Dominant seismic noise sources in the Southern Ocean and West Pacific, 2000–2012, recorded at the Warramunga Seismic Array, Australia Anya M. Reading 1 , Keith D. Koper 2 , Martin Gal 1 , Leon S. Graham 1 , Hrvoje Tkalčić 3 , and Mark A. Hemer 4 1 School of Physical Sciences (Earth Sciences) and CODES Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania, Australia, 2 Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA, 3 Research School of Earth Sciences, Australian National University, Acton, ACT, Australia, 4 Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia Abstract 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 (2000–2012). 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. 1. Introduction Seismic noise, also known as ambient energy, and as microseisms, is the term given to the background signal observed continuously, at relatively low, fluctuating amplitudes, on seismic stations worldwide. These stations were originally intended for the recording of higher-amplitude transient signals from earthquakes or man-made explosions. As most seismic stations record continuously, a wealth of archival data exists for seismic noise analysis. A spectral analysis of seismic noise [Aster et al., 2010; Berger et al., 2004; Stutzmann et al., 2000] reveals two distinct peaks in power spectral density at periods of approximately 14 s (primary microseisms, PM) and 7 s (secondary microseisms). The source of these peaks may be traced to excitation pathways with ocean sources, direct pressure on the seafloor and standing waves from the interaction of incident and reflected waves in the deep ocean [Cessaro, 1994]. The frequency doubling effect for the latter case, secondary microseisms, is responsible for their commonly used alternative name: double frequency (DF) microseisms. The connection between ocean storm activity and “hum,” lower frequency seismic noise [Rhie and Romanowicz, 2006], has been established, and a large data set from USArray has been used in the analysis of hum, PM, and DF microseisms over a whole year [Traer et al., 2012]. Seismic noise sources have a dual significance to the global geophysical community. First, as a source of energy for earth structure determination [Harmon et al., 2010; Poli et al., 2012; Prieto et al., 2009; Romanowicz, 2008; Tkalčić et al., 2012; Zhang et al., 2010]. Such analysis techniques have become popular following the realization that Rayleigh waves may be extracted from ambient seismic noise, and the dispersion properties used to model the seismic structure [Campillo and Paul, 2003; Shapiro and Campillo, 2004]. An improved understanding of the location and variability of dominant sources will enable optimization of Earth structure studies that use this ambient seismic noise. Seismic noise has a second significance as it provides a strong indication of storm activity in the deep ocean. Deep ocean storm activity is of global interest owing to its role in the uptake of atmospheric CO 2 [Canadell et al., 2007; Le Quere et al., 2009]. The variability of CO 2 uptake under different storm conditions [Gruber et al., 2009] is one reason for concern regarding the changing storminess of the Southern Ocean and its investigation using both in situ waverider buoy observations and satellite data [Hemer , 2010; Hemer et al., 2010; Young et al., 2011]. The seismic noise record has the potential to provide an independent data stream to monitor Southern Ocean storm activity [Aster et al., 2010; Stutzmann et al., 2009]. In this data-poor region, additional evidence may aid the future resolution of the apparent discrepancy READING ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1 PUBLICATION S Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL060073 Key Points: • Long-duration (2000–2012) seismic array study of Southern Ocean noise sources • Three P wave sources dominate: South Atlantic, Kerguelen, and Philippine Sea • Noise sources show strong seasonality and slight interannual variability Supporting Information: • Readme • Text S1 • Figure S1 • Figure S2 • Figure S3a • Figure S3b Correspondence to: A. M. Reading, anya.reading@utas.edu.au Citation: Reading, A. M., K. D. Koper, M. Gal, L. S. Graham, H. Tkalčić, and M. A. Hemer (2014), Dominant seismic noise sources in the Southern Ocean and West Pacific, 2000–2012, recorded at the Warramunga Seismic Array, Australia, Geophys. Res. Lett., 41, doi:10.1002/2014GL060073. Received 3 APR 2014 Accepted 8 MAY 2014 Accepted article online 13 MAY 2014