Collaborative Research: High Resolution Imaging of Deep Mantle Structure Using USArray Data
Principal Investigators:
Thorne Lay, Professor of Earth Sciences, UCSC Ed Garnero, Associate Professor of Geological Sciences, ASU
This project utilizes USArray Transportable Array (TA) and Backbone Array data in high-resolution seismic wave imaging of deep mantle structure. The research plan is designed to scale naturally with the progressive deployment of TA stations in the western U.S, building upon initial results obtained using California regional network data. We are analyzing teleseismic earthquake waves that traverse the deep mantle in three subregions: beneath the southern U.S.-Gulf of Mexico-Central America-Caribbean, under the Central Pacific, and beneath Alaska and northern North America. Seismic methodologies that exploit the relatively dense 70-km station spacing provided by the TA data are being applied. Double-array waveform stacking, Kirchhoff scattering migration in 3D tomographic models, shear-wave splitting analysis allowing for azimuthal anisotropy, and finite frequency tomographic arrival time inversion procedures are the primary methods being applied to broadband P and S waveforms. When the initial TA deployment is completed, it will enable 5-fold increase in deep mantle spatial sampling relative to recently developed localized high resolution models for beneath the Cocos Plate and regional framework structures that we have developed. Particular effort is being focused on the mid- and deep mantle beneath Central America and the Gulf of Mexico, where there is evidence in tomographic models for tabular high velocity structures extending through the lower mantle and spreading broadly in the D" Region, possibly associated with the Farallon slab and with the long history of subduction beneath North America. The velocity gradients, structural continuity, structural fabric, and spatial extent of the deep mantle structure is being examined using arrival time analysis and migration methods, with 2D and 3D synthetic modeling being used to quantify structures and to verify the reliability of the imaging procedures. Corresponding structure beneath the subduction zone under Alaska is being analyzed, and these two convergent zone regions are being compared to the isolated structure under the Central Pacific. We are assessing contributions to deep mantle structure from slab material, a post-perovskite phase transition, deep mantle dynamic flow, and partial melting. These analyses will be closely coordinated with Earthscope investigations of shallow mantle structure, incorporating corrections based on models found by others for receiver structure, lithospheric anisotropy, and upper mantle travel time variations. Our goal is to exploit USArray data to make breakthrough advances in understanding of deep mantle structure and its relationship to mid- and upper-mantle flow associated with the westward movement of the North American continent.
