Rayleigh Waves From OHANA OBSs in the Northeast Pacific Ocean Reveal Low Deep Shear Velocities and Pervasive Azimuthal Anisotropy

Abstract The 2021–2023 OHANA ocean‐bottom seismometer deployment in the northeast Pacific Ocean provides a rich data set for seismic studies to explore the crust, lithosphere and asthenosphere in a 600 km wide region about 1,500 km northeast of Hawaii, west of the Moonless Mountains. The study area covers mainly 40‐to‐55 Myr‐old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of typical oceanic lithosphere that has a normal plate cooling history or if there is evidence for plate rejuvenation from a secondary mantle‐dynamic process. We present the analysis of path‐averaged low‐frequency (10–60 mHz) Rayleigh‐wave phase‐velocity curves from earthquake records retrieved for 210 two‐station pairs, and its inversion for depth‐dependent shear velocity, Vs . Forward modeling and the inversion of the average phase velocity indicate lower shear velocities than anticipated for 50‐Myr old lithosphere. Velocities in the mid‐to‐lower lithosphere and in the asthenosphere are 2%–3% lower than expected. We also observe significant and internally consistent azimuthal anisotropy of up to 5%, where the “fast direction” places between the fossil and modern plate motion directions. A preliminary analysis of ambient‐noise cross‐correlation functions reveals similar patterns for the fundamental mode as well as the first overtone.