Seismic Anisotropy below the Juan De Fuca Plate: Results from the Cascadia Initiative

Abstract:

Investigation of seismic anisotropy using shear wave splitting is typically used to infer information about asthenospheric flow geometry. In order to understand how such flow varies below the Juan de Fuca plate and is affected by its subduction along the Cascadia forearc, stacked splitting results are determined for 90 offshore and 27 onshore broadband seismometers. These instruments were deployed as part of phases one and two of the Cascadia Initiative, which is an ongoing effort to understand tectonic processes in the Pacific Northwest. The onshore results conform to those of previous studies, suggesting a uniformly trench-perpendicular flow field and thus implying a thick layer of material entrained beneath the slab. The offshore results offer suggestions about how the orientation of asthenospheric flow evolves across the width of the Juan de Fuca plate, and are unique in that they document variation in anisotropy from the ridge to the trench. The results indicate ridge-perpendicular flow geometry close to the Juan de Fuca ridge, which appears to rotate towards the direction of absolute plate motion as one moves towards the trench. As suggested by the latest tomographic images the subducting Juan de Fuca plate is segmented, with ‘gaps’ between the segments. These gaps may act as channels for the flow of mantle material.