UC San Diego

Resolving oceanic crustal structure contributes to our understanding of crustal magma supply, intraplate deformation, and dynamics in the underlying mantle. Accurate structural models can also provide insight into the mechanisms of processes caused by tectonic movements such as earthquakes and volcanism. This study presents the initial data analysis of the OHANA array, an array of ocean bottom seismometers (OBSs) deployed in the Pacific Ocean halfway between Hawaii and California. OHANA provides data from a dense seismic array for an area of oceanic lithosphere that previously had little data coverage. We measured phase velocity dispersion curves from ambient noise Rayleigh waves for the fundamental mode and first overtone branches. The resulting dispersion curves are used in inversions for maps of phase velocity at several frequencies and vertical 1D shear velocity models. We find that our phase velocity maps do not show significant lateral differences across the array area. In addition, our lower frequency maps have to be adjusted to take into account the effects of the overlying water layer on phase velocity. We also find that our 1D shear velocity model is faster than we expect for the age of the lithosphere within our study area, 40-50 Myr, with shear velocities more similar to 110+ Myr old lithosphere. This significant finding requires further evaluation to determine its validity or identify a mechanism for these anomalously fast shear velocities.