New Normal Mode Constraints on the Ratio Between Shear- and Compressional-Wave Velocity Heterogeneity in Earth's Mantle

Abstract:

Free oscillations, or normal modes, of the Earth provide important constraints on large-scale structures in the mantle. Spheroidal normal modes are sensitive to both shear (Vs) and compressional (Vp) wave velocity and have the advantage that they do not suffer from different S-wave and P-wave data coverage as is the case for body waves. Thus, modes are ideal for constraining the ratio R=δln(Vs)/δln(Vp), which may contain information on the presence of thermal or chemical heterogeneity of the mantle and is crucial in the debate on the origin and nature of the two lower mantle Large Low Shear wave Velocity Provinces (LLSVPs) in the lower mantle. According to mineral physics experiments, R is lower than 2-2.5 in an isochemical lower mantle (Karato & Karki, 2001) in the absence of phase transitions. Most of the previously estimated R-values in seismological studies increase to about 3 in the bottom ∼1000 km of the mantle (e.g. Su & Dziewonski, 1997; Romanowicz, 2001; Houser et al., 2008; Koelemeijer et al., 2016), generally interpreted to be due to the presence of chemical heterogeneity.

Here, we will use normal mode data to make tomographic models of 3D variations in Vs and Vp and constrain R in the Earth's mantle. Normal mode spectra can be inverted in two ways, using either 1) a direct spectrum one-step inversion or 2) a two-step inversion with splitting function measurements as intermediate step. Most previous studies have incorporated normal modes by including splitting function coefficients in the two-step inversion. However, this method may suffer from possible non-uniqueness or inconsistencies of the splitting functions. Alternatively, normal mode spectra can also be inverted directly in a one-step inversion, but this method has not been used extensively in normal mode studies because of its high computational cost. We investigate how the mantle R-value depends on the inversion approach, comparing models obtained with either method. We will present a new state-of-the-art model of R for the mantle, based on normal mode data, including Stoneley modes (sensitive to the lowermost mantle) and higher order fundamental modes (sensitive to the uppermost mantle) using the direct one-step inversion method.