Tracking Cyclones in the Southwest Indian Ocean with an Ocean-Bottom Seismometer Network

Abstract:

The French-German RHUM-RUM project deployed 57 broadband ocean-bottom seismometers (OBS) over an area of 2000 x 2000 km² between September 2012 and December 2013, spread around La Reunion Island and along the Central and the Southwest Indian Ridges. During this period, seven tropical cyclones propagated over the OBS network, providing the unique opportunity for in situ analysis and spatio-temporal tracking of this source of secondary (i.e twice the frequency of the ocean waves) microseismic noise and pressure fluctuations induced on the seafloor. We performed spectral analysis, seafloor pressure and ground polarization analyses on the continuous OBS data, focusing on cyclone Dumile, which passed directly over the OBS network. We observe that microseisms strongly increase in amplitude in the 0.1-0.45 Hz frequency band as the cyclone approaches and propagates over the instruments, and that this noise amplitude is directly related to the distance and intensity of the cyclone. Analysis of the temporal noise variations across the network permit to locate and track the area of maximum noise amplitude, which points towards the cyclone centre with good accuracy. Polarization analyses show that cyclones generate compressional waves in the water column, which give rise to both compressional and surface waves that propagate through the solid earth. In addition to atmospheric, oceanographic and satellite observations, microseisms recorded on the seafloor may therefore be considered a means for monitoring cyclone evolution and intensity.