Rayleigh Waves from the OHANA Project Indicate Low Mantle Velocities as well as Anisotropy in the Northeast Pacific

The 2022-2023 OHANA OBS deployment in the northeast Pacific Ocean provides a rich dataset for comprehensive seismic studies to explore the crust, lithosphere and asthenosphere in a 600-km wide region west of the Moonless Mountains. The study area covers mainly 40-to-50 Myr old Pacific lithosphere. A fundamental question to be addressed is whether this particular area has the signature of a typical oceanic lithosphere that has a normal plate cooling history. Alternatively, we seek evidence for a previously proposed reheating process, e.g. resulting from small-scale shallow-mantle convection. Given its location, the OHANA experiment contributes crucial data to the Pacific Array initiative.We present the analysis of path-averaged Rayleigh wave dispersion curves obtained from earthquake records. The average dispersion across the OHANA network indicates lower shear velocities than expected for a 50-Myr old crust and lithosphere. Velocities in the mid-to-lower lithosphere appear to be 2-3% lower than expected. Compared to other recent broadband OBS deployments in the Pacific Ocean, we image a profound reduction in shear velocity throughout the entire asthenosphere though imaging fidelity declines with depth.We observe strong and internally consistent azimuthal anisotropy, where the ‘fast direction’ places between fossil and modern plate motion directions. This anisotropy dominates over isotropic lateral heterogeneity in the crust and lithosphere.