Characteristics of ambient seismic noise as a source for 2 surface wave tomography

6 [1] Interstation cross correlations of ambient seismic noise from 1 year of continuous data at periods 7 between 6 and 50 s are used to study the origin of the ambient noise using stations located in Europe, 8 southern Africa, Asia, and three regions within North America. The signal-to-noise ratios (SNR) of 9 Rayleigh waves for positive and negative correlation time lags at periods of 8, 14, 25 and 50 s are used to 10 determine the azimuthal distribution of strong ambient noise sources. Ambient noise in both the primary 11 (10–20 s) and secondary microseism bands (5–10 s) comes dominantly from the directions of relatively 12 nearby coastlines with stronger noise occurring in the Northern Hemisphere in northern winter and in the 13 Southern Hemisphere in southern winter, consistent with the hypothesis that oceanic microseisms are 14 generating this noise. The observed differences in the directivity of noise in the primary and secondary 15 microseism bands are the consequence of propagation and attenuation, rather than the location of 16 generation. At intermediate and long periods (>20 s), there is much less seasonal variation in both signal 17 strength and directivity. We argue that our results are explained most simply by near-coastal sources rather 18 than deep ocean sources at all periods. Although the dominant ambient noise sources are distributed 19 inhomogeneously in azimuth, strong ambient noise emerges from most directions when using recordings 20 that are 1 year in duration. Simulations illustrate that this is what ensures the accuracy of the empirical 21 Green’s functions and ambient noise tomography.