A Reference Vs and Vp/Vs Model for Young Oceanic Crust From Controlled-Source OBS Data

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Determining the properties of oceanic lithosphere and how they vary as the lithosphere ages is one of the primary methods that scientists use to infer its composition and evolution. Reference models of oceanic crust use the speed at which seismic waves travel through the Earth (i.e., seismic velocity) as their main parameter. However, current models use primarily compressional seismic waves (waves that vibrate in the same direction as they travel). This project will build a shear velocity (waves that vibrate at right angles to the direction of travel) reference model for oceanic crust and upper mantle. The model will be derived from data collected in 2012 across the Juan de Fuca tectonic plate offshore Oregon and Washington. Crustal ages in this region vary from 0 to 9 million years. This model will provide a reference framework to interpret ocean crust properties in terms of composition, modification by magmatic activity, and extent of deformation, alteration, and fracturing. The model can also be used to quantify the amount of water in oceanic crust entering deep-sea trenches that influence natural hazards such as megathrust earthquakes and arc volcanism at subduction zones. This project will support a postdoctoral investigator and a graduate student. By making use of existing seismic datasets, this project will also leverage previous investments in marine seismic acquisition.The 2012 ocean bottom seismometer (OBS) dataset includes high-quality recordings of converted waves that can be exploited to constrain shear-wave (S) velocity structure through the sediments, crust, and uppermost mantle at a plate scale from the Juan de Fuca Ridge near-axis region to the Cascadia deformation front, encompassing crustal ages from 0 to ~9 Ma. This dataset will be used to: (1) Constrain S-wave velocity structure in the shallowmost igneous crust by modeling the amplitude vs. offset of compressional (P) and P-to-S reflections from the basement and performing advanced pseudo-1D nonlinear Bayesian waveform inversion of OBS data based on a Markov Chain Monte Carlo method and time-domain elastic finite-difference simulation of P-Sv wave propagation. (2) Advanced processing of the OBS records using the theory of supervirtual refraction interferometry for improving the signal-to-noise ratio and detectability of mantle refracted S waves. (3) Conduct traveltime tomography modeling of the 2-D S-wave velocity structure of the crust and uppermost mantle along the Ridge-to-Trench transects using wide-angle crustal and mantle refracted/reflected P-to-S converted phases. (4) Investigate the azimuthal anisotropic character of the crustal and mantle S-wave velocity structure. The results will be interpreted in terms of hydrothermal alteration and porosity in-filling due to mineral precipitation within the crust and mantle as a function of age using effective medium theory. The final deliverable product of this project will be a reference model describing the shear-wave structure of oceanic crust and uppermost mantle for crustal ages of 0-9 Ma built from synthesizing the project’s results and integrating them with existing results from other seismic experiments in the region.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.