Advances in analysing the composition of the microseismic wavefield have benefitted from the deployment of the Large Aperture Seismic Array (LASA) in Montana, North America in the early 60’s. Utilising LASA and beamforming methods to separate the microseisms wavefield in slow- ness space (or frequency-wavenumber space) fundamental and higher-mode Rayleigh, Love and P waves were reported (Toksöz and Lacoss, 1968; Lacoss et al., 1969). In the PM band, Love waves with twice the amplitude of Rayleigh waves were identified, while in the SM band<0.2Hz fundamental mode Rayleigh waves dominate. Between 0.2 0.3 Hz a mix of Rayleigh and P waves were observed and at 0.3 0.6 Hz the P waves dominate. However, as the authors point out, arrays closer to the coast will likely show a higher portion of fundamental mode Rayleigh waves at higher frequencies. These results are confirmed by an independent study on the same array (Haubrich and McCamy, 1969) and additionally show that P waves generation regions differ with frequency.
Rayleigh waves
The generation regions of Rayleigh waves are predominantly near coastlines (e.g. Friedrich et al., 1998; Bromirski and Duennebier, 2002; Bromirski et al., 2005; Chevrot et al., 2007; Bromirski et al., 2013) where SM excitation can occur (i.e. where swell/wind sea is present). Generation of Rayleigh waves occurs in the deep ocean far off coastlines (Cessaro, 1994; Stehly et al., 2006; Chevrot et al., 2007; Kedar et al., 2008; Obrebski et al., 2012), but is rarely observed on land. The generation locations of higher mode Rayleigh waves do not have to coincide with fundamental mode Rayleigh wave generation directions (Brooks et al., 2009) and a seasonal influence can oc- cur (Tanimoto et al., 2006). Multiple locations, North America (Toksöz and Lacoss, 1968; Lacoss et al., 1969), New Zealand (Brooks et al., 2009) and the Netherlands (Kimman et al., 2012) find higher mode Rayleigh waves in the frequency range of⇠0.15 0.2 Hz. This frequency range is also consistent with a study performed in the South Central Pacific with ocean bottom seismome- ters (OBS) (Harmon et al., 2007), where higher mode Rayleigh waves are observed between 3.5 - 7s, while the fundamental mode is observed between 2-16s. In general, the generation regions of Rayleigh waves are variable with time, depending on the swell / wind sea conditions, but gen- eration regions that are dominant over the period of a full year have been observed as well (e.g. Tanimoto et al., 2006).
Love waves
The presence of Love waves in the microseisms spectrum, is now well established (e.g. Toksöz and Lacoss, 1968; Lacoss et al., 1969; Haubrich and McCamy, 1969; Capon, 1973; Friedrich et al., 1998; Campillo and Paul, 2003; Chevrot et al., 2007; Lin et al., 2008; Nishida et al., 2008a,b;
Hadziioannou et al., 2012; Matsuzawa et al., 2012; Behr et al., 2013), but is to date not fully understood. For the PM where direction interaction between the ocean bottom and the ocean gravity wave occurs, Saito (2010) derived that shear traction of ocean waves acting on the sea bottom can excite Love waves. For SM, a vertical point pressure force acting on the sea bottom, such as the one assumed for the wave-wave interaction (Longuet-Higgins, 1950; Hasselmann, 1963), should not excite Love waves. Hence, the mechanism has so far not been determined. Apart from fundamental mode Love waves, higher mode Love waves are also present in the microseisms wavefield (e.g. Nishida et al., 2008b).
The generation region for Love waves differs with frequency. For the PM band, the gen- eration regions of Rayleigh and Love waves can be the same, but when observed from a single array, the generation region differ (e.g. Matsuzawa et al., 2012; Behr et al., 2013). The excitation mechanism in the PM band proposed by Saito (2010) is subject to a radiation pattern for Love waves, which are generated perpendicular to the propagation direction of the ocean gravity wave and no generation occurs inline with the propagation direction. This radiation is visible in obser- vational analysis (e.g. Matsuzawa et al., 2012; Behr et al., 2013), although not explicitly stated in the latter work. For the SM band, the generation regions of Love waves seem to be closely related to Rayleigh waves (Toksöz and Lacoss, 1968; Lacoss et al., 1969; Haubrich and McCamy, 1969; Friedrich et al., 1998; Nishida et al., 2008a; Hadziioannou et al., 2012; Behr et al., 2013) and the H/Z component ration is<1 (Friedrich et al., 1998; Nishida et al., 2008a) unlike in the PM band where it is>1 (Toksöz and Lacoss, 1968; Lacoss et al., 1969; Nishida et al., 2008a).
Lgwaves
Recently, theLgphase was observed in the short period microseisms wavefield (Koper et al., 2009,
2010). TheLgphase is a supercritical S wave trapped in the crustal wave guide. Since theLgphase
does not propagate over ocean-continent margins (Zhang and Lay, 1995), the generation of this phase should be connected to near coastal generation or some sort of scattering process from a different wave type along its ray path towards the seismic array. Analysing 18 International Mon- itoring Stations (IMS) arrays, Koper et al. (2010) found theLg phase to be the dominant part of
the short period microseisms band (0.25-2.5s), with a contribution of about 50% to the vertical microseisms wavefield.
Compressional body waves
Apart from surface waves, P waves are present in the wavefield (e.g. Toksöz and Lacoss, 1968; Lacoss et al., 1969; Haubrich and McCamy, 1969; Capon, 1973; Gerstoft et al., 2008; Koper et al., 2009, 2010; Zhang et al., 2009; Landès et al., 2010; Zhang et al., 2010; Traer et al., 2012; Boue et al., 2013; Obrebski et al., 2013). A difference between surface and body waves is their localiziation. While surface waves propagate along great circle paths on of the Earth’s surface, assuming no refraction due to anisotropic geology occurs, body waves travel through the Earth and their angle of incident carries information on the location of generation. The apparent velocity of the body wave or the angle of incident can be use to backproject the ray path to the location of origin with the help of traveltime tables such as ak135 (Kennett, 2005). Apart from the direct P
wave andPn/Pg(e.g. Koper et al., 2010), PP and inner core phases PKP are present (e.g. Gerstoft
et al., 2008; Koper and de Foy, 2008; Landès et al., 2010) and more exotic phases (Boue et al., 2013).
The generation regions of body waves are predominantly in deep oceans (e.g. Lacoss et al., 1969; Gerstoft et al., 2008; Zhang et al., 2010) and dependent on the frequency (e.g. Haubrich and McCamy, 1969; Zhang et al., 2009). In general, P wave generation regions are in agreement with the main storm paths in the northern and southern hemisphere, while little generation is found around the equatorial region. Similar to surface waves, P wave sources are subject to seasonal variation (e.g. Zhang et al., 2010) and show strongest excitation during the winter months of the particular hemisphere. In the short period microseisms range, strong P waves have been observed (Zhang et al., 2009) in close proximity to the array and correlate well with local wind conditions, i.e. are generated by wind swell in shallow locations.
Shear body waves
To date, no array analysis observed S body waves with beamforming to infer their generation lo- cation. The absence of S waves in three component (3C) beamforming studies suggests that the power of these waves is relatively weak.