The frequency dependence and locations of short-period microseisms generated in the Southern Ocean and West Pacific

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 (R_g) dominate for lower frequencies (<0.55 Hz) while higher frequencies (>0.55 Hz) show a transition to higher mode surface waves (L_g). 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.