Azimuthal anisotropy quantified by teleseismic SKS, SKKS, PKS (“XKS”) and Local S wave splitting parameters is used to investigate mantle deformation and flow beneath the boundary of the North American and Caribbean plates and adjacent areas. A total of 2556 XKS and 299 pairs of local S wave splitting parameters were obtained at 23 stations. The observations can be divided into two groups based on the spatial distribution of the resulting fast polarization orientations. Those observed on the Caribbean Plate are mostly WNW-ESE which are mostly trench-parallel. In contrast, the fast orientations observed on the North American Plate are dominantly NNE-SSW which are approximately trench-orthogonal and are consistent with those previously observed in southern Mexico to the north of the area of the current study. At most of the stations at which XKS and local S wave splitting parameters are available, the splitting parameters from the two types of shear waves are comparable, suggesting that the observed azimuthal anisotropy is mostly from the mantle wedge above the slab. The observations especially those obtained at recently deployed stations in Guatemala provide new insights into the complicated mantle flow system associated with slab subduction and rollback, as well as lithospheric shearing along the southern boundary of the North American Plate.

To discern spatial and explore possible existence of temporal variations of upper crustal anisotropy in an ~15 km section of the San Jacinto Fault Zone (SJFZ) that is composed of the Buck Ridge and Clark faults in southern California, we conduct a systematic shear wave splitting investigation using local S-wave data recorded by three broadband seismic stations located near the surface expression of the SJFZ. An automatic data selection and splitting measurement procedure is firstly applied, and the resulting splitting measurements are then manually screened to ensure reliability of the results. Strong spatial variations in crustal anisotropy are revealed by 1694 pairs of splitting parameters (fast polarization orientation and splitting delay time), as reflected by the dependence of the resulting splitting parameters on the location and geometry of the raypaths. For raypaths traveling through the fault zones, the fast orientations are dominantly WNW-ESE which is parallel to the faults and may be attributed to fluid-filled fractures in the fault zones. For non-fault-zone crossing raypaths, the fast orientations are dominantly N-S which are consistent with the orientation of the regional maximum compressive stress. A three-dimensional model of upper crustal anisotropy is constructed based on the observations. An apparent increase in the raypath length normalized splitting times is observed after the 03/11/2013 M4.7 earthquake, which is largely attributable to changes in the spatial distribution of earthquakes before and after the M4.7 earthquake rather than reflecting temporal changes of upper crustal anisotropy.