Seismic detection of a low-velocity layer atop the 410-km discontinuity: verification of an electromagnetically detected layer of partial-melt atop the 410-km discontinuity beneath SE Arizona

Receiver functions calculated from a ten-station subset of the Earthscope Transportable Array (TA) show a well-defined 45 km thick low-velocity layer atop the 410-km discontinuity (“410”) beneath the Tucson, Arizona, region. This observation is consistent with a previously detected layer of increased electrical conductivity in the same region, from which the existence a 5-30 km thick layer of partial melt atop the 410 was inferred. Relative to IASP91 the low-velocity layer is characterized with respect to increasing depth as: a 3.4% shear wave velocity decrease over a 10-km interval, underlain by a 25-km thick constant velocity interval, followed by a diffuse 30 km thick bottom gradient over which the shear wave velocity increases by 3.5%. The shear wave velocity increase over the bottom interval is less than predicted by the IASP91 earth model, suggesting the olivinewadsleyite transformation takes place in water-saturated conditions. The diffuse bottom gradient is consistent with a hydrated mantle transition zone, thus the lowvelocity layer is interpreted as a manifestation of the Transition Zone Water Filter model.