Strong phases are sometimes observed between P and PP for earthquakes at epicentral distances 80¿--120¿. The present study analyzes as an example broadband data from Chilean earthquake recorded in Europe by th NARS network, and by worldwide distributed GEOSCOPE stations. Clear phases are observed in the P-coda only for the European stations.

Underside reflections at upper mantle discontinuities beneath the PP bounce point a the Mid-Atlantic Ridge are unable to explain these phases. A discontinuity in the lowermost mantle at the top of D' is also discarded as a possible explanation.

Reflections and conversions at a strong reflector about about 200 km deep beneath Chile provide a satisfying explanation of the observations. The P-coda is dominated by waves leaving the focus as S-waves. The focal solution and the resulting radiation pattern explain the variations with azimuth and epicentral distance of the phases observed in the P-coda.

The reflector at 200 km beneath Chile may be the upper boundary of a piece of slab detached from the Nazca plate. Alternatively, it may result from the chemical differenciation of the slab during its subduction. Modelling P-coda generated in other subduction zones may help to specify this differenciation process. ¿ American Geophysical Union 1988