ALBERT

Description: INTRODUCTION As early as the 1920s, Gutenberg (1926) equated the low-viscosity asthenosphere with a seismic low-velocity zone. Regions of high and low velocity in the mantle are today usually determined with tomographic methods. These methods are, however, not very sensitive to sharp boundaries. Converted waves have been used for many years to study boundaries in the mantle. A velocity reduction with depth (possibly the lithosphere-asthenosphere boundary, LAB) was found with P-to-S converted waves (P-receiver functions) beneath North America at ∼100 km depth with a sharpness of about 10 km (Rychert and Shearer 2009). A comparison of the LAB depth from S-receiver functions (see, e.g., Yuan et al. 2006 and Kumar et al. 2006 for a description of the method and data examples) with that from surface-wave tomography found good agreement in the western United States (Li et al. 2007) and along the East Coast (Rychert et al. 2007). Our results from the California coast are also consistent with the P-to-S and S-to-P images earlier analyzed by Kumar and Kawakatsu (2011), along a profile spanning ocean to continent. A discrepancy exists in the central part of the craton, where the sharp velocity decrease at about 100 km in P-receiver functions (PRF) was confirmed with S-receiver functions (SRF) in contrast to the smooth velocity decrease around 200 km depth known from surface-wave data (Yuan and Romanowicz 2010; Abt et al. 2010; Yuan et al. 2011). The petrophysical nature of the LAB is still very much a subject of discussion (e.g., Eaton et al. 2009; Fischer et al. 2010). We think our results from USArray data significantly contribute to this discussion. The earlier results were obtained with relatively few, sparsely distributed seismic stations (Abt et al. 2010). We apply...