Description: Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.

Description: At a convergent margin large amounts of structurally bound water are carried into the Earth’s interior and - as the subducting plate descends and the temperature rises - are driven off to some extent into the mantle wedge, where they are thought to trigger intermediate-depth earthquakes in the Wadati-Benioff zone and melting under volcanic arcs. However, a largely uncertain fraction outlasts sub-arc fluid release and hence enters the deeper mantle, which leads to a connection between the oceans and the Earth’s deep water cycle. Thus, a detailed knowledge of the water budget of a subduction zone is not only important to understand arc volcanism, but as well to comprehend the chemical development of the Earth’s mantle. For this purpose, profound information about the amount of water that is subducted along with the oceanic plate is indispensable. The present thesis uses geophysical methods to determine the degree of hydration of the Cocos Plate offshore Nicaragua, which is subducted beneath the Caribbean Plate. In general it was assumed that structured water is transported into the slab in sediments and the upper crust only, though in recent years growing evidence suggested that lower crust and upper mantle might contain capacious amounts of fluids as well, since the bending of the incoming oceanic plate leads to a reactivation or creation of normal faults (bend-faults), which are visible in batrymetric data and have been inferred to cut deep enough into the plate to provide a pathway for seawater to penetrate into the lithosphere, changing ”dry” peridotites to ”wet” serpentinites, which contain up to 13% of water. Such a mechanism could transport much more fluids into the earth’s interior than any other considered possibility. However, the cutting depth of these bend-faults and hence the depth that seawater could penetrate into the mantle was not well-defined, for one reason since focal depth of earthquakes associated with the bend-faults were poorly known. Yet previous studies assumed cutting depths such that serpentinization is firstly restricted by its thermal limit of 600± C. This study uses openly accessible, global broadband data of earthquakes offshore Central America as well as an unique dataset from a local long-period earthquake monitoring network offshore Nicaragua, to determine typical focal depths off earthquakes at the trench-outer rise and further relates these focal depths to the cutting depths of bend-faults. In addition, a full 3d-tomographic inversion that consistently integrates seismic airgun blasts and local as well as regional seismicity, could show reduced seismic mantle velocities at the outer rise and nearby the deep sea trench with an evolutionary trend towards it. Best explained is this by a fractured and ii partly serpentinized lithosphere. The use of regional sources (i.e. earthquakes in distances of ¸200 km from the seismic network) in the tomographic inversion process made it possible, for the first time, to reflect the entire brittle lithosphere. In a second approach, relative arrival times of large earthquakes that occurred during the deployment of the seismic network were investigated. Again, it could be shown that seismic mantle velocities decrease in accordance with the onset of bend-faults in the bathymetry. But not only seismic velocities decrease nearby the trench, the average moment magnitude of outer rise earthquakes does as well, though the number of events increases significantly. We explain this a weakened lithosphere and hence a reduced yield strain, which again suggests an occurrence of serpentinite. However, tomographic images suggest that the area of reduced seismic velocities and in turn possible serpentinization does not reach the cutting depth of bend-faults nor the depth of the 600± C isotherm. Focal mechanisms of several earthquakes were determined via moment tensor inversion and forward modelling respectively and it could be shown that where seismic velocities are reduced only tensional ruptures occur, which allow for water infiltration, meanwhile the area beneath is dominated by compressional rupture behaviour, which presents a barrier for seawater. This result does not only confirm and enlarge flexure models of subducting plates [Chapple and Forsyth, 1979; Christensen and Ruff, 1988], but also establishes a coherent connection between stress distribution in the incoming plate and penetration depth of seawater and is the first study in this vein.

