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We propose a new approach capable of measuring local seismic anisotropy from 6C (three‐component translation and three‐component rotation) amplitude observations of ambient seismic noise data. Our recent theory demonstrates that the amplitude ratio of 6C cross‐correlation functions (CCFs) enables retrieving the local phase velocity. This differs from conventional velocity extraction methods based on the travel time. Its local sensitivity kernel beneath the 6C seismometer allows us to study anisotropy from azimuth‐dependent CCFs, avoiding path effects. Such point measurements have great potential in planetary exploration, ocean bottom observations, or volcanology. We apply this approach to a small seismic array at Pin˜ $\widetilde{n}$on Flat Observatory (PFO) in southern California, array‐deriving retrieves rotational ground motions from microseismic noise data. The stress‐induced anisotropy is well resolved and compatible with other tomography results, providing constraints on the origin of depth‐dependent seismic anisotropy. Plain Language Summary: In contrast to the well‐known translational displacement of seismic waves, rotation measures the angle of rotation of a point in a medium as it deforms. Traditionally, the travel time of cross‐correlation functions of translational displacement of ambient seismic noise and its corresponding propagation distance is used to calculate the velocity of the subsurface medium. However, our recent theory shows that the amplitude ratio of translational displacement and rotation allows us to reveal the local velocity of the medium beneath the receiver from ambient seismic noise data and to study anisotropy from earthquake events. This paper points out for the first time that the amplitude observations of 6C cross‐correlation functions of ambient seismic noise provide a new approach to measure the azimuth‐dependent velocity in anisotropic media. Key Points: We show a new approach enabling the extraction of local phase velocity from 6C cross‐correlation functions of ambient noise dataAzimuth‐dependent 6C cross‐correlation functions allow us to study local seismic anisotropy and its depth dependenceLocal anisotropy at Pin˜ $\widetilde{n}$on Flat Observatory is compatible with compression stress, providing constraints on stress‐induced anisotropy [ABSTRACT FROM AUTHOR]

We present new P and S teleseismic tomography models of the upper mantle beneath California and Nevada. Improved tomographic methods and the incorporation of new data increase image resolution and seismic structure definition. Technical advancements include the use of 3‐D ray tracing, ray weighting and finite‐frequency sensitivity kernels. Together with the incorporation of modern crustal velocity models to account for crustal delay time, these tomographic advancements reveal slightly higher amplitudes and narrower structures beneath prominent tectonic features in California and Nevada. The addition of new data from the southern California Borderland and northwestern California are particularly useful. Based on our imaging results, we discuss the effects of temperature and partial melt on upper mantle seismic velocity beneath California and Nevada. We interpret the major seismically fast structures in our study area to be Juan de Fuca slab and its downward continuation beneath central Nevada and foundered Farallon lithosphere beneath the southern Sierra Nevada and Transverse Ranges. The seismically slow structures beneath the slab‐free window of northern California Coast Range, the Salton Trough extensional basin, and the Cascadia back arc, are all consistent with the presence of partial melting of decompressing mantle. The imaged seismically fast and slow anomalies imply buoyancy variations and melt productivity that ultimately act on the overlying crust. Plain Language Summary: We present new images of the structure of the upper mantle beneath California and Nevada based on the travel times of seismic waves from around the world. Modern methods and new data increase image resolution and seismic structure definition, and these advancements reveal stronger and narrower structures beneath prominent geologic features in California and Nevada. Based on our results we discuss the effects of temperature and partial melt on upper mantle seismic velocity beneath this region. We interpret structures in the upper mantle beneath central Nevada, the southern Sierra Nevada, the Transverse Ranges, and the Salton Trough. The imaged regions of faster seismic wave speeds are associated primarily with the Juan de Fuca‐Farallon slab. Structures imaged with slower seismic wave speeds indicate the presence of partial melt in the upper mantle. These seismic anomalies imply variations in the upper mantle that ultimately impact the overlying crust in California and Nevada. Key Points: We image upper mantle structures beneath California and Nevada with improved seismic techniques and an expanded data setThe major seismically fast anomalies likely represent fragments of Farallon oceanic lithosphereSlow anomalies are consistent with elevated temperatures and partial melt of decompressing mantle [ABSTRACT FROM AUTHOR]