A Synoptic View of the Formation, Evolution, and Shallow Subduction of the Juan de Fuca and Gorda Plates

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The installation of a large array of ocean bottom and land seismometers for the Cascadia Initiative provides unprecedented coverage of the Juan de Fuca and Gorda plates, offshore, and in Cascadia, onshore. The array makes it possible to study the structure and processes of an oceanic plate from its formation at the spreading center, through its evolution, and ultimate subduction beneath the continent. The scientific goals are to image the magma producing regions beneath the Juan de Fuca and Gorda spreading centers, to understand the thermal evolution of an oceanic plates and to determine how the plate deforms over time. In the region where the plate begins to bend downward at the onset of subduction, special attention will devoted to the Locked and Transition Zones of the major fault separating the downgoing plate from the over-riding plate. The new 3-D model that will emerge will encompass much of the locked zone that fails in major earthquakes across Cascadia, which pose perhaps the greatest seismic hazard to the US.This study focuses on the development of a 3D model of the crust and uppermost mantle to a depth of between 60 km and 100 km, depending on the maximum period of observation. The model will encompass isotropic structures as well as the radial (or polarization) and azimuthal anisotropy of shear velocities. The work will be based primarily on Rayleigh and Love wave dispersion data derived from ambient noise and earthquakes, which will be interpreted within the framework of a Bayesian Monte Carlo inversion to generate a distribution of accepted models from which uncertainties will be estimated. Receiver functions and Rayleigh wave horizontal/vertical amplitude ratios will be assimilated in the inversion wherever practical. The seismological models, and the data used to construct them, will be archived and shared with the broader geoscience community. High level data products such as dispersion maps, 3-D models, receiver functions, other surface wave observables like H/V ratios, and research codes are delivered through dynamically updated, internet-accessible databases. The proposal will also support one graduate student.