A ubiquitous low‐velocity layer at the base of the mantle transition zone

Abstract

Abstract Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of ~600 km, indicating a low‐velocity layer at the base of the mantle transition zone. The slant‐slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X‐discontinuity at ~300 km depth is also observed in our global stacks, which can be explained by the coesite‐stishovite phase transformation.