The characteristics of ambient seismic noise as a source for surface wave tomography

In this study, we cross-correlate waveforms of ambient seismic noise between 6 and 100 sec to study the origin of ambient seismic noise. We use one year of data recorded at numerical stations located in Europe, North America, and southern Africa. We measure the signal-to-noise ratios (SNR) of Rayleigh waves for positive and negative correlation time lags at periods of 8, 14, 25 and 50 sec to determine the azimuthal distribution of strong ambient noise sources. Ambient noise at both primary (10-20 sec) and secondary microseism bands (5-10 sec) comes dominantly from the direction of nearly coastlines with stronger noise occurring in the northern hemisphere in northern winter and in the southern hemisphere in southern winter, consistent with the hypothesis that oceanic microseisms are generating this noise. The observed differences in the directivity of noise at primary and secondary microseismic bands are the consequence of propagation and attenuation, rather than the generation mechanism. At intermediate and long periods (> 20 sec), there are no appreciable seasonal variations in both signal strength and directivity. Our results are consistent with near-coastal sources rather than deep ocean sources. Even with the inhomogeneous distribution of the dominant noise sources, there is nearly azimuthally homogenous ray coverage in surface wave tomography from one year of ambient noise data.