Rayleigh wave phase velocities beneath the oceanic and continental margin of the North American and Pacific plate boundary

The Pacific-North America plate boundary, located in Southern California, presents a unique opportunity to study tectonic stability and deformation of the Pacific and North America plates from a migrating oceanic spreading center and its subduction beneath the North American plate up until 30 Ma (Atwater, 1989). The ongoing rotation of the Transverse ranges and Borderlands following this event make this an active plate boundary that presents tectonic stresses and seismic hazards for the local area. I consider offshore seismometer coverage to the west side of the plate boundary system. I use Rayleigh waves recorded by an array of 34 ocean bottom seismometers deployed offshore southern California for 12 month duration from August 2010 to September 2011. The array recorded teleseismic earthquakes filtered for 16 s – 78 s at distances ranging from 30° to 120° with good signal-to-noise ratios for magnitudes Mw ≥ 5.8. The inversion technique considers non-great circle path propagation by representing the arriving wave field as two interfering plane waves. Phase velocities averaged over the study area are 1.3% lower than the velocities found by other surface wave studies of a seafloor ages of 20-52 Ma. Phase velocities offshore southern California in the oceanic mantle align in the N-S direction and show increasing velocities with older seafloor ages to the west at periods of up to 45 s. At 55 s, the velocity structure changes to an E-W alignment indicating a transition from oceanic lithospheric below 40 s (53 km depth) to asthenospheric mantle flow aligned with Pacific plate motion above 55 s (73 km depth). ). Variations in the N-S pattern of velocities that increase with age indicate a slightly sinusoidal pattern consistent with spacing between offshore fracture zones that suggests these surface features involve deformation at lithospheric depths to at least 50 km. Anisotropy is consistent with Pacific plate motion, N 78.5˚ W, in a NW-SE direction for all regions in my study area, except for the periods above 40 s in the Borderland where anisotropy demonstrates N-S alignment. Phase velocities at lithospheric depths are 5 % higher in the oceanic mantle compared to the continental mantle indicating compositional and structural differences due to formation history in the two tectonic environments.