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Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Reunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Reunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate.

The 2010 Haiti earthquake: A complex fault pattern constrained by seismologic and tectonic observations

After the January 12, 2010, Haiti earthquake, we deployed a mainly offshore temporary network of seismologic stations around the damaged area. The distribution of the recorded aftershocks, together with morphotectonic observations and mainshock analysis, allow us to constrain a complex fault pattern in the area. Almost all of the aftershocks have a N‐S compressive mechanism, and not the expected left‐lateral strike‐slip mechanism. A first‐order slip model of the mainshock shows a N264°E north‐dipping plane, with a major left‐lateral component and a strong reverse component. As the aftershock distribution is sub‐parallel and close to the Enriquillo fault, we assume that although the cause of the catastrophe was not a rupture along the Enriquillo fault, this fault had an important role as a mechanical boundary. The azimuth of the focal planes of the aftershocks are parallel to the north‐dipping faults of the Transhaitian Belt, which suggests a triggering of failure on these discontinuities. In the western part, the aftershock distribution reflects the triggering of slip on similar faults, and/or, alternatively, of the south‐dipping faults, such the Trois‐Baies submarine fault. These observations are in agreement with a model of an oblique collision of an indenter of the oceanic crust of the Southern Peninsula and the sedimentary wedge of the Transhaitian Belt: the rupture occurred on a wrench fault at the rheologic boundary on top of the under‐thrusting rigid oceanic block, whereas the aftershocks were the result of the relaxation on the hanging wall along pre‐existing discontinuities in the frontal part of the Transhaitian Belt.

Modern mermaids: New floats image the deep Earth

Scientists get a window into deep Earth structures by using a method called seismic tomography. Similar to computed tomography (CT) scans of the brain, seismic tomography uses delays in the arrival times of seismic P waves to make scans and three-dimensional images of the variations in seismic wave speed in the Earths interior. Patterns in the delays indicate thermal or compositional anomalies in the Earths mantle and core, such as those caused by sinking cold oceanic lithosphere or rising hot thermal plumes. A large number of observations of such delays are available for continental regions; the number of observations for the United States is especially high due to a dense deployment of stations currently being installed temporarily and moved across a large area as part of the USArray project, a branch of the U.S. National Science Foundations EarthScope program. In contrast, no comparable sensor density has been available in the oceans

Seismic monitoring in the oceans by autonomous floats.

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M≥6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments.

P‐Delays from Floating Seismometers (MERMAID), Part I: Data Processing

We present methods of data analysis adapted to Mobile Earthquake Recorder in Marine Areas by Independent Divers (MERMAID) seismograms, obtained with hydrophones mounted on moving underwater floats. If the MERMAID float comes immediately to the surface after recording an earthquake signal, the seismogram location is obtained from the first Global Positioning System (GPS) position, using a correction for the surface drift of the float. In the case of earthquakes recorded without an immediate surfacing, the location is estimated using a linear interpolation between GPS positions. We performed a Bezier interpolation of the GPS positions to estimate a location error. In 67% of the cases, the distance between the two trajectories was less than 500 m. We tested the method on six months of data acquired in the Ligurian basin (Mediterranean Sea). To validate the (manually) picked onset times for P waves, we performed a preliminary tomographic inversion beneath the Ligurian basin of MERMAID data together with a much larger volume of picks from nearby land and ocean‐bottom seismometer stations. After inversion we found that 67% of MERMAID data have a misfit between ±0.17  s, but the distribution of misfits is not Gaussian and shows outliers. We conclude that floating seismometers are an excellent and accurate means for covering oceanic areas for P‐wave tomography.

A real time seismological station at 2500 m depth in front Toulon

In the frame of a collaboration with ANTARES project, a broad band seismological sensor was installed on sea floor at 2500 m and connected to a data center for real time analysis. As expected, the long period seismic noise recorded is quite large, but the presence of a large set of others observations as temperature, pressure and current velocity provide some help to interpret the observed variation in time. The main improvement on the noise level was obtained in burring completely the sensor. The observations are still not at the level of the land observations with similar sensor, but the experience provided us some guides for the development of rules for the development of seismic sea floor permanent observation to complement land observatories in case of offshore seismicity.