EVIDENCE FOR PARTIAL MELT AT THE CORE-MANTLE BOUNDARY NORTH TONGA FROM THE STRONG SCATTERING OF SEISMIC-WAVES

Scattered waves that precede the seismic phase PKP (which traverses th e Earth's core) have been used to identify and locate small-scale hete rogeneity in the Earth's mantle(1-6). A recent study has demonstrated that the global data set of these precursors is consistent with weak h eterogeneity (about 1 per cent r.m.s. velocity variation) distributed throughout the mantle(7). Here we show, however, that anomalously larg e PKP precursors from earthquakes in northern Tonga require much stron ger heterogeneity (10-15 per cent r.m.s. velocity variation) in a laye r about 60 km thick near the core-mantle boundary below Tonga. This re gion of the core-mantle boundary is also marked by low shear-wave velo cities in the lower mantle(8) and is near an area of very low compress ional-wave velocity in the lowermost tens of kilometres of the mantle( 9), which has been interpreted as evidence for the presence of partial melt(10). The strength of the scattering that we observe provides str ong support for the presence of partial melt in this area, and also su ggests that vigorous small-scale convection is taking place at the cor e-mantle boundary.