The upper-mantle discontinuities underneath the GRF array from P-to-S converted phases
Main Article Content
A data-processing method is applied which includes a rotation of the three components, normalization and delay-and-sum of broadband records of earthquakes from a large distance and azimuth distribution, recorded at a single station (or an array). Clear P-to-S converted phases at the mantle discontinuities are observed in the Grafenberg records, after this data processing. Theoretical seismograms are computed for the PREM model and processed in the same way as the observed data. A comparison with the data shows that the depth interval between the two discontinuities in the mantle transition zone (those at 400 and 670 km depth in PREM) is around 240 km. The 670-km discontinuity is sharper than the 400-km discontinuity and is comparable in sharpness with the crust-mantle transition, as far as it is possible to judge from the broadband data used. There are indications of pronounced lateral heterogeneity of the 400-km transition, underneath GRF. We have also observed converted and multiply reflected shear waves in the crust, which set sensitive limits to the average crustal model underneath the array. These data suggest that the velocity jump at the Moho is smaller than indicated by refraction studies.
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