Monitoring austral and cyclonic swells in the “Iles Eparses” (Mozambique channel) from microseismic noise

Abstract

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.

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

“…The source of this noise is located in the southern Ocean, as demonstrated by Davy et al (2015) from the analysis of permanent seismic stations located in the Indian Ocean and also modelled from wave dynamics (Ardhuin et al 2011;. This particular swell event was recently analysed at the seismic stations located in the Mozambique channels (Barruol et al 2016) and the map of this swell propagation in the SW Indian Ocean was presented in their Fig. 3.…”

“…It has been shown that cyclones in the SW Indian Ocean region generate large microseismic noise in both the PM and the SM frequency band. PM sources are generated in coastal areas by direct interaction of the swell with the sloping sea floor, such as shown by the example of the cyclone Felleng (January 2013) hitting the island of Tromelin located 600 km NNW of La Réunion (Barruol et al 2016). SM sources are dominantly generated in the deep ocean by standing waves developing mainly in the neighbourhood of storm areas occurring at high latitudes (Stutzmann et al 2009;Ardhuin et al 2012), but also in low latitude areas and associated to tropical storms such as demonstrated for cyclone Dumile that occurred in early January 2013 (Davy et al 2014).…”

“…When high-amplitude ocean waves of 12-25 s period generated by a distant storm impact a shoreline, interactions of incident waves with opposing components reflected from the coast may generate a local source of long-period secondary microseism, referred here as LPSM but also defined as LPDF (Bromirski et al 2005;Beucler et al 2014;Koper & Burlacu 2015). Analysing the seismic noise in the PM or the LPSM frequency bands, which are both generated near the coast, may therefore provide a way to characterize the ocean waves locally in terms of amplitude, period, and sometimes, direction of propagation, as well as the local impact of swell on the shore (Barruol et al 2016).…”

Seismological Observations of Ocean Swells Induced by Typhoon Megi Using Dispersive Microseisms Recorded in Coastal Areas

“…In recent years, a new method of observing swells emerged, based on seismic records of typhoon-generated noise (i.e., "microseisms") [15][16][17][18][19][20]. Seismic noise is used here as a proxy for swell monitoring in areas where ocean wave gauges are poorly instrumented.…”

Multi-trace-element sea surface temperature coral reconstruction for the southern Mozambique Channel reveals teleconnections with the tropical Atlantic

“…A mangrove formation ranging from shrub to forest stands grows in the salt waters of the back and windward edge of the lagoon (Lambs et al, 2016). Europa is characterized by a high-energy environment under the influence of south to southeasterly trade winds (strongest in austral winter) and occasionally impacted by tropical cyclones in austral summer (Barruol et al, 2016). A train of anticyclonic ocean eddies traveling through the Mozambique Channel transports tropical water southward, eventually feeding the Agulhas Current (Beal et al, 2011).…”