannot occur because the fault is pinned by the deeper un-slipped zone. However, on the shallower side, the edge of the fault becomes free when the rupture approaches the free surface. In this region, such a large slip can occur without releasing a large amount of stress

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

Vers l'arrêt spontané de la rupture en dynamique de la source : non-élasticité du milieu et loi de friction hétérogène

Des papiers en anglais sont associés à cette thèse - merci de me contacter par email pour en disposer.Some scientific papers (written in english) about this work can be obtained on demand. Please send me an email.During an earthquake, the rupture grows and propagates on the fault. When it stops, the seism reaches its final size. Understanding what, in natural cases, determines the rupture ability to propagate or stop is a critical issue in seismology, because earthquakes size can span over a wide range, but only the biggest ones are threatful. In this thesis, we study the impacts of several different ways to stop or perturbate the rupture propagation, through numerical dynamic simulations of the earthquake process. First, we included a limit to elasticity of the bulk surrounding the fault, in ordre to simulate the rupture propagation in a fractured medium, which dissipates a part of the energy released. For the first time, we have included and studied the impact of this dissipation inside a 3D rupture model. In these conditions, the rupture becomes more sensitive to barriers, and consequently stops more easily. The rupture kinematics are remarkably modified : the rupture velocity is slower, and the slip velocity is limited. Surface motions are less important. Thus, plastic behaviour of the bulk shows up as an important phenomena to take into account for seismic rupture modeling. Second, we studied the impact of a spatial variability of the rupture resistance on the fault. Introducing such an heterogeneity leads to slip profiles shapes that are closer from natural observations, showing off linear trends. Moreover, the rupture propagation and arrest location loose their predictability, as a consequence of the gradual stop of the rupture front on the small barriers included. For a same statistic of barrier size, a wide range of rupture size has been obtained. A power law distribution similar to a Gutemberg-Richter law can be obtained, if the mean fracture energy on the fault is a function of the rupture size, as it has been observed in the calculations including plasticity. Finally, we studied the scaling between the final slip and the asperity size, using smooth asperity/barrier models. We show that the dynamics control one part of the maximum slip scaling law, and also that the fault segmentation has to be taken into account to fit properly the scaling law.Durant un tremblement de terre, la rupture grandit et se propage sur la faille. Lorsqu'elle s'arrête, le séisme atteint sa taille finale. Comprendre ce qui, dans la nature, détermine la capacité à se propager ou à s'arrêter de la rupture est fondamental en sismologie, puisqu'il existe des séismes de toutes les tailles, mais que seuls les plus grands sont dévastateurs. Dans cette thèse, nous étudions l'impact de différentes façons d'arrêter ou de perturber la propagation de la rupture, à travers des études numériques dynamiques. Tout d'abord, nous avons inclus une limite à l'élasticité du milieu entourant la faille, afin de simuler la propagation de la rupture dans un milieu fracturé, qui dissipe une partie de l'énergie libérée. Nous avons, pour la première fois, inclus et étudié l'impact de cette dissipation dans un modèle de rupture 3D. La rupture, dans ces conditions, est beaucoup plus sensible aux barrières, et s'arrête plus facilement. La cinématique de la rupture est remarquablement modifiée (vitesse de propagation plus lente, vitesse de dislocation maximale limitée). Les mouvements engendrés en surface sont atténués. La plasticité est, de fait, un phénomène crucial à prendre en compte dans la modélisation de la rupture sismique.Dans une deuxième partie, nous avons étudié l'impact d'une hétérogénéité spatiale de résistance à la rupture sur la faille. L'introduction de l'hétérogénéité permet d'obtenir des profils de glissement dont la forme se rapproche des formes observées dans les cas naturels. La propagation et l'arrêt de la rupture perdent leur caractère prédictible lorsque ce sont de petites barrières qui arrêtent la rupture progressivement. On peut obtenir une grande variété de tailles d'événements pour une même statistique de taille des barrières. L'obtention d'une loi puissance de type Gutemberg-Richter sur toute la gamme des tailles d'événements est conditionnée par l'augmentation progressive de l'énergie de fracturation avec la taille de la rupture, d'une façon similaire à ce qui est obtenu en considérant un comportement plastique du milieu. Enfin, l'étude des relations glissement final - taille de l'aspérité rompue, dans des modèles lisses de type aspérité/barrière, a montré que la dynamique contrôlait une partie de la loi d'échelle du glissement maximum, et que la segmentation des failles modifie sensiblement la loi d'échelle

Vers l'arrêt spontané de la rupture en dynamique de la source : non-élasticité du milieu et loi de friction hétérogène

