Constraining large-scale lower mantle attenuation and anisotropy structure using novel observations of whole Earth oscillations

Abstract:

Seismic body wave tomography has been very successful in imaging Earth's internal variations in seismic wave velocity for the lower mantle. These studies show evidence for large low-shear velocity provinces (LLSVP's) under the Pacific and Africa, but it is still debated if these are mainly thermal or compositional in origin. There have also been numerous observations of shear wave anisotropy in the D" region, but its regional extend is still unclear because body waves have limited global coverage. Body wave and surface wave studies have been able to image upper mantle variations in 3D attenuation, but information on lower mantle attenuation is limited. Thus, many fundamental question regarding the origin of lower mantle structure remain unanswered, such as the presence of post-perovskite, compositional heterogeneity or partial melt.

Here, we will use long period whole Earth oscillations, or normal modes, to constrain lower mantle attenuation and anisotropy and attempt to answer some of these questions. The advantage of normal modes is that they do not require approximations to include scattering and focussing, they are sensitive to the whole mantle including the D" region and also have full global coverage because they are not limited by uneven distribution of earthquakes and seismometers. Using a data set of recent large earthquakes, we measure both elastic splitting functions (dependent on velocity, density and anisotropy) and anelastic splitting functions (dependent on attenuation) for spheroidal and toroidal modes and use these to make interpretations regarding lower mantle attenuation and anisotropy structure. Our anelastic splitting functions show that the LLSVP's are weakly attenuating and surrounded by strongly attenuating regions potentially due to the presence of post-perovskite. By combining our observations of spheroidal modes (mainly sensitive to P and SV velocity) with new observations of toroidal modes (mainly sensitive to SH velocity) we also investigate the global existence of radial anisotropy in the D" region, in comparison with previous body wave studies.