Rayleigh Wave Azimuthal Anisotropy Beneath Hawaii Using PLUME Ocean-Bottom Seismometers

Hawaii is an ideal location at which to study mantle plume dynamics because its central location on the Pacific plate enables analysis of plume-related geophysical anomalies separate from the effects of nearby plate boundaries. Previous understanding of the interaction of a proposed rising plume at Hawaii with flow patterns in the surrounding mantle was limited by a lack of data needed to perform high-resolution imaging of the anisotropic structure around the plume. Multiple geodynamical models show a parabolic flow pattern from the interaction of a rising plume with a moving plate, but the shear wave splitting of SKS waves shows a fast axis that is aligned with the fossil spreading direction. A possible explanation for this surprising result could be that flow in the asthenosphere is confined to a thin layer to which SKS data are insensitive. We prefer to measure the frequency-dependent azimuthal anisotropy of Rayleigh waves to understand variation in azimuthal anisotropy with depth. We use data from 185 events during the second PLUME ocean-bottom seismometer deployment to calculate Rayleigh wave azimuthal anisotropy at ~300 locations around Hawaii by comparing arrivals across a triangle of stations at frequencies between 10-50 mHz. We also use data from the first PLUME deployment to increase resolution near Hawaii. In initial inversions, we investigate how anisotropy changes through lithospheric and upper asthenospheric depths ranging from ~50 to 150 km. Results indicate that the orientation of the fast axis in the lithosphere aligns with the fossil spreading direction, but that at deeper depths this orientation shifts towards the direction of current plate motion. Additionally, a region of circling fast axis directions and low shear velocity anomalies to the west of the Island of Hawaii is indicative of a circular mantle flow associated with a rising mantle plume that does not reach into the upper lithosphere.