Analysis of waves converted from S to P in the upper mantle beneath the Baltic Shield

Teleseismic data recorded by broadband stations of the Swedish National Seismic Network have been used in a study of waves converted from S to P in the uppermost mantle. S to P converted waves are recorded as precursors to the direct S arrival. This enables us to study seismic discontinuities at depths of 100–300 km where the traditional receiver function technique based on P to S conversion fails due to disturbances from crustal reverberations. Clear signals of S to P conversion are seen for several horizons in the 50–200 km depth range. This is interpreted as a layered lithosphere with alternating high and low velocity bodies. A sharp contrast is imaged at depths around 160 km and continuous for almost 1000 km in the southern part of the profile. This structure coincides with the velocity contrast interpreted by several authors as the lower limit of a prominent low velocity zone in the 95 to 160 km depth range. A feature which we identify as the lithosphere–asthenosphere boundary is clearly imaged at depths around 200 km.