Observations of S410p and S350p phases at seismograph stations in California

We analyze a new set of seismic data from seismograph stations in California. This data set consists of nearly 5000 S receiver functions for 47 seismograph stations. As a rule, the stacked SRFs display a distinct S410p seismic phase (S wave converted to P at the 410 km discontinuity). The wave paths of S410p sample the upper mantle beneath California and the neighboring region of the Pacific. In northernmost California the S410p travel times are close to those of the IASP91 global model. Further south, S410p usually arrives about 2 s earlier than predicted by the IASP91 model. This early arrival can be explained either by an anomalously high Vp/Vs velocity ratio (1.9 in a 125 km thick layer of the upper mantle versus 1.8 in IASP91), by a depression of the 410 km discontinuity of 15 km, or by a combination of both effects with smaller amplitudes. We observe systematically S350p phase which is converted from a negative discontinuity (with a lower S velocity at the lower side) near a depth of 350 km. The observations of S350p are indicative of a low S velocity layer a few tens of kilometers thick atop the 410 km discontinuity beneath southern California and the neighboring oceanic region. Some receiver functions also display S480p phase, which is interpreted as evidence of an intermittent low-velocity layer in the transition zone.