Origin of deep ocean microseisms by using teleseismic body waves
Résumé
Recent studies of oceanic microseisms have concentrated on fundamental mode surface waves. Extraction of fundamental mode Rayleigh and Love wave Green functions from station-station correlations of ambient seismic noise has recently been demonstrated to be a very powerful tool for imaging of the Earth's crust and uppermost mantle. In this study we concentrate on energetic arrivals in two frequency bands around the primary (14 s) and the secondary (7 s) microseismic peaks that appear at near-zero times in noise cross correlations. Thanks to a polarization analysis of data from the Eastern Turkey Seismic Experiment network, we identify this “near-zero time” signal as an upcoming P wave in the secondary microseismic frequency band (5-10 s). In a second step, analyzing noise cross correlations from three different arrays (in Yellowstone, in Turkey, and in Kyrgyzstan), we determine the origin of these signals by means of beam-forming analysis and its projection on the Earth. Our results show that in the 0.1-0.3 Hz frequency band, the energetic “near-zero” time arrivals in seismic noise cross correlations are mainly formed by teleseismic P, PP, and PKP waves. Generation of this ambient body waves in the secondary microseismic band presents a marked seasonal behavior with sources located in southern and northern oceans during summer and winter, respectively. Moreover, body wave array analysis is accurate enough to confirm that significant amount of the microseism energy is generated far from the coast in deep oceans.
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