A. Eu-marie-curie, C. , «. Rhum-rum-»-to, and K. S. , Additional support was provided by We thank DEPAS (Deutsche Geräte-Pool für Amphibische Seismologie , Germany), GEOMAR (GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Germany) and INSU-IPGP (Institut National des Sciences de l'Univers -Institut de Physique du Globe de Paris, France) for providing the broad-band ocean-bottom seismometers (44 DEPAS, 4 GEOMAR, 9 INSU-IPGP). The RHUM-RUM data set) has been assigned the FDSN network code YV and is hosted and served by the French RESIF data centre (http://seismology.resif.fr) Data will be freely available by the end of 2017. We thank cruise participants and crew members on R/V Marion Dufresne, cruise MD192 and on R/V Meteor, cruise M101. We acknowledge support from/discussions within TIDES COST Action ES1401. We used the open-source toolboxes We are grateful to Martin Schimmel for providing his 'polfre' toolbox (v, Python v.2.7 Schimmel et al. 2011), and for his patience in answering our questions. This manuscript was greatly improved thanks to the suggestions of two anonymous reviewers, p.2015, 1995.

C. Amante and B. W. Eakins, NOAA Technical Memorandum NESDIS NGDC-24, National Geophysical Data Center, pp.10-7289, 2009.

P. N. Anderson, F. K. Duennebier, and R. K. Cessaro, Ocean borehole horizontal seismic sensor orientation determined from explosive charges, Journal of Geophysical Research, vol.77, issue.B5, pp.92-3573, 1987.
DOI : 10.1029/JB092iB05p03573

G. E. Baker and J. L. Stevens, Backazimuth estimation reliability using surface wave polarization, Geophysical Research Letters, vol.90, issue.9, pp.10-1029, 2004.
DOI : 10.1029/2004GL019510

G. Barruol, RHUM-RUM Marion Dufresne MD192 cruise report, 2012.

G. Barruol, D. Suetsugu, H. Shiobara, H. Sugioka, S. Tanaka et al., Mapping upper mantle flow beneath French Polynesia from broadband ocean bottom seismic observations, Geophysical Research Letters, vol.34, issue.44, p.3610, 1029.
DOI : 10.1029/2009GL038139

URL : https://hal.archives-ouvertes.fr/hal-00420907

P. Berens, CircStat: a MATLAB toolbox for circular statistics, J. Stat. Softw, issue.10, pp.31-32, 2009.

M. Beyreuther, R. Barsch, L. Krischer, T. Megies, Y. Behr et al., ObsPy: A Python Toolbox for Seismology, Seismological Research Letters, vol.81, issue.3, pp.530-533, 2010.
DOI : 10.1785/gssrl.81.3.530

K. De-meersman, M. Van-der-baan, and J. Kendall, Signal Extraction and Automated Polarization Analysis of Multicomponent Array Data, Bulletin of the Seismological Society of America, vol.96, issue.6, pp.2415-2430, 2006.
DOI : 10.1785/0120050235

A. M. Dziewonski and D. L. Anderson, Preliminary reference Earth model, Physics of the Earth and Planetary Interiors, vol.25, issue.4, pp.297-356, 1981.
DOI : 10.1016/0031-9201(81)90046-7

G. Ekstrom and R. W. Busby, Measurements of Seismometer Orientation at USArray Transportable Array and Backbone Stations, Seismological Research Letters, vol.79, issue.4, pp.554-561, 2008.
DOI : 10.1785/gssrl.79.4.554

F. R. Fontaine, G. Barruol, A. Tommasi, and G. H. Bokelmann, Upper-mantle flow beneath French Polynesia from shear wave splitting, Geophysical Journal International, vol.170, issue.3, pp.1262-1288, 2007.
DOI : 10.1111/j.1365-246X.2007.03475.x

URL : https://hal.archives-ouvertes.fr/hal-01249275

F. R. Fontaine, G. Barruol, B. L. Kennett, G. H. Bokelmann, and D. Reymond, Upper mantle anisotropy beneath Australia and Tahiti from P wave polarization: implications for real-time earthquake location, J. geophys. Res, vol.114, issue.B3, p.10, 1029.
DOI : 10.1029/2008jb005709

URL : https://hal.archives-ouvertes.fr/hal-00413141

F. R. Fontaine, G. Barruol, H. Tkal?i´tkal?i´c, I. Wölbern, G. Rümpker et al., Crustal and uppermost mantle structure variation beneath La R??union hotspot track, Geophysical Journal International, vol.203, issue.1, pp.107-126, 2015.
DOI : 10.1093/gji/ggv279

URL : http://hal.univ-reunion.fr/hal-01236131/document

F. Grigoli, S. Cesca, T. Dahm, and L. Krieger, A complex linear least-squares method to derive relative and absolute orientations of seismic sensors, Geophysical Journal International, vol.188, issue.3, pp.1243-1254, 2012.
DOI : 10.1111/j.1365-246X.2011.05316.x

K. Hosseini, obspyDMT (Version 1.0.0) [software]'. Available at: https://github.com/kasra-hosseini/obspyDMT, 2015.

B. N. Kennett and E. R. Engdahl, Traveltimes for global earthquake location and phase identification, Geophysical Journal International, vol.105, issue.2, pp.429-465, 1991.
DOI : 10.1111/j.1365-246X.1991.tb06724.x

