ScP; A probe of ultralow velocity zones at the base of the mantle

The core-reflected phase ScP is a sensitive probe of the properties of an ultralow velocity zone (ULVZ) at the base of the mantle. In synthetic experiments, a ULVZ with large depressions in P and S velocities results in ScP having two diagnostic precursors and one later arrival, whose timing depends on ULVZ V_P, V_S, and thickness. Amplitudes of the additional arrivals can be large if ULVZ shear velocity has extreme reductions, such as 30% and greater. Some data examples are presented for southwest Pacific events recorded at short period and broadband arrays in Hawaii, which show evidence for a ULVZ. Resolving all three additional arrivals will help in addressing the strength and ratio of V_S and V_P reductions in ULVZ, and thus the issue of partial melt.