SP12RTS: a degree-12 model of shear- and compressional-wave velocity for Earth's mantle

We present the new model SP12RTS of isotropic shear-wave (V _S ) and compressional-wave (V _P ) velocity variations in the Earth's mantle. SP12RTS is derived using the same methods as employed in the construction of the shear-wave velocity models S20RTS and S40RTS, and the same data types. SP12RTS includes additional traveltime measurements of P-waves and new splitting measurements: 33 normal modes with sensitivity to the compressional-wave velocity and 9 Stoneley modes with sensitivity primarily to the lowermost mantle. Contrary to S20RTS and S40RTS, variations in V _S and V _P are determined without invoking scaling relationships. Lateral velocity variations in SP12RTS are parametrised using spherical harmonics up to degree 12, to focus on long-wavelength features of V _S and V _P and their ratio R. Large-lowvelocity provinces (LLVPs) are observed for both V _S and V _P . SP12RTS also features an increase of R up to 2500 km depth, followed by a decrease towards the core-mantle boundary. A negative correlation between the shear-wave and bulk-sound velocity variations is observed for both the LLVPs and the surrounding mantle. These characteristics can be explained by the presence of post-perovskite or large-scale chemical heterogeneity in the lower mantle.