Des papiers en anglais sont associés à cette thèse - merci de me contacter par email pour en disposer.Some scientific papers (written in english) about this work can be obtained on demand. Please send me an email.During an earthquake, the rupture grows and propagates on the fault. When it stops, the seism reaches its final size. Understanding what, in natural cases, determines the rupture ability to propagate or stop is a critical issue in seismology, because earthquakes size can span over a wide range, but only the biggest ones are threatful. In this thesis, we study the impacts of several different ways to stop or perturbate the rupture propagation, through numerical dynamic simulations of the earthquake process. First, we included a limit to elasticity of the bulk surrounding the fault, in ordre to simulate the rupture propagation in a fractured medium, which dissipates a part of the energy released. For the first time, we have included and studied the impact of this dissipation inside a 3D rupture model. In these conditions, the rupture becomes more sensitive to barriers, and consequently stops more easily. The rupture kinematics are remarkably modified : the rupture velocity is slower, and the slip velocity is limited. Surface motions are less important. Thus, plastic behaviour of the bulk shows up as an important phenomena to take into account for seismic rupture modeling. Second, we studied the impact of a spatial variability of the rupture resistance on the fault. Introducing such an heterogeneity leads to slip profiles shapes that are closer from natural observations, showing off linear trends. Moreover, the rupture propagation and arrest location loose their predictability, as a consequence of the gradual stop of the rupture front on the small barriers included. For a same statistic of barrier size, a wide range of rupture size has been obtained. A power law distribution similar to a Gutemberg-Richter law can be obtained, if the mean fracture energy on the fault is a function of the rupture size, as it has been observed in the calculations including plasticity. Finally, we studied the scaling between the final slip and the asperity size, using smooth asperity/barrier models. We show that the dynamics control one part of the maximum slip scaling law, and also that the fault segmentation has to be taken into account to fit properly the scaling law.Durant un tremblement de terre, la rupture grandit et se propage sur la faille. Lorsqu'elle s'arrête, le séisme atteint sa taille finale. Comprendre ce qui, dans la nature, détermine la capacité à se propager ou à s'arrêter de la rupture est fondamental en sismologie, puisqu'il existe des séismes de toutes les tailles, mais que seuls les plus grands sont dévastateurs. Dans cette thèse, nous étudions l'impact de différentes façons d'arrêter ou de perturber la propagation de la rupture, à travers des études numériques dynamiques. Tout d'abord, nous avons inclus une limite à l'élasticité du milieu entourant la faille, afin de simuler la propagation de la rupture dans un milieu fracturé, qui dissipe une partie de l'énergie libérée. Nous avons, pour la première fois, inclus et étudié l'impact de cette dissipation dans un modèle de rupture 3D. La rupture, dans ces conditions, est beaucoup plus sensible aux barrières, et s'arrête plus facilement. La cinématique de la rupture est remarquablement modifiée (vitesse de propagation plus lente, vitesse de dislocation maximale limitée). Les mouvements engendrés en surface sont atténués. La plasticité est, de fait, un phénomène crucial à prendre en compte dans la modélisation de la rupture sismique.Dans une deuxième partie, nous avons étudié l'impact d'une hétérogénéité spatiale de résistance à la rupture sur la faille. L'introduction de l'hétérogénéité permet d'obtenir des profils de glissement dont la forme se rapproche des formes observées dans les cas naturels. La propagation et l'arrêt de la rupture perdent leur caractère prédictible lorsque ce sont de petites barrières qui arrêtent la rupture progressivement. On peut obtenir une grande variété de tailles d'événements pour une même statistique de taille des barrières. L'obtention d'une loi puissance de type Gutemberg-Richter sur toute la gamme des tailles d'événements est conditionnée par l'augmentation progressive de l'énergie de fracturation avec la taille de la rupture, d'une façon similaire à ce qui est obtenu en considérant un comportement plastique du milieu. Enfin, l'étude des relations glissement final - taille de l'aspérité rompue, dans des modèles lisses de type aspérité/barrière, a montré que la dynamique contrôlait une partie de la loi d'échelle du glissement maximum, et que la segmentation des failles modifie sensiblement la loi d'échelle

Dynamic Overshoot Near Trench Caused by Large Asperity Break at Depth

International audienceIn an attempt to explain the large shallow slip that occurred near the trench during the 2011 Tohoku-Oki earthquake, numerical simulations of earthquake dynamic rupture were carried out using a fault model with a subduction interface containing a bump-shaped asperity, which might result from subduction of an old submarine volcano or seamount. It was assumed that during the interseismic period, slip only occurs outside the bump area and that stress accumulates inside the bump, creating a seismogenic asperity. We roughly evaluated the amount of slip outside the bump during the interseismic period, assuming a constant long-term subduction rate. Then we could estimate the accumulated stress inside the bump. We constructed the initial stress distribution based on the stress change caused by the slip-deficit distribution. A constitutive relation was constructed based on a slip-weakening friction law and was used to compute spontaneous ruptures. The results indicate that a large slip can occur between the trench and the bump, even though a very small amount of stress is accumulated there before the rupture. This is due to an interaction between the free surface and the fault that causes slip overshoot. On the region of the fault below the bump, such overshoot cannot occur because the fault is pinned by the deeper un-slipped zone. However, on the shallower side, the edge of the fault becomes free when the rupture approaches the free surface. In this region, such a large slip can occur without releasing a large amount of stress

