The origin of the microseismic wavefield is associated with deep ocean and coastal regions where, under certain conditions, ocean waves can excite seismic waves that propagate as surface and body waves. Given that the characteristics of seismic signals generally vary with frequency, here we explore the frequency‐ and azimuth‐dependent properties of microseisms recorded at a medium aperture (25 km) array in Australia. We examine the frequency‐dependent properties of the wavefield, and its temporal variation, over two decades (1991–2012), with a focus on relatively high‐frequency microseisms (0.325–0.725 Hz) recorded at the Warramunga Array, which has good slowness resolution capabilities in this frequency range. The analysis is carried out using the incoherently averaged signal Capon beamforming, which gives robust estimates of slowness and back azimuth and is able to resolve multiple wave arrivals within a single time window. For surface waves, we find that fundamental mode Rayleigh waves (Rg) dominate for lower frequencies (<0.55 Hz) while higher frequencies (>0.55 Hz) show a transition to higher mode surface waves (Lg). For body waves, source locations are identified in deep ocean regions for lower frequencies and in shallow waters for higher frequencies. We further examine the association between surface wave arrivals and a WAVEWATCH III ocean wave hindcast. Correlations with the ocean wave hindcast show that secondary microseisms in the lower‐frequency band are generated mainly by ocean swell, while higher‐frequency bands are generated by the wind sea, i.e., local wind conditions.

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Seismic noise is important in determining Earth structure and also provides an insight into ocean wave patterns and long-term trends in storm activity and global climate. We present a long-duration study of seismic noise focused on the Southern Ocean using recordings from the Warramunga Seismic Array, Northern Territory, Australia. Using high-resolution analysis, we determine the seismic slowness and back azimuth of observed seismic noise, microseisms, at hourly intervals through over a decade (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012). We identify three dominant sources of body wave (P) noise in the Southern Ocean which we interpret to originate from a South Atlantic source propagating as PP waves, and Kerguelen Island and Philippine Sea sources propagating as P waves. We also identify surface waves from around the Australian coast. All sources show distinct seasonality and a low, but discernable, interannual variability.

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We extend three‐component plane wave beamforming to a more general form and devise a framework, which incorporates velocity heterogeneities of the seismic propagation medium and allows us to estimate accurately sources that do not follow the simple plane wave assumption. This is achieved by utilizing fast marching to track seismic wave fronts for given surface wave phase velocity maps. The resulting matched field processing approach is used to study the surface wave locations of Rayleigh and Love waves at 8 and 16 s based on data from four seismic arrays in the western United States. By accurately accounting for the path propagation effects, we are able to map microseism surface wave source locations more accurately than conventional plane wave beamforming. In the primary microseisms frequency range, Love waves are dominant over Rayleigh waves and display a directional radiation pattern. In the secondary microseisms range, we find the general source regions for both wave types to be similar, but on smaller scales differences are observed. Love waves are found to originate from a larger area than Rayleigh waves and their energy is equal or slightly weaker than Rayleigh waves. The energy ratios are additionally found to be source location dependent. Potential excitation mechanisms are discussed which favor scattering from Rayleigh‐to‐Love waves.