SEISMIC EVIDENCE FOR A 920-KM DISCONTINUITY IN THE MANTLE

Analysis of hundreds of seismograms from a seismic array in the Japane se islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the Tonga subduction zone. This discontinuity is also evident beneath subduction zones in the Japan and Flores seas. T he discontinuity probably represents either a change in mantle chemica l composition or the signature of the subducted slab's garnet layer.