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.

S U M M A R YOcean waves activity is a major source of microvibrations that travel through the solid Earth, known as microseismic noise and recorded worldwide by broadband seismometers. Analysis of microseismic noise in continuous seismic records can be used to investigate noise sources in the oceans such as storms, and their variations in space and time, making possible the regional and global-scale monitoring of the wave climate. In order to complete the knowledge of the Atlantic and Pacific oceans microseismic noise sources, we analyse 1 yr of continuous data recorded by permanent seismic stations located in the Indian Ocean basin. We primarily focus on secondary microseisms (SM) that are dominated by Rayleigh waves between 6 and 11 s of period. Continuous polarization analyses in this frequency band at 15 individual seismic stations allow us to quantify the number of polarized signal corresponding to Rayleigh waves, and to retrieve their backazimuths (BAZ) in the time-frequency domain. We observe clear seasonal variations in the number of polarized signals and in their frequencies, but not in their BAZ that consistently point towards the Southern part of the basin throughout the year. This property is very peculiar to the Indian Ocean that is closed on its Northern side, and therefore not affected by large ocean storms during Northern Hemisphere winters. We show that the noise amplitude seasonal variations and the backazimuth directions are consistent with the source areas computed from ocean wave models.

S U M M A R YOcean wave activity excites seismic waves that propagate through the solid earth, known as microseisms, which, once recorded on oceanic islands, can be used to analyse the swell. Here, we analyse the microseismic noise recorded in different period ranges by the permanent seismic station RER on La Réunion Island and by a temporary network of 10 broad-band seismic stations deployed on the island to analyse extreme swell events. We perform a comparative analysis of cyclonic and austral swell events by analysing not only the primary (PM, ∼10-20 s period) and secondary (SM, ∼3-10 s) microseisms but also the long-period secondary microseisms (LPSMs, ∼ 7-10 s), which may result from the interaction between incident ocean waves and the reflected waves off the coast. We compare the microseismic observations with buoy data when available and with hindcasts from numerical ocean wave models. We show that each cyclone is characterized by its own individual signature in the SM, which depends not only on its distance and intensity but also on its dynamics and trajectory. Thus, the SM contains relevant information for cyclone detection and monitoring. Analysing the PM and the LPSM, and comparing it to direct buoy observations and/or wave numerical models allows characterizing the local impact of the swell with the island in terms of amplitude, period, and sometimes, direction of propagation, making possible to use a seismic station as an ocean wave gauge. The microseisms, which link the atmosphere, the ocean and the solid Earth, can thus provide valuable observations on extreme swell events, in addition to oceanic and meteorological data.

a b s t r a c tWe deployed five broadband three-components seismic stations in the Iles Eparses in the south-west Indian Ocean and on Mayotte Island, between April 2011 and January 2014. These small and remote oceanic islands suffer the effects of strong ocean swells that affect their coastal environments but most islands are not instrumented by wave gauges to characterize the swells. However, wave action on the coast causes high levels of ground vibrations in the solid earth, so-called microseismic noise. We use this link between the solid earth and ocean wave activity to quantify the swells locally. Spectral analyses of the continuous seismic data show clear peaks in the 0.05e0.10 Hz frequency band (periods between 10 and 20 s), corresponding to the ocean wave periods of the local swells. We analyze an example of austral swell occurring in August 2013 and a cyclonic event (Felleng) that developed in January 2013, and quantify the ground motion at each station induced by these events. In both cases, we find a linear polarization in the horizontal plane with microseismic amplitude directly correlated to the swell height (as predicted by the global swell model WaveWatchIII), and a direction of polarization close to the predicted swell propagation direction. Although this analysis has not been performed in real time, it demonstrates that terrestrial seismic stations can be efficiently used as wave gauges, and are particularly well suited for quantifying extreme swell events. This approach may therefore provide useful and cheaper alternatives to wave buoys for monitoring swells and the related environmental processes such as beach erosion or coral reef damages.