Sources of Secondary Microseisms in the Indian Ocean

Abstract:

Ocean waves activity is a major source of micro-vibrations that travel through the solid Earth, known as microseismic noise and recorded worldwide by broadband seismometers. Storms are accepted to represent an important source of noise in the ocean basins, and thus, microseisms analysis of continuous seismic records can be used to localize the noise sources in the ocean and to follow their variations in space and time. In order to locate and quantify the noise sources in the Indian Ocean, we analyzed one year (2011) of continuous data recorded by permanent seismic stations localized in the Indian Ocean. From the Rayleigh wave polarization analysis performed at each individual stations, we retrieved the number of polarized signals in the time-frequency domain and their back-azimuths. Polarization spectra show that secondary microseisms are more polarized between 6 and 10 s of period. We observe seasonal variations in the number of polarized signals with much more detections during the austral winter. On the other hand, we do not observe seasonal variations in the noise back-azimuth directions, suggesting that the dominating microseismic noise sources are always located in the southernmost Indian Ocean, except for cyclonic episodes that are restricted in space and time. Compared to the Pacific and Atlantic oceans, this Indian Ocean property can be explained by its closed geometry on the North and by the absence of large storms in the Northern part of the basin during the boreal winter. We show that the results of this polarization analysis are in good agreement with the expected source areas computed from ocean wave numerical model.