Rayleigh wave phase velocities, small‐scale convection, and azimuthal anisotropy beneath southern California

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We use Rayleigh waves to invert for shear velocities in the upper mantle beneath southern California. A one‐dimensional shear velocity model reveals a pronounced low‐velocity zone (LVZ) from 90 to 210 km. The pattern of velocity anomalies indicates that there is active small‐scale convection in the asthenosphere and that the dominant form of convection is three‐dimensional (3‐D) lithospheric drips and asthenospheric upwellings, rather than 2‐D sheets or slabs. Several of the features that we observe have been previously detected by body wave tomography: these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern Sierra Nevada and Walker Lane region; sinking lithosphere beneath the southern Central Valley; upwelling beneath the Salton Trough; and downwelling beneath the Transverse Ranges. Our new observations provide better constraints on the lateral and vertical extent of these anomalies. In addition, we detect two previously undetected features: a high‐velocity anomaly beneath the northern Peninsular Range and a low‐velocity anomaly beneath the northeastern Mojave block. We also estimate the azimuthal anisotropy from Rayleigh wave data. The strength is ∼1.7% at periods shorter than 100 s and decreases to below 1% at longer periods. The fast direction is nearly E‐W. The anisotropic layer is more than 300 km thick. The E‐W fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms: pure shear in the lithosphere due to N‐S tectonic shortening and simple shear in sublithosphere mantle due to mantle flow.