3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period

Structure sismique et évolution de la lithosphère au Tibet (réflexions grand-angle et conversions d'ondes télésismiques)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Structure sismique de la zone de subduction des Petites Antilles (implications sur les dimensions de la zone sismogène interplaque)

Les Petites Antilles présentent un contexte géodynamique caractérisé par la subduction à très faible vitesse (_2 cm/an) d'une lithosphère océanique âgée (_84-100 Ma). L'activité sismique y est relativement faible, notamment à l'interplaque, et l'aléa sismique lié à un éventuel séisme de méga-chevauchement reste encore mal contraint. Cette thèse se veut être une première étape dans l'évaluation de la capacité de la zone de subduction des Petites Antilles à générer un tel événement. Dans ce but, ces travaux tentent d'appréhender l'extension en profondeur du domaine sismogène de l'interplaque. Le manque de couverture des stations sismologiques permanentes est un inconvénient majeur dans l'exploration des Petites Antilles. Il s'explique en raison du peu de terres émergées et de leur éloignement de la zone potentiellement sismogène de l'interplaque. La région a donc fait l'objet de plusieurs campagnes océanographiques qui ont permis, notamment, le déploiement de sismomètres fond de mer (OBS) ; certains instruments étant restés immergés plusieurs mois afin de procéder à une écoute de la sismicité. Les travaux réalisés au cours de cette thèse se sont focalisés sur deux jeux de données de sismique grand-angle acquis au large des îles de la Dominique et de la Martinique. Leur analyse a permis la construction de modèles tomographiques 3D et 2D respectivement à l'échelle de l'avant-arc et de l'ensemble de la subduction. Ces modèles renseignent sur la structure sismique des plaques en convergence ainsi que sur leur géométrie. Ils permettent de discuter le rôle de la structure dans le fonctionnement de la subduction et d'obtenir une première estimation de l'extension en profondeur de la zone sismogène en considérant la portion de l'interplaque comprise entre le butoir et le Moho de la plaque supérieure. L'interprétation conjointe des modèles tomographiques et des localisations des séismes a permis, dans un second temps, d'affiner cette estimation. La répartition des épicentres montre en effet une sismicité qui se concentre pour l'essentiel sous une portion de l'avant-arc plus rigide présentant des vitesses plus rapides. La limite amont de la zone sismogène serait donc plus profonde que la position du butoir car elle se situe en retrait de ce dernier. Plus en profondeur, des séismes interplaque observés entre 35 et 45 km marquent la limite aval de la zone sismogène. Celle-ci pourrait ainsi atteindre une profondeur jusqu'à 10 km sous la jonction du Moho et de l'interplaque et, par conséquent, se trouver au contact du manteau lithosphérique. Ces résultats suggèrent une extension en profondeur importante de la zone sismogène qui pourrait atteindre une largeur de prés de 70 km face aux îles de la Dominique et de la Martinique. Ces travaux doivent cependant être poursuivis afin d'évaluer pleinement la capacité de la zone de subduction des Petites Antilles à générer un éventuel séisme de méga- chevauchement. Le taux de couplage de la zone sismogène devra être précisé ainsi que l'extension latérale de celle-ci en tenant compte d'une éventuelle segmentation liée, par exemple, à l'entrée en subduction de rides océaniques.The Lesser Antilles is a case study of a very slow subduction (_2 cm/yr) of an old oceanic lithosphere (_84-100Ma). The region presents a relatively low seismic activity, especially along the interplate contact, and the seismic hazard associated with a possible mega-thrust earthquake is still poorly known. This PhD thesis is a first step toward assessing the ability of the Lesser Antilles subduction zone to produce such a large subduction event. To do so, it aims at constraining the downdip width of the interplate's seismogenic zone. The lack of coverage of permanent seismological stations is a major limitation in the exploration of the Lesser Antilles subduction zone. It is due to the presence of only small aligned islands at far distances from the potentially seismogenic interplate area. Several oceanographic cruises were therefore planned that notably allowed the repeated deployment of ocean bottom seismometers ; some of them being left for a few months of background seismicity recording. This thesis specifically focuses on two sets of wide-angle seismic data acquired offshore the Dominica and Martinique islands. From their analysis 3D and 2D tomographic models were produced respectively over the forearc region and across the whole subduction complex. These models constrain the plates' seismic structure as well as their geometry. They allow the discussion of how the imaged structures affect the subduction processes and give a first estimation of the downdip width of the seismogenic zone, defined as the segment of the interplate between the backstop and the upper plate's Moho. The joint interpretation of seismic models and earthquake localizations then refine this first assessment. Epicenter distribution shows indeed that seismicity concentrates beneath the rigid part of the forearc, where higher velocities are observed. The updip limit of the seimogenic zone would then lie deeper than the backstop's depth as seismicity locates it further arcward. At depth, interplate earthquakes observed between 35 and 45 km depth reveal the downdip limit of the seismogenic zone. The latter would reach a depth over 10 km deeper than the contact of the Moho and the interplate, suggesting it lies at the contact of the interplate with the mantle wedge. All together, these results imply a large downdip width of the seismogenic zone (_70 km) offshore the Dominica and Martinique islands. Further work is, however, needed in order to fully comprehend the ability of the Lesser Antilles subduction zone to produce a possible mega-thrust earthquake. This would necessitate the evaluation of seismic coupling at the interplate contact or the possible segmentation of the seismogenic zone due to, fro instance, the subduction of oceanic ridges.NICE-BU Sciences (060882101) / SudocSudocFranceF

