Characteristics of seismic noise: Fundamental and higher mode energy observed in the northeast of the Netherlands

Abstract

We study seismic noise recorded in the northeast of the Netherlands by beamforming and by using empirical Green’s functions obtained by seismic interferometry. From beamforming we found differences in noise directions in different frequency bands. The main source region for primary microseisms (0.05–0.08 Hz) is in the west-northwest direction, while the secondary microseisms (0.1–0.14 Hz) have a west-southwest back azimuth. Furthermore, we observed a fast (~4 km/s) arrival corresponding to the Rayleigh wave overtone. This arrival is also in the secondary microseism band (between 0.15 and 0.2 Hz), but has a west-northwest back azimuth. Both arrivals in the secondary microseism band gain in strength during winter, as does the average wave height in the North Atlantic.We measured phase velocity dispersion curves from both beamforming and noise cross-correlations, as well as group velocity from the latter. These are then jointly inverted for an average 1D S-wave model. The results show how the combination of different methods leads to a more complete characterization of the propagation modes and an improved knowledge of the subsurface, especially as the group velocity measurements increase the upper frequency limit of analysis, providing valuable information of the shallowest subsurface.