Towards Toroidal Mode Constraints on Large-Scale Mantle Anisotropy

Abstract:

Seismic anisotropy provides constraints on Earth’s dynamic properties, which helps to understand Earth’s mantle flow and deformation. A key method to investigate seismic anisotropy in the deep mantle are whole Earth oscillations or normal modes, which are standing waves along Earth’s surface and radius. They are excited after strong earthquakes and are sensitive to Earth’s large scale structure. Normal modes can be divided in (i) spheroidal modes, involving a combination of horizontal and vertical surface motion and (ii) toroidal modes, dominated by horizontal surface motion. Toroidal modes are our main interest here. In combination with spheroidal modes, they provide important large-scale constraints on Earth’s anisotropic mantle structure.

Normal modes are most conveniently measured using the splitting function approach (e.g. Giardini et al., 1986). They enable us to recover radial and azimuthal anisotropy for P- and S-velocity simultaneously. Since the studies by Resovsky & Ritzwoller (1998) and Tromp & Zanzerkia (1995), toroidal mode splitting functions haven’t been measured. Here, we will expand the more recent spheroidal mode studies by Deuss et al. (2013) and Koelemeijer et al. (2013), by focussing specifically on toroidal mode observations by adding horizontal component data for all new large earthquakes from the last 35 years.

We extend self-coupling splitting function measurements for isolated toroidal fundamental modes and overtones using horizontal data recordings and will interpret these for radial anisotropy in for example de D” region. In addition, toroidal mode energy may also become visible on the vertical component instead of only the horizontal components due to cross-coupling (i.e. exchange of energy) between fundamental toroidal and spheroidal modes. The effect of rotation of the Earth on toroidal-spheroidal cross-coupling is well known. Here, we will investigate the occurrence of additional cross-coupling due to radial and azimuthal anisotropy which may provide important information of the anisotropic structure of Earth’s mantle (Beghein et al., 2008). Furthermore we investigate the influence of toroidal-spheroidal mode cross-coupling on core-mantle boundary sensitive Stoneley modes, which may affect the interpretation of lower mantle density (Koelemeijer et al. 2017).