Attenuation in the upper mantle beneath Southern California: Physical state of the lithosphere and asthenosphere

We invert phase and amplitude data of Rayleigh waves for attenuation (Q^-1) and shear wave velocities beneath southern California using teleseismic sources recorded by the TriNet/USArray network. Fundamental mode surface wave studies from 25 to 143 s period allow us to constrain the vertical variation of shear quality factor Q_μ in the upper mantle. We use 2-D sensitivity kernels for surface waves based on single-scattering (Born) approximation to account for the effects of scattering on amplitude. A one-dimensional shear velocity model reveals a pronounced low velocity zone (LVZ) from ∼80 km to ∼200 km underlying a high velocity lid. Q_μ shows a similar pattern; large Q_μ at depths shallower than 80 km and much smaller Q_μ at depths greater than 100 km. Models that attribute the variations of attenuation and shear velocities with depth solely to temperature and pressure effects predict too low Q_μ values if they match the shear velocities. Alternative models considering the presence of partial melt can explain the observed very low Vs velocities in the asthenosphere. Partial melt in the asthenosphere could be generated due to decompression and the reduced solidus for damp mantle when the asthenosphere rose to fill the space left by the subducted Farallon plate.