D" structure beneath Alaska, more evidence for (Mg,Fe)O phase

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 post-perovskite (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 of 6 MPa/K and a velocity jump of 1.5% when corrected by tomographic predictions, it does not explain the first feature. Sun and Helmberger [2008] expanded on this mapping approach by attempting a new parametrization that requires a sample of D" near the ScS bounce point (δV_S) where the phase height and velocity jump are functions of δV_S. These parameters are determined by modeling dense record sections collected from PASSCAL data and most sampling regions concentrated beneath Central America. Here, we expand our sampling region beneath Alaska with the dense USArray records, providing coverage into the shadow zone beyond 90°. Both SH and SV are studied where 1D modelling suggests that a δV_SH = 3.5% increase ~240 km above the CMB with δV_SV about half. The distorted waveforms at distances large than 85° require the addition of a strong reduction of shear velocities, 6 to 10%, with a thikcness of about 60 km, which could caused by various amounts of iron-rich (Mg,Fe)O-perovskite mixtures. To map-out the lateral variation of D" layer in this region,we apply a recently developed 2D stagged-grid Finite-difference code to generate synthetics for hybrid D" models transformed from the latest global tomographic images.