New Constraints on Mantle Shear Velocity Structure Offshore Cascadia From the Joint Analysis of Teleseismic Body and Rayleigh Wave Data

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

To better constrain mantle dynamics offshore Cascadia, we jointly analyze teleseismic body wave delay times and Rayleigh wave phase and amplitude observations for 3D shear velocity (Vs) structure. Rather than simultaneously invert both seismic datasets, we instead progress through an alternating series of inversions in which the body and Rayleigh wave data are independently inverted for Vs perturbations. At each step in the series, the starting model is taken as the solution from the previous step thus coupling the two datasets. Preliminary results demonstrate that this simple approach is effective in yielding solutions that fit both datasets while also highlighting the consistencies and inconsistencies between them. A notable feature required by both datasets is a sharp gradient in seismic velocity east of the Juan de Fuca (JdF) Ridge that cannot be attributed to conductive cooling alone (e.g. Bell et al., 2016; Byrnes et al., 2017). The body wave data illuminate a deeper (~200 km) low velocity region beneath the JdF Ridge than recovered by the Rayleigh wave data alone. In the upper 100-200 km of the mantle beneath the Gorda plate, northern and southern ends of the JdF Ridge, and near the shelf, we observe a negative correlation among sequential velocity corrections in later iterations required by the two datasets, indicating fundamental inconsistencies. These differences are small compared to the absolute velocity variations, but still are significant. We consider the role of seismic anisotropy and the contrasting sensitivities of these seismic phases to Earth structure as possible explanations for these discrepancies.

Presenting Author: Douglas Toomey

Authors