Imprints of time-dependent microseism source distributions on the coda of correlations

SUMMARY Seismic interferometry, applied to continuous seismic records, yields correlation wavefields that can be exploited for information about Earth’s subsurface. The coda of the correlation wavefield has been described as multiply scattered waves that are highly sensitive to crustal heterogeneity and its changes. Therefore, the coda of consecutive correlation wavefields allows to monitor velocity variations over time to detect weak changes in the medium at depth. Ocean microseisms, generated by ocean–land interactions, are the dominant continuous source of seismic energy at frequencies below 0.5 Hz. It is well-understood that these oceanic sources are not homogeneously distributed over Earth and change over the seasons, which commonly results in asymmetric correlation wavefields from seismic data. The impact of these seasonal changes on the coda of the correlation wavefield is typically considered negligible. In contrast, we demonstrate that oceanic noise sources and their changes directly impact the composition of the coda. We compute correlation wavefields between several master stations throughout Europe and the Gräfenberg array in Germany. We beam form these correlation wavefields, in the microseism frequency band, to detect coherent waves arriving at the Gräfenberg array. We perform this analysis for a 2-yr period, which enables us to compare variations in source direction over the seasons. We find seismic waves arriving from dominant sources to the North–Northwest of Gräfenberg in boreal winter (with slownesses corresponding to surface waves) and towards the South in summer (with slownesses corresponding to body waves) throughout the entire correlation wavefield, including its late coda. Beam forming the original recordings before cross-correlation confirms that the seasonally dominant source regions are directly detected also in the correlation wavefield coda. We derive that seismic waves propagating from isolated microseism source regions will be present in correlation wavefields even if the master station, or ‘virtual source’, used for correlation recorded no physical signal at all. The findings we present raise concerns about velocity monitoring approaches relying on the coda being comprised exclusively of scattered waves. Our results also suggest that higher order correlations do not achieve an effectively more homogeneous source distribution, and instead may even enhance such bias.