Seismic heterogeneity and small‐scale convection in the southern California upper mantle

Abstract

We invert traveltime residuals of teleseismic P and S phases for 3‐D perturbations in VP, VS, and VP/VS structures in the southern California upper mantle. The tomographic inversion uses frequency‐dependent 3‐D sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals. In addition to separate VP and VS tomography, we jointly invert the P and S data sets for VP/VS perturbations by imposing a smoothness constraint on the δlnVS/δlnVP field. The regional upper mantle is very heterogeneous with the greatest amplitude of velocity and VP/VS perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern California. Our imaging improves the resolution of the geometry and amplitude of these features. In addition to the major structures imaged and discussed in previous papers, we find a large high‐velocity anomaly at depths between ∼340 and 500 km beneath the Borderland, Transverse Ranges, and northern Peninsular Ranges. This anomaly is separate from the well‐known uppermost mantle Transverse Ranges anomaly, which extends no deeper than ∼175–200 km. The strongest low‐velocity, high VP/VS anomaly is found beneath and west of the Salton Trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning. The magnitudes of VP, VS, and VP/VS perturbations and knowledge of regional 1‐D velocity and attenuation lead us to conclude that the asthenosphere contains up to ∼1% partial melt extending to depths as great as 150–200 km.