Description: The project SFB574 “Volatiles and Fluids in Subduction Zones” aims to better understand the processes and quantities involved in the exchange of fluids in convergent margins. The current phase concentrates on the accretionary margin of south-central Chile, a region that was affected after our data mining phase in 2008 by the rupture of the great 2010 Maule Earthquake (Mw 8.8). Within the project, an “amphibious” network of 15 ocean bottom seismometers and 22 land stations was operated from April to October 2008 along 350 km from the outer-rise to the magmatic arc. Additional readings from 11 permanent stations of the Chilean Seismological Service were included in the database improving the coverage to the north and south. One of the goals of the project is to gain a detailed image of the structure of the crust and upper mantle and the seismogenic zone by analyzing precise local earthquake locations and combined passive and active source seismic tomographic images. To achieve precise earthquake locations and to serve as an initial model for local earthquake tomography, we derived a P- and S-wave minimum-1D model using a very high-quality subset of 260 events with ~4000 P-wave and ~2000 S-wave arrivals. Most of the ~1500 earthquakes recorded over the sixmonth period were originated within the subducting slab down to ~140 km depth, with a higher concentration beneath the main cordillera, at depths of 80-100 km. We observe for the first time with a local network a double Benioff zone in this area. Fewer events were generated at the outer-rise, at depths of ~20-40 km, closely following the NE-SW trend of the oceanic plate faulting. The upper-plate seismicity occurred mainly beneath the main cordillera, and within the margin backstop offshore and SW of Pichilemu, where high activity was clustered in a ~1200 km2 area, between 10-30 km depth. The sparse interplate microseismicity nucleated from ~45 up to ~10 km depth and the upper limit is in good agreement with 2D refraction velocity modeling of the shallow part of the margin. The lack of seismicity supports the models reporting nearly complete interplate locking before the great earthquake. From the relocated catalogue, a subset of events has been selected for the 3D inversion of seismic velocities. We review the first tomographic results to present valuable insights into the structure and stress distribution in the Maule region before the 2010 earthquake.

Description: OS53A-1343 The West Nile Delta forms part of the source of the large turbiditic Nile Deep Sea Fan. Since the late Miocene sediments have formed an up to 10 km thick pile, which includes about 1 - 3 km of Messinian evaporates. The sediment load of the overburden implies strong overpressures and salt-related tectonic deformation. Both are favourable for fluid migration towards the seafloor guided by the fractured margin. The western deltaic system, Rosetta branch, has formed an 80 km wide continental shelf. Here at 700 m water depth the mud volcano North Alex (NA) developed his circular bathymetric feature, which proved to be an active gas and mud-expelling structure. A 3-D high-resolution multichannel seismic survey (IFM-GEOMAR P-Cable system) was completed across the mud volcano. 3-D time migration provided a 3-D data cube with a 6.25 m grid. Vertical seismic sections did reveal a large set of faults located within the main mud volcano as well as surrounding the structure. Internal faults are mainly related to episodic mud expulsion processes and continuous gas and fluid production. Deep cutting external faults surround the structure in a half circle shape. Horizontal amplitude maps (time slices) of indicate recent activity of these faults even up to the seafloor. High gas saturation of the sediments is indicated by inverted reflection events. In the centre the gas front cuts into the seafloor reflection while it dips down with increasing radius. Only with the small grid resolution inward dipping reflections become visible, which form an upward opened concave reflector plane underlying the top gas front. The interpretation assumes an oval lens shaped body (conduit) saturated with gas at the top of the mud volcano. It provides the upper termination of the mud chimney. This separation is further supported by passive seismic observations. Distant earthquakes can stimulate long-period harmonic oscillations in mud volcanoes. Such oscillations are detectable with three-component ocean bottom seismometers (OBS) and are best explained by a gas-saturated volume at - or in close proximity to - the surface. The period of these oscillations is directly linked to the composition and dimension of this volume. Further, these oscillations are associated with pressure changes in the gas volume, which are thought to disturb the balance between gas pressure and water pressure strongly enough to cause degassing in the upper sediment layers of the mud volcano. This way, gas transport and release in mud volcanoes might be triggered by external seismic sources. Additionally, tremors of higher frequencies can be observed at NA Mud Volcano, and are most likely generated in the mud chimney beneath the top gas volume. Further evidence for the existence of a rather deep chimney (〉750m) comes from S-wave observations of regional earthquakes. Records from OBS that were placed at the volcano’s centre (1), differ from OBS in greater distance (2). S-wave arrivals suggest the existence of a cylindrical-shaped waveguide beneath OBS of type (1). Such features cannot be seen on OBS of type (2). Thus, S-wave velocities need to be lower in the chimney than in the surrounding, which is a reasonable assumption. Modelling of these waveguides can give the dimension of the chimney (width and depth).