Assessing swells in La Réunion Island from terrestrial seismic observations, oceanographic records and offshore wave models

Abstract

SUMMARY La Réunion Island in the southwest Indian Ocean is seasonally affected by austral swells among which some extreme events may have strong impacts on coastal infrastructures. The very limited number of sensors available on and around the island and in the whole SW Indian Ocean impedes any direct monitoring of the swell activity. In this study, we analyse direct observations of the ocean swell by combining terrestrial measurements of the microseismic noise with in situ oceanographic observations issued from two pressure gauges and an Acoustic Doppler Current Profiler (ADCP), together with swell numerical modelling. The reliability of the terrestrial seismic station to characterize the ocean activity in both the primary and secondary microseisms peaks (PM and SM, respectively), and also in the long period secondary microseismic peak (LPSM) for the case of La Réunion Island is presented and discussed here. By computing the hourly RMS of the PM and LP(SM) amplitudes, we establish a transfer function between the PM and (LP)SM amplitude and the maximum wave height, which appears to be valid for any PM and LPSM amplitudes >0.15 μm and >1.0 μm, respectively. The correlation coefficient between the PM amplitude and the wave height is >0.92. It suggests that the PM amplitude can be used as a robust proxy for the swell height and may help calibrating the wave heights from other independent observable. For some swell events, we observe LPSM that correlate well (>0.91) with the local wave height suggesting a generation by coastal swell reflection. From polarization and spectral analyses, directions and periods of swells are also well retrieved from seismic data. Finally, continuous measure of the SM amplitude shows that it can be used as precursor information for distant swells that may hit La Réunion Island a few days after their generation in the southern Indian Ocean.

Cyclone Signatures in the South-West Indian Ocean from Two Decades of Microseismic Noise

“…The black dashed-dotted line indicates the median of daily average during the quiet period in 2019, i.e., in the absence of cyclone or austral swell activities. Austral swells are indeed generated by strong winds and long-distance fetches, in general by depressions moving around Antarctica, which affected La Réunion mainly in winter season, between April and October e.g., [5,29]. Some of them occurring in September and October 2018 are indicated by the pink boxes in Figure 2c.…”

“…A CpH of 1 corresponds to a perfectly polarized signal in the corresponding plane (i.e., a linear ground motion), while a value of 0 characterizes a random (circular) ground motion. More details about these parameters are presented in Section S1 and can be found in previous works [5,7,26,27]. In this work, we focused our attention to the azimuth of significantly polarized signal corresponding to CpH > 0.6.…”

“…Combining the azimuth issued from polarization analysis with the delays between the arrival time of PM and SM, it should be feasible to get an approximate location of the SM source, using a single seismic station e.g., [5]. However, polarization must be used with caution since, as explained above, the results may be biased by simultaneous SM sources from other storms worldwide, and in the case of La Réunion, by the strong depressions moving around Antarctica and generating austral swells [29].…”

“…There are two types of microseisms, which differ from their frequencies and origins: the Primary Microseisms (hereafter called PM, with frequency ∼0.05-0.1 Hz) and Secondary Microseisms (SM, with frequency range ∼0.1 to 1 Hz). PM have the same period as the ocean wave e.g., [3] and have been used as a wave proxy e.g., [4,5]. They are accepted to be generated in the shallow waters through the interaction between the sloping seafloor pressurization by ocean waves e.g., [6,7].…”

“…In addition, SM are also created in the vicinity of storm centers due to the interactions of two waves from opposite directions, which are generated by the same storm at different times e.g., [8,9,12]. SM sources have also been detected in coastal regions where the sea waves reflected at the coastal slope may interact with the incoming swell e.g., [5,8,13,14]. This last origin of SM is also known as long period secondary microseisms (LPSM).…”