Shallow anisotropy in the Mediterranean mantle from surface waves

Abstract

[1] We present new evidence for the existence of the LoveRayleigh discrepancy in the Mediterranean region and constrain the average polarization anisotropic structure of the Mediterranean mantle. We analysed regional Rayleigh and Love waveforms recorded at 3-component broadband seismic stations. None of the 3-component seismograms could be fit with a single 1D isotropic and smooth velocity model. However, satisfactory fits for individual Rayleigh and Love waveforms could often be obtained using realistic 1D velocity models. We used these 1D path-average velocity structures to derive 3D S-velocity models for the Mediterranean region. Our results show that Love waves require higher velocities (about 200 m/s) compared to Rayleigh waves between 30 and 120 km depth. We relate the observed anisotropy to lattice-preferred orientation of crystallographic axes of elastically anisotropic minerals such as olivine.

An upper-mantleS-wave velocity model for Northern Europe from Love and Rayleigh group velocities

“…Given the sensitivity of Love waves in the frequency range of our data, we restrict our discussion in terms of anisotropy to 50 and 90 km depths. The strongest anomalies at 90 km depth in Central Europe correlate nicely with similar anomalies found by Marone et al (2004b). At 50 km depth, positive and large values of anisotropy cover almost the entire Northeast Atlantic, eventually extending underneath the continent in Southern and Central Norway.…”

“…Although this level of heterogeneity is commonly considered to be high for a mantle model, similar levels of heterogeneities have been found in comparable regional studies (e.g. Lévêque et al 1998 for the Indian Ocean, Debayle & Kennett 2000 for Australia, Marone et al 2004a,b for the Mediterranean region or Feng et al 2004, 2007 for South America). We have made a series of tests, as detailed in the previous section, to ensure that the location and amplitude level of the heterogeneities is not a result of underdamping in the inversion.…”

Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions

“…Global seismic anisotropy has been detected by Tanimoto (1990, 1991), Park and Levin (2002), and Shapiro and Ritzwoller (2002); and anisotropy in limited regions has been reported by Montagner and Jobert (1988), Ribe (1989), Ekstro m and Dziewonski (1998), Silveira et al (1998), Ritzwoller et al (2001), Villaseň or et al (2001, Meissner et al (2002), Raykova and Nikolova (2003), and Marone et al (2004). Some evidences for anisotropy in the specific area of Tibet has been reported by Song et al (1991), Lavé et al (1996), Huang et al (2004), Ozacar and Zandt (2004), Shapiro et al (2004), and Lev et al (2006).…”

“…In other words, the Rayleigh and Love dispersion curves can be well fitted simultaneously by the same 1D isotropic, elastic depth-dependent shear-wave velocity model in an isotropic medium. However, when horizontally polarized Love waves and vertically polarized Rayleigh waves propagate with distinct speeds (V SH -V SV ), the Love-Rayleigh (L-R) discrepancy can be explained in terms of transverse isotropy of the elastic medium (Lévêque et al, 1998;Debayle and Kennett, 2000;Muyzert and Snieder, 2000;Silveira and Stutzmann, 2002;Raykova and Nikolova, 2003;Marone et al, 2004;Fouch and Rondenay, 2006). In a previous paper (Chen et al, 2009), we computed averaged Rayleigh and Love wave group-velocity dispersion curves for each main tectonic unit of the Tibetan Plateau from the local group velocities determined on each 2°Â 2°grid-cell falling into the target tectonic Shapiro et al (2004) reported the Tibetan middle crust requires the presence of relatively strong radial anisotropy according to the observed group-velocity dispersion.…”

Theory and Observations - Seismic Anisotropy

“…It is absent from some regions, for example, northern Europe (Muyzert et al, 1999). The values of x $ 1.08, corresponding to 4% difference in SH and SV velocity, are however very common below the Moho and down to at least 200 km depth (Freybourger et al, 2001;Gaherty, 2007;Marone et al, 2004;Panning and Romanowicz, 2006) (see Figure 19 for some examples). Larger values of x $ 1.18 (Debayle and Kennett, 2000;Maupin and Cara, 1992) are challenging to reconcile with olivine LPO and may require additional fine-horizontal layering.…”