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In this article the authors have analyzed seismic noise to obtain new information on the structure of Earth's crust. By correlating the data from a month of ambient noise due in part to wave-wave interactions in the ocean 2 detected by 62 long-period seismograph stations in southern California, they determined the seismic response that they would have obtained from Earth's crust if they had applied forces at each of their stations. In particular, they measured the times that it took for seismic surface waves to propagate between every pair of stations. They then used lomographic techniques to create a map of seismic wave velocity with an unprecedented horizontal resolution of 75 to 100 km.

The observation of continuously excited free oscillations of the Earth, in the absence of earthquakes, was first made by Japanese scientists in 1998. Since then, attention has focused on elucidating the physical mechanism responsible for them. The mechanism must be shallow, as fundamental modes appear to be preferentially excited and it shows seasonal variability. An array-based method has been developed to detect and locate sources of very long period surface wave energy, utilizing the dispersive properties of Rayleigh waves and data from two large aperture arrays of very long period seismometers, in California and in Japan. It is shown that, for each array, there is a well defined preferential direction, which is stable over one season but changes significantly from winter to summer. The fluctuations as a function of time of the maximum stack amplitudes are correlated across the two arrays and point to the northern Pacific Ocean in the northern hemisphere winter and the southern oceans in the summer, correlating with the distribution of maximum wave height. It is inferred that the background oscillations originate primarily in the oceans, and are caused by a non-linear coupling mechanism involving the atmosphere (winds), the oceans (infragravity waves) and the seafloor. [ABSTRACT FROM AUTHOR]