Comparison of broadband and short-period seismic waveform stacks: Implications for upper-mantle discontinuity structure

Stacks of short-period and broadband seismic waveforms from four deep (h > 550 km) earthquakes in South America recorded in California exhibit small amplitude signals between the direct P and surface-reflected pP phases. The anomalous and variable slownesses of these signals in short-period stacks, as well as differences between short-period and broadband stacks suggest that the weak arrivals originate by reflection from laterally varying structure in the mantle near the subduction zone. The only structure consistently producing near-source and near-receiver reflections is the “410 km” discontinuity, which varies in depth by ±10 km, and has an impedance contrast comparable with that predicted by reference Earth models.