URL : http://gji.oxfordjournals.org/cgi/content/short/105/2/429

G. Laske, Global observation of off-great-circle propagation of Long-Period surface waves, Geophysical Journal International, vol.123, issue.1, pp.245-259, 1995.
DOI : 10.1111/j.1365-246X.1995.tb06673.x

D. Mainprice, Seismic Anisotropy of the Deep Earth from a Mineral and Rock Physics Perspective, Treatise on Geophysics, pp.487-538, 2015.
URL : https://hal.archives-ouvertes.fr/hal-00408321

F. Niu and J. Li, Component azimuths of the CEArray stations estimated from P-wave particle motion, Earthquake Science, vol.52, issue.1, pp.3-13, 2011.
DOI : 10.1007/s11589-011-0764-8

Ø. Pettersen and V. Maupin, Lithospheric anisotropy on the Kerguelen hotspot track inferred from Rayleigh wave polarisation anomalies, Geophysical Journal International, vol.149, issue.1, pp.225-246, 2002.
DOI : 10.1046/j.1365-246X.2002.01646.x

D. Reymond, Différentes approches pour une estimation rapide des paramètres de source sismique: application pour l'alerte aux tsunamis, 2010.

G. Rossum, Python Reference Manual, CWI (Centre for, Mathematics and Computer Science, 1995.

P. J. Rousseeuw and C. Croux, Alternatives to the Median Absolute Deviation, Journal of the American Statistical Association, vol.69, issue.424, pp.1273-1283, 1993.
DOI : 10.1080/01621459.1984.10477105

J. Rueda and J. Mezcua, Orientation Analysis of the Spanish Broadband National Network Using Rayleigh-Wave Polarization, Seismological Research Letters, vol.86, issue.3, pp.929-940, 2015.
DOI : 10.1785/0220140149

C. Scheingraber, K. Hosseini, R. Barsch, and K. Sigloch, ObsPyLoad: A Tool for Fully Automated Retrieval of Seismological Waveform Data, Seismological Research Letters, vol.84, issue.3, pp.525-531, 2013.
DOI : 10.1785/0220120103

M. Schimmel and J. Gallart, Degree of Polarization Filter for Frequency-Dependent Signal Enhancement Through Noise Suppression, Bulletin of the Seismological Society of America, vol.94, issue.3, pp.1016-1035, 2004.
DOI : 10.1785/0120030178

M. Schimmel, E. Stutzmann, F. Ardhuin, and J. Gallart, Polarized Earth's ambient microseismic noise, Geochemistry, Geophysics, Geosystems, vol.10, issue.4, pp.10-1029, 2011.
DOI : 10.1029/2011GC003661

V. Schlindwein and F. Schmid, Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere, Nature, vol.34, issue.348, pp.535-276, 2016.
DOI : 10.1038/nature18277

J. Scholz, Local seismicity of the segment-8 volcano at the ultraslow spreading Southwest Indian Ridge, 2014.

V. Schulte-pelkum, G. Masters, and P. M. Shearer, Upper mantle anisotropy from long-period P polarization, J. geophys. Res, vol.106, issue.21, pp.917-938, 2001.

J. C. Stachnik, A. F. Sheehan, D. W. Zietlow, Z. Yang, J. Collins et al., Determination of New Zealand Ocean Bottom Seismometer Orientation via Rayleigh-Wave Polarization, Seismological Research Letters, vol.83, issue.4, pp.704-713, 2012.
DOI : 10.1785/0220110128

S. C. Stähler, Performance report of the RHUM-RUM ocean bottom seismometer network around La R??union, western Indian Ocean, Advances in Geosciences, vol.41, pp.43-63, 2016.
DOI : 10.5194/adgeo-41-43-2016-supplement

X. Wang, Q. Chen, J. Li, and S. Wei, ???Wave Particle Motion: An Application to the NECsaids Array, Seismological Research Letters, vol.87, issue.4, pp.901-911, 2016.
DOI : 10.1785/0220160005

S. C. Webb, Broadband seismology and noise under the ocean, Reviews of Geophysics, vol.22, issue.24, pp.105-142, 1998.
DOI : 10.1029/97RG02287

P. Wessel, W. H. Smith, R. Scharroo, J. Luis, and F. Wobbe, Generic mapping tools: improved version released, EOS, Trans. Am. geophys. Un, issue.45, pp.94-409, 2013.
DOI : 10.1002/2013eo450001

J. M. Whittaker, A. Goncharov, S. E. Williams, R. D. Müller, and G. Leitchenkov, Global sediment thickness data set updated for the Australian-Antarctic Southern Ocean, Geochemistry, Geophysics, Geosystems, vol.11, issue.8, pp.14-3297, 2013.
DOI : 10.1002/ggge.20181

T. Williams and C. Kelley, Gnuplot 5.0?an interactive plotting program, 2015.

P. Young, Everything You Wanted to Know About Data Analysis and Fitting But Were Afraid to Ask, 2015.
DOI : 10.1007/978-3-319-19051-8

URL : http://arxiv.org/abs/1210.3781

Y. Zha, S. C. Webb, and W. Menke, Determining the orientations of ocean bottom seismometers using ambient noise correlation, Geophysical Research Letters, vol.9, issue.2, pp.3585-3590, 2013.
DOI : 10.1029/2007GC001814