Seismic Imaging of the Ecuadorian Subduction Zone : From Conventional Seismic Processing to Advanced Imaging Workflow

International audienc

Pervasive compression and deep structure of the Hellenic subduction forearc, west of Crete, revealed by penetrative long offset multichannel seismic data

International audienceThe Hellenic subduction system exhibits a fairly atypical structure, resulting from the intense radial extension undergone by the Aegean domain from Eocene to Miocene. It features a very wide fore-arc, that itself includes an external non-volcanic arc between southeastern Peloponnesus and southwestern Anatolia, with Crete occupying a frontal position midway along it. South of it, separated from the Aegean domain by a large escarpment, the Hellenic trenches marking the most advanced position of the forearc Alpine nappes and the wide abutting accretionary wedge are found. The Hellenic subduction is widely regarded as poorly coupled from historical seismicity. Yet, a large tsunamigenic destructive event is reported in 365 CE just west of Crete, for which various rupture processes are proposed. In order to gain better insight into the structures of this major event, some deep penetrating seismic acquisition was performed in 2015 during the SISMED project, using the leading-edge equipment of R/V Marcus Langseth along a 210 km profile crossing through the fore-arc; an 8 km-long streamer and a voluminous source were used. Pre-stack depth imaging was applied to the recorded data set, with a strong effort to build a reliable velocity model using common image focusing analysis, both in the time and depth domains. Good imaging conditions thus were reached down to a maximum depth of 25 km. From south to north, our results reveal that: (1) a 6-7 km-thick oceanic crust with hints of compressive deformation enters the subduction, carrying no sediment past the backstop; (2) it crosses the continental Moho as shallow as 13 km depth; (3) above, a set of reactivated antithetic reverse faults controlling the Matapan forearc basin is clearly imaged; (4) no evidence is found supporting a speculated inverse splay fault outcropping at the toe of the Hellenic scarp and tentatively related to the 365 CE event; (5) some steep faulting is observed at mid-escarpment, whose downward continuation coincides with a jump in the forearc Moho’s depth, and that likely accommodates some of the required dextral strike slip partitioning motion; (6) possible clues are found for some compressive reactivation of the Maleas basin, north of the external arc

Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles

Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics