Lower mantle tomography and phase change mapping

Abstract

A lower mantle S wave triplication (Scd) has been recognized for many years and appears to be explained by the recently discovered perovskite (PV) to postperovskite (PPV) phase change. Seismic observations of Scd display (1) rapid changes in strength and timing relative to S and ScS and (2) early arrivals beneath fast lower mantle regions. While the latter feature can be explained by a Clapeyron slope (γ) of 6 MPa/K and a velocity jump of 1.5% when corrected by tomographic predictions, it does not explain the first feature. Here, we expand on this mapping approach by attempting a new parameterization that requires a sample of D" near the ScS bounce point (δVS) where the phase height (hph) and velocity jump (β) are functions of (δVS). These parameters are determined by modeling dense record sections collected from USArray and PASSCAL data where Grand's tomographic model is the most detailed in D" structure beneath Central America. We also address the range of γ to generate new global models of the phase boundary and associated temperature variation. We conclude that a γ near 9 MPa/K is most satisfactory but requires β to be nonuniform with a range from about 1.0 to 4.0% with some slow region samples requiring the largest values. Moreover, the edges of the supposed buckled slabs delimitated by both P and S waves display very rapid changes in phase boundary heights producing Scd multipathing. These features can explain the unstable nature of the Scd phase with easy detection to no detection commonly observed. The fine structure at the base of the mantle beneath these edges contains particularly strong reflections indicative of local ultralow velocity zones, which are predicted in some dynamic models.