SHARPNESS OF THE MANTLE DISCONTINUITIES

Seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep Earth but these data are difficult to obtain. We explore a possibility to determine sharpness of the major mantle discontinuities (those at depths around 400 and 650 km) from the broad-band records of phases converted from P to S underneath the receiver. Our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the P waves in the teleseismic records of BRV (Kazakhstan), GRF (Germany), NRE0 (Norway) and YKW (Canada). The data indicate that the shape of the pulse converted at the 400-km discontinuity is close 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 discontinuity is sharp (width of the transition zone is less than 5-7 km). The 650-km transition is modelled by a linear gradient zone 20 - 30 km thick.