273 research outputs found
Relation between subduction megathrust earthquakes, trench sediment thickness and upper plate strain
No full text
Relation between subduction megathrust earthquakes, trench sediment thickness and upper plate strain
No full textInternational audienceGiant earthquake (moment magnitude M-w >= 8.5) forecasts for subduction zones have been empirically related to both tectonic stresses and geometrical irregularities along the subduction interface. Both of these controls have been suggested as able to tune the ability of rupture to propagate laterally and, in turn, exert an important control on giant earthquake generation. Here we test these hypotheses, and their combined influence, by compiling a dataset of trench fill thickness (a proxy for smoothing of subducting plate relief by sediment input into the subduction channel) and upper plate strain (a proxy for the tectonic stresses applied to the subduction interface) for 44 segments of the global subduction network. We statistically compare relationships between upper plate strain, trench sediment thickness and maximal earthquake magnitude. We find that the combination of both large trench fill (>= 1 km) and neutral upper plate strain explains spatial patterns of giant earthquake occurrence to a statistically significant degree. In fact, the concert of these two factors is more highly correlated with giant earthquake occurrence than either factor on its own. Less frequent giant earthquakes of lower magnitude are also possible at subduction zones with thinner trench fill and compressive upper plate strain. Extensional upper plate strain and trench fill < 0.5 km appear to be unfavorable conditions, as giant earthquakes have not been observed in these geodynamical environments during the last 111 years. Citation: Heuret, A., C. P. Conrad, F. Funiciello, S. Lallemand, and L. Sandri (2012), Relation between subduction megathrust earthquakes, trench sediment thickness and upper plate strain, Geophys. Res. Lett., 39, L05304, doi: 10.1029/2011GL050712
Subduction dynamics as revealed by trench migration
No full textInternational audienceNew estimates of trench migration rates allow us to address the dynamics of trench migration and back-arc strain. We show that trench migration is primarily controlled by the subducting plate velocity V-sub, which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: V-t = 0.5V(sub) - 2.3 (in cm a(-1)) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending
Plate kinematics, slab shape and back-arc stress: A comparison between laboratory models and current subduction zones
No full textA combination of statistical studies on present-day subduction zones and three-dimensional (3D) laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones. In 3D laboratory models, the analogue of a two layer linearly viscous lithosphere–upper mantle system is achieved by means of silicon putty glucose syrup tank experiment. The subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics. Both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior: (1) trenches migrate at a rate close to the overriding plate motion, but always move slower than the overriding plates. The mechanism at work is a direct consequence of “slab anchoring” opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability. (2) An overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench. (3) Shallow and steep dips are found if the overriding plate moves toward and away from the trench, respectively
Trench migration, net rotation and slab-mantle coupling
No full textInternational audienceLaboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales. Our models are set up with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (mantle). For our study, the lithosphere/upper mantle viscosity contrast has been systematically varied, from similar to 10 to similar to 10(5) in order to explore the parameter space between weak and strong slab dynamics. We found that subduction is characterized by a retreating mode for viscosity ratios > 10(4), by the coexistence of a retreating mode and an advancing mode for viscosity ratios between similar to 10(4) and similar to 10(2), and quasi-stationary, Rayleigh-Taylor like behaviour for ratios < 10(2). By combining our experimental results and kinematic data from current subduction zones in four reference frames which differ in the amount of net rotation, we infer that a lithosphere/upper mantle viscosity contrast of 150-500 is necessary to obtain realistic trench/subducting plate velocity ratios as well as the variability of subduction styles observed in nature
Where giant earthquakes may come
No full textGiant earthquakes (Mw≥8.5) usually occur on the boundary between subducting and overriding plates of converging margins, but it is not yet clear which (if any) subduction zones are more prone to produce such a kind of events. Here we show that subduction zones may have different capabilities to produce giant earthquakes. We analyze the frequency-magnitude distribution of the interplate earthquakes at subduction zones that occurred during 1976–2007 and calculate the propensity (defined as the average annual rate) of giant events for about half of the subduction zones. We find that the b value of interplate earthquakes is significantly different among the subduction zones, and out-of-sample giant earthquakes (before 1976 and after 2007) have occurred preferentially in high-propensity areas. Besides the importance for seismic hazard assessment and risk mitigation, our results seem to indicate that a higher seismicity rate does not necessarily imply a higher likelihood to generate giant earthquakes, and the way in which the stress is released at a subduction interface does not change significantly after the occurrence of such events
Towards an integrative taxonomy of the genus Cecropia Loefl
No full textLe genre Néotropical Cecropia Loefl. regroupe des arbres pionniers qui colonisent les milieux ouverts et perturbés. Ils constituent un modèle d’étude dans des champs disciplinaires très variés. Malgré le volume et la diversité des recherches menées sur ce genre, il est paradoxal de constater que l’identification des espèces reste extrêmement difficile et constitue un frein à l’ensemble de ces études. La monographie de Berg & Franco-Rosselli (2005) retient 61 espèces et 2 sous-espèces. La circonscription proposée repose essentiellement sur des caractères morphologiques.L’objectif de thèse est d’organiser un cadre de travail par une série d’outils logiciels et analytiques pour i) extraire la substantifique moelle de la monographie par une approche numérique et ii) revoir la circonscription des espèces par une approche en taxonomie intégrative couplant taxonomie numérique et modèle de niches à une échelle continentale. Nos résultats montrent que la monographie est cohérente, avec des taxons bien séparés d’un point de vue morphologique, mais avec une mise en synonymie massive qui n’est pas argumentée. Notre analyse permet d’identifier les caractères les plus discriminants à utiliser en taxonomie numérique et permis de construire une clef d’identification multientrées pour le genre. La modélisation de la niche climatique montre un fort recouvrement entre les espèces et pas d’identité climatique réellement propre à l’une d’entre elles. Nous identifions également plusieurs espèces à haut potentiel invasif.Ce travail appelle à intégrer des approches en biologie moléculaire pour mieux comprendre la dynamique évolutive où l’hybridation fréquente génère un continuum morphologiqueRÉSUMÉ DE LA THÈSE EN ANGLAISThe Neotropical genus Cecropia Loefl. includes pioneer trees that colonize open gaps and disturbed environments. They constitutes a model for study in a wide variety of disciplinary fields. In spite of the volume and diversity of research carried out on this genus, it is paradoxical that the core ability to identify species remains problematic and impedes progress. The monograph by Berg & Franco-Rosselli (2005) describe 61 species and 2 subspecies. The proposed circumscription is essentially based on morphological characters. The objective of this thesis is to organize a framework through a series of software and analytical tools to i) extract the “substantial marrow” of the monograph by a numerical approach and ii) to review the circumscription of species by an integrative taxonomy approach coupling numerical taxonomy and niche models on a continental scale.Our results show that the monograph is coherent, with taxa that are morphologically well delineated, but with a massive synonymy that is not argued. Our analysis allows us to identify the most discriminating characters to be used in a numerical taxonomy and allowed us to build a multi-entry identification key for the genus. The modelling of the climatic niche shows a strong overlap between the species and no climatic identity really specific to one of them. We also identify several species with highly invasive potential.This work calls for the integration of molecular biology approaches that seem indispensable to help circumscribe species and better understand the evolutionary dynamics of what appears to be a species complex, where frequent hybridization generates a morphological continuum
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