Long wavelength structure of Earth's lowermost mantle from normal mode splitting function inversions

The lowermost part of the Earth's mantle forms a thermal boundary layer atop the core-mantle boundary (CMB). This D'' layer has a strong influence on the nature of mantle convection and the core heat flow. Its complex seismic structures provide an insight into the material properties and processes occurring at the bottom of the mantle. Large-low-shear velocity provinces (LLSVPs) dominate tomographic shear wave velocity (Vs) models. The question remains whether these long wavelength features are dominantly thermal or thermochemical structures. Information on their density and shear to compressional wave velocity (Vp) ratio is vital for assessing their influence on mantle dynamics. Earth's normal modes are particularly useful to study these long wavelength features due to their global character. More importantly, the splitting of modal spectra is affected by density structure in addition to velocity. In particular, Stoneley modes, which are modes confined to solid-liquid interfaces such as the CMB, are mainly sensitive to structures in the D''. Observations of these modes provide an improved depth resolution and unique constraints on the long wavelength structures in the deep mantle. Several large magnitude earthquakes have occurred in the last decade increasing the amount of available data for normal mode observations significantly. We make use of recent normal mode splitting function observations derived from model spectra up to 10 mHz for 93 events with Mw larger than 7.4 in the period 1978-2011. Our data include 33 modes sensitive to compressional wave velocity and 8 CMB Stoneley modes. We combine our normal mode data with independent constraints from body waves and surface waves and invert independently for lateral Vs and Vp structures in Earth's mantle. We particularly focus our analysis on the relative distribution of Vs and Vp anomalies and perform tests for possible density structures in order to determine the long wavelength characteristics of the LLSVPs.