Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method-the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P-wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core-mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10-15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.
- Body waves
- Dynamics of lithosphere and mantle
- Mantle processes
- Subduction zone processes
- Wave scattering and diffraction
ASJC Scopus subject areas
- Geochemistry and Petrology