Analyses of extreme swell events on La Réunion Island from microseismic noise.

Ocean 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 SMcontains 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. [ABSTRACT FROM AUTHOR]