Rayleigh-wave Ambient Noise Analysis for the OHANA Experiment 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.

Continuous 4-component data (broadband ground motion and pressure) were recovered at 24 sites.  In a top-down approach, we start with the assembly and analysis of ambient-noise cross-correlation functions (CCFs) of the vertical components, between 5 and 35 s. The CCFs contain prominent waveforms from overtones that can help improve resolution as a function of depth.

We present the analysis of path-averaged dispersion curves for the fundamental mode. Forward modeling and the inversion of the average dispersion across the OHANA network both indicate normal oceanic crust over a fairly typical mature oceanic lithosphere though shear velocities in the upper lithosphere are a few percent lower than is expected for a 50-Myr old lithosphere. Velocities in the mid-to-lower lithosphere may be 2-3% higher than expected but resolution degrades with increasing depth. We observe significant and internally consistent azimuthal anisotropy in both the fundamental mode as well as the first overtone. We juxtapose this analysis to an earthquake-based analysis that reaches deeper into the lower lithosphere and upper asthenosphere.