Dynamic rupture scenarios of anticipated Nankai-Tonankai earthquakes, southwest Japan

International audienceWe investigated dynamic rupture scenarios of anticipated megathrust earthquakes on the Nankai-Tonankai subduction zone, southwest Japan. To improve the scenario reliability, the model parameters should be constrained by available data, or derived from their analysis. We employed the three-dimensional plate interface geometry and the slip-deficit rate on the interface. Accumulated slip-deficit was used to obtain the stress drop distribution of anticipated earthquakes. The estimated stress drop distribution is consistent with the seismogenic asperity locations known from the analysis of past earthquakes. Fault friction constitutive parameters, however, had to be assumed from indirect observations because they cannot be constrained directly by the data. Based on various geophysical observations, we defined three regions where larger fracture energy is required. These are the eastern edge of the Tonankai area, the western edge of the Nankai area, and the region between the Tonankai and Nankai areas (beneath the Kii peninsula). Such lateral heterogeneity promoted the segmented rupture along the Nankai trough. With predefined stress drop and constitutive parameters, various rupture scenarios for Tonankai and Nankai asperities were obtained for different initiation locations. In some cases, a single segment is ruptured, while in other cases, all the segments are broken due to dynamic linkage at the segment boundary, causing a giant earthquake. The initiation location is a critical parameter that controls the rupture propagation across the segment boundary. These scenarios will be extremely useful to evaluate deterministically the strong ground motions and tsunami hazards caused by the next major earthquakes in southwest Japan

Calibration of subsurface dynamic parameters and fault geometry from surface fault rupture observations: an example from the shallow 2019 Mw4.9 Le Teil (France) event.

International audienceWe investigate the impact of several friction, stress drop and fault geometry on surface fault rupture amount and patterns. Based on a shallow reverse surface rupturing Mw4.9 earthquake which occurred in southeastern France in November 2019, and models derived from data collected (InSAR, waveforms), we set up a rupture scenario that is consistent with the observations. From this kinematic scenario we constrain the dynamic parameters of the deeper part of the rupture (300-2 km depth), while we test the shallow part parameters (<300m). The surface rupture produced by the different models are then compared to the surface deformation patterns and amplitude. We show that the shallow surface layers are likely slip-strengthening, but also that the surface rupture is not a passive marker of the deeper rupture process: they are both linked. The frictional behavior (Dc, Stress drop, weakening or strengthening) directly modulates the amount of surface rupture. Dynamic rupture history notably differs from the kinematic model, although the friction evolution of the first was directly derived from the second. Adding a secondary structure in the northern part improves significantly the surface rupture fit, as well as the rupture history. Finally, such a shallow reverse fault earthquake seen through its dynamics emphasizes a puzzling question: what is the absolute level of stress on a seismogenic fault so close to the surface

The effect of thermal pressurization on dynamic fault branching

International audienceWe numerically investigate the effect of thermal pressurization (TP) on fault branch behaviour during dynamic rupture propagation, a situation likely to occur during large earthquakes at subduction interfaces. We consider a 2-D mode II (in-plane) rupture in an infinite medium that propagates spontaneously along a planar main fault and encounters an intersection with a pre-existing branching fault. The fault system is subjected to uniform external stresses. We adopt the values used by Kame et al. We use the 2-D boundary integral equation method and the slip-weakening friction law with a Coulomb failure criterion, allowing the effective normal stress to vary as pore pressure changes due to TP. We reveal that TP can alter rupture propagation paths when the dip angle of the main fault is small. The rupture propagation paths depend on the branching angle when TP is not in effect on either of the faults, as described by Kame et al. However, the dynamic rupture processes are controlled more by TP than by the branching angle. When TP is in effect on the main fault only, the rupture propagates along the main fault. It propagates along the branch when TP is in effect on both faults. Finally, we considered the case where there is a free surface above the branch fault system. In this case, the rupture can propagate along both faults because of interaction between the free surface and the branch fault, in addition to TP on the main fault

Off-fault plasticity favors the arrest of dynamic ruptures on strength heterogeneity : Two-dimensional cases

International audienceWe study the effects of a plastic behavior of the volume around the fault on in-plane and anti-plane 2D rupture dynamics. Both rupture modes exhibit similar answer to off-fault yielding, in terms of modification of the kinematics of the rupture front, and in terms of energy lost outside the fault plane. We then compare the ability of the rupture to propagate through a barrier on the interface. The plastic behavior, responsible for a linear increase of the global fracture energy during dynamic crack growth, enhances the rupture front sensitivity to a static resistance increase on the fault. Consequently, the rupture arrest is more easily provoked in heterogeneous models that include a plastic yielding, even with relatively small variations of frictional resistance along the fault plane