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Periodic Report Summary 1 - RHUM-RUM (RHUM-RUM: Imaging a mantle plume under the hotspot of La Réunion)

RHUM-RUM is a seismological experiment designed to image an oceanic mantle plume – or lack of plume – from crust to core beneath La Réunion Island, and to understand these results in terms of material and heat flows. The Réunion hotspot is one of the most active volcanoes in the world, and its hotspot track leads to the Deccan Traps of India, one of the largest flood basalt provinces on Earth, which erupted 65 Ma ago. The genesis and the origin at depth of this mantle upwelling are controversial, and La Réunion stands exemplarily for the entire class of hotspot-type volcanism that occurs mostly in the world's oceans, far removed from plate boundaries. It remains unclear whether oceanic hotspots are fed by deep mantle plumes, and whether plumes are significant contributors to the solid earth's heat budget. From 2011 to 2014, we deployed 57 broadband seismometers on the ocean floor, complemented by 37 seismic stations on the surrounding land masses of Madagascar, Mauritius, the Seychelles, and the Iles Eparses. Since 2014, analysis of the data has been underway.
RHUM-RUM has become the largest effort worldwide to image a plume under an oceanic hotspot. It started as a French-German cooperative effort that pools the marine resources, funding, scientific expertise, and local infrastructure needed to realize this large experiment. I initiated the project during my time as an Assistant Professor at LMU Munich, together with my French co-Principal Investigator Guilhem Barruol from the Université de la Réunion/CNRS. My move in 2013 to the University of Oxford as an Associate Professor meant that some of the German funding had to be left behind, which this Marie Curie Career Integration Grant has been used to mitigate, so that RHUM-RUM could continue uninterrupted. The CIG grant is facilitating the mobility and stipends of a postdoctoral associate and several PhD students to enable the close collaboration and supervision originally intended. The part of the project carried out in my group at Oxford uses the most advanced methods of waveform tomography to turn conventional and unconventional body wave phases (P, S, Pdiff, Sdiff,...) into 3-D structural models of the entire mantle column under La Réunion.
Data analysis during this first reporting period was mainly concerned with denoising, i.e., the understanding and removal of signals in the ocean-bottom and island seismograms that do not originate from the deep subsurface. This is challenging because the ocean column hosts many different noise sources, which are however interesting for their own sake. Peer-reviewed, published results so far include assessments of seismometer issues, their remedies, and statistics of data yields; the tracking of tropical storms using the ocean-bottom seismometer network; the use of near-shore island seismometers as proxies for tides gauges during storms and strong swell episodes; and the first observation of the seismic hum on ocean bottom seismometers. M.Sc. and B.Sc. theses have focused on the characterization and tracking of ships passing above the ocean-bottom seismometers; investigation of sound propagation in the deep sea below the SOFAR channel; the use of OBS as proxies for ocean-bottom current meters; and the analysis of whale song.

With the data denoising effort largely concluded, work is well underway toward the primary objectives of RHUM-RUM, the structural imaging of the subsurface beneath Réunion from crust to core, in order to shed light on the hotspot’s long-lived heat source.

Public website of RHUM-RUM project:

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