SHARPNESS OF THE MANTLE DISCONTINUITIES

Seismic estimates of sharpness of the mantle discontinuities are impor tant for constraining models of composition and temperature in the dee p Earth but these data are difficult to obtain. We explore a possibili ty to determine sharpness of the major mantle discontinuities (those a t depths around 400 and 650 km) from the broad-band records of phases converted from P to S underneath the receiver. Our estimates are obtai ned from a comparative analysis of waveforms of the converted phases a nd those of the P waves in the teleseismic records of BRV (Kazakhstan) , GRF (Germany), NRE0 (Norway) and YKW (Canada). The data indicate tha t the shape of the pulse converted at the 400-km discontinuity is clos e to that of the P wave whereas the pulse of the 650-km conversion is substantially broader. We infer from this comparison that the 400-km d iscontinuity is sharp (width of the transition zone is less than 5-7 k m). The 650-km transition is modelled by a linear gradient zone 20 - 3 0 km thick.