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

Abstract

Tropical Cyclones (TC) represent the most destructive natural disaster affecting the islands in the South-West Indian Ocean (SWIO) each year. Monitoring ocean activity is therefore of primary importance to secure lands, infrastructures and peoples, but the little number of oceanographic instruments makes it challenging, particularly in real time. Long-term seismological records provide a way to decipher and quantify the past cyclonic activity by analyzing microseisms, seismic waves generated by the ocean activity and propagating through the solid Earth. In the present study, we analyze this microseismic noise generated by cyclones that develop in the SWIO basin between 1999 and 2020, using broadband seismic stations in La Réunion. The power spectral density (PSD), together with the root mean square (RMS) analyses of continuous seismic data recorded by the permanent Geoscope RER seismic station, indicate the intensification of the microseismic noise amplitude in proportion to the cyclone intensity. Thus, we establish a relationship between the cyclone intensity and the PSD of the Secondary Microseisms (SM) in frequency band ∼0.14 to 0.25 Hz (4 to 7 s period). The Pearson coefficient between the observed and estimated TC intensity are >0.8 in the presence of a cyclone with mean wind speeds >75 km/h and with a seismic station distance-to-storm center D < 3000 km. A polarization analysis in the time and frequency domains allows the retrieval of the backazimuth of the SM sources during isolated cyclone events and well-polarized signal, i.e., CpH > 0.6. We also analyzed the RMS of the Primary Microseisms (PM frequency between ∼0.05 and 0.1 Hz, i.e., for 10 to 20 s period) for cyclones passing nearby La Réunion (D < 500 km), using the available temporary and permanent broadband seismic stations. We also found high correlation coefficients (>0.8) between the PM amplitude and the local wave height issued from the global hindcast model demonstrating that the PM amplitude can be used as a robust proxy to perform a real-time wave-height monitoring in the neighboring ocean. Transfer functions are calculated for several cyclones to infer wave height from the seismic noise amplitude recorded on land. From the analysis of two decades of data, our results suggest that it is possible to quantify the past ocean activity for as long as continuous seismic archives are available, emphasizing microseismic noise as a key observable for quantifying and understanding the climate change.

Seismic Signature of Rain and Wind Inferred From Seismic Data

“…Better monitoring of the river activity and the sediment transport could help to characterize the processes at play and to minimize the induced damages.During the last few years, the cyclone impact on tropical islands has been the subject of a large multidisciplinary project called ReNovRisk involving atmosphere, hydrosphere, solid Earth and human sciences (Barthe et al, 2021;Bousquet et al, 2021;Tulet et al, 2021). In this frame, seismology has been used to analyze and quantify oceanic swells from terrestrial records (Rindraharisaona et al, 2020(Rindraharisaona et al, , 2021 and river activities during cyclonic floods (e.g., Gonzalez, 2019) to develop proxies for sediment transports, motivating finer signal analyses…”

Impact of Tropical Cyclones on Inhabited Areas of the SWIO Basin at Present and Future Horizons. Part 1: Overview and Observing Component of the Research Project RENOVRISK-CYCLONE

“…In the Indian Ocean, most remote sources of seismic noise are located in the southernmost part of the Austral Ocean basin and are associated with storm systems moving around Antarctica [84,87,88]. Some noise sources may also develop at tropical latitudes in association with tropical cyclones [41] (this Special Issue). Recent seismic deployments on the ocean floor enabled the making of in situ observations of SM underneath TCs in the neighborhood of Réunion Island [37] and confirmed the possibility to track TC and TS from the ocean bottom.…”

“…Another original approach, based on the previous work of [37][38][39], was also further investigated to quantify extreme swell phenomena from microseismic noise measurements recorded by ground seismometers (see Section 3.1.2.). The preliminary assessment of terrestrial seismic observations collected in Réunion Island against oceanographic records and offshore wave model data have demonstrated that land-based seismic stations could be particularly useful to observe both austral [40] and cyclonic swell [41] (this Special Issue).…”

“…The swell-noise amplitude correlation can be used to build a transfer function to translate the amplitude of the seismic noise in terms of swell height. Although such a relation is station-related, and therefore not universal, it was nevertheless shown to provide particularly good results for strong swell events [40,41,81].…”

“…Terrestrial seismic stations therefore provide alternative and complementary observations of both TC and ocean activities. In some cases, the availability of several decades of seismic archives may also provide new opportunities to derive cyclone climatologies [41].…”