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Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseismic noise. Here we use a network of 57 ocean bottom seismometers (OBS) deployed around La Réunion Island in the southwest Indian Ocean to investigate the noise generated in the secondary microseismic band as a tropical cyclone moved over the network. Spectral and polarization analyses show that microseisms strongly increase in the 0.1–0.35 Hz frequency band as the cyclone approaches and that this noise is composed of both compressional and surface waves, confirming theoretical predictions. We infer the location of maximum noise amplitude in space and time and show that it roughly coincides with the location of maximum ocean wave interactions. Although this analysis was retrospectively performed, microseisms recorded on the seafloor can be considered a novel source of information for future real-time tracking and monitoring of major storms, complementing atmospheric, oceanographic, and satellite observations.

In order to investigate the lithospheric structure of the Mozambique channel (Indian Ocean), we deployed 5 broadband three-components seismic stations between April 2011 and January 2014 on the Islands of Europa (EURO), Juan de Nova (JNOV), Mayotte (MAYO), and Glorieuses (GLOR) in the Mozambique channel and on Tromelin Island (TROM) located ca. 450 km east of Madagascar. We performed measurements of teleseismic shear wave splitting using SKS and SKKS phases and receiver function analyses to characterize the nature and thickness of the crust and the underlying upper mantle structure. Seismic anisotropy is observed at all seismic stations and display a rather homogeneous pattern: average values of the splitting parameters show fast polarization trending between N112°E (EURO) and N120°E (JNOV) to N81°E (MAYO). Observed delay times vary between 0.8 (JNOV) and 1.0 s (MAYO). In Mayotte, analysis of individual backazimuthal variation of splitting parameters suggests an upper mantle more complex than a single anisotropic layer. Station TROM located on Tromelin Island shows similar fast polarization azimuth of N123°E but slightly higher delay time (1.4 s). Fast polarization directions are compared with values predicted by drag-induced anisotropy and absolute plate motion direction and with lithospheric fossil spreading directions. Results are also compared to orientation predicted by large-scale mantle convection models. Receiver functions observations and modelling of P-to-S conversions at the Mohorovičić (Moho) discontinuity have been employed to investigate the variations in the Moho depth, the nature of the crust and of the crust-mantle transition. Preliminary results suggest a Moho depth of 16 km beneath Europa Island