The processes associated with magma injection at shallow depths within the crust have been the topic of many geophysical studies, some investigating the seismicity that accompanies volcanic activity and others attempting to map the subsurface extent and geometry of the resulting magma bodies. The aim of this study is to obtain a better understanding of these processes by investigating the nature of seismic signals that accompany volcanic eruptions and by seismically imaging a magma body beneath a mid-ocean ridge, both located on, or adjacent to Iceland. The seismic phenomena associated with the 1996 Vatnajӧkull subglacial eruption in central Iceland, have been studied using data recorded by both temporary (HOTSPOT) and permanent (SIL) seismic networks. These networks comprise 60 broadband and short-period three-component seismographs and cover most parts of the country. Two very active volcanic systems, Bárdarbunga and Grimsvӧtn, are situated underneath the Vatnajokull ice cap. The volcanoseismic signals recorded there were categorised according to their waveform shape and frequency content, into three groups: (a) low-frequency events (1-2 Hz); (b) mixed-frequency events (1-4 Hz); and (c) volcanic tremor. The eruption was preceded by intense seismic activity which began with a = 5.6 earthquake located at the Bárdarbunga volcanic system. The epicentres of the earthquake swarm that followed the M(_w), = 5.6 event initially delineated the Bárdarbunga caldera rim and then migrated towards Grimsvӧtn, to a place where a fissure was later observed. Pre-eruptive tremor started at least two days before the eruption as a harmonic signal around five narrow frequency bands (0.5-0.7, 1.6, 2.2, 2.8 and 3.2 Hz). Co-eruptive tremor started as a broadband, continuous signal which evolved into low-amplitude background tremor interrupted by high-amplitude, cigar-shaped bursts. Further analysis revealed that continuous tremor and the cigar-shaped bursts had all the characteristics of low- dimensional chaotic signals. Geophysical and geochemical evidence suggest that a lateral migration of magma from Bárdarbunga facilitated the rupture of the roof of a magma chamber, situated at the fissure area, which subsequently erupted as tephra on the glacier. The second phase of the RAMESSES (Reykjanes Ridge Axial Melt Experiment: Structural Synthesis from Electromagnetic and Seismics) experiment involved the acquisition of multichannel seismic reflection data from 39 along- and across-axis lines shot over the magmatically active 57º 45'N axial volcanic ridge. The data from one along-axis line were processed using a variety of techniques that mainly aimed at reducing the large amount of coherent noise present, a result of scattered energy at the rough seabed. The final processed section revealed a number of reflection events that could be interpreted as intra-crustal reflections, originating from the interface between pillow lavas and sheeted dykes, and from the top part of a thin melt lens.

Depuis maintenant une dizaine d'années, la vision du cycle sismique en zone de subduction a beaucoup évolué. Des découvertes récentes ont mis en évidence une grande diversité des régimes de glissement dans ces zones, avec notamment des glissements asismiques transitoires appelés « séismes lents » (SSE) et des vibrations de faibles amplitudes, persistantes dans le temps, appelées « trémors non volcaniques » (NVT). Ce travail a pour objectif l'étude des trémors non volcaniques afin de caractériser ces nouvelles manifestations des zones de faille. Nous avons abordé ce problème avec deux approches distinctes :1. Observer les trémors dans le milieu naturel afin de déterminer leurs caractéristiques. La zone étudiée correspond à la lacune sismique de Guerrero le long de la subduction mexicaine. Nous avons développé une méthode de détection et de localisation des NVT au Mexique grâce à des analyses d'antennes par formation de voie sur les corrélations. Cette méthode permet de mettre en évidence cer taines caractéristiques des NVT : une complexité des sources pour un épisode de trémors, une corrélation entre les activités de NVT et les pics de vitesse des glissements lents à plus long terme. Par ailleurs, l'étude de l'impact du séisme de Maule (2010, Chili, Mw 8.8) au Mexique montre qu'il a déclenché le second sous évènement du séisme lent de 2009-2010. Ce déclenchement d'un SSE s'ac- compagne de fortes activités de trémors, modulées par les ondes du séisme de Maule dans un premier temps, puis simplement associées au SSE.2. Modéliser les trémors expérimentalement et numériquement pour mieux com- prendre leur origine physique et leurs évolutions sur le long terme. Nous avons en particulier utilisé une expérience de frottement à faible vitesse qui indique une corrélation systématique entre les accélérations d'un glissement et l'émission de signaux qui ressemblent à des NVT. Une modélisation numérique de la zone de subduction mexicaine est également présentée et montre la possibilité de reproduire des trémors en considérant une transition d'affaiblissement critique associée à un processus de décrochage. / The vision of the seismic cycle in subduction zones has considerably evolved over the last 10 years. New discoveries has pointed the diversity of slip behaviors in these zones with aseismic slow slip called « slow slip events » (SSE) and persistent low amplitudes vibrations called « non-volcanic tremors » (NVT). The goal of this thesis is to study the non-volcanic tremors in order to characterize these new manifestations of fault zones. We used two different approaches: 1. We first observed the non-volcanic tremors in the nature in order to characterize this phenomenon. The area of interest is the Guerrero seismic gap along the Mexican subduction zone. We develop a new detection and location method based on beamforming of correlations of seismic signals. This new method exhibits some characteristics of NVT: a complex source for a single tremor episode and a correlation between the NVT episodes and the long-term peak of movement velocity in southwards direction. Moreover, the study of the consequences of the Maule earthquake on the Mexican subduction zone showed that this earthquake triggered the 2009-2010 SSE in Guerrero. This triggering of slow slip is accompanied by strong seismic tremor actvity that are first modulated by the passing waves and then associated to the SSE. 2. We model numerically and experimentally the tremors in order to better understand their physical origin and their long-term evolution. We used a very slow friction experiment that indicates a systematic correlation between slip acceleration of a slider and emission of acoustic signals that are similar to NVT. A numerical modeling of the Mexican subduction zone is also presented and shows the possibility to reproduce NVT with a critical depinning transition.