Anisotropy and Deformation Processes in Southern California From Rotational Observations

Seismic anisotropy in the upper mantle reveals geodynamic processes and the tectonic evolution of the Earth. The two most powerful methods, surface wave tomography, and shear-wave splitting observations, cannot investigate the deep local anisotropy with good vertical and lateral resolution, resulting in poor constraints on plate deformation processes of the complex plate boundary beneath the Southern California region. Here, we show that the amplitude ratio of translational displacement and rotation makes it possible to retrieve the local anisotropy in the upper mantle. Azimuthal anisotropy in the asthenosphere is well determined and resolved in lateral and vertical directions. The fast axis retrieved from amplitude observations indicates the local rapid changes in plate deformation and complex pattern of mantle flow, which is compatible with the distributions of horizontal mantle flow illuminated by geodetic measurements, providing new insights on geodynamic processes of the Southern California region. Rotational motion is the angle of ground rotation observed during Earth's deformation, and the ratio of amplitude to translational motion is sensitive to local structure. In the past few decades, study on the mantle structure inside the Earth has mainly relied on the time difference of seismic waves to calculate azimuth-dependent velocity changes, namely azimuthal anisotropy. Due to the correlation between the direction of maximum velocity propagation and the direction of mantle flow and plate deformation, the study of mantle anisotropy can provide evidence for the evolution of the Earth. However, studying anisotropy based on seismic wave travel time is often affected by heterogeneity, especially in extremely complex structures such as Southern California. The splitting of shear waves can effectively constrain the anisotropy of the mantle in the lateral direction, but its depth resolution is poor. Additional rotational amplitude observations with local sensitivity and depth resolution can provide better constraints on the study of mantle anisotropy, providing new evidence for the direction of mantle flow and plate motion. Local seismic anisotropy revealed for the first time from rotational amplitude observationsDepth dependence of anisotropy in the local upper mantle of Southern California region is well resolved in lateral and vertical directionsThe asthenospheric fast axis matches absolute plate motions, providing new insights on geodynamic processes in Southern California