This project focuses on 1) the development of a new tomographic method that integrates the complementary resolving power of seismic surface waves and scattered body wave phases and 2) the application of this method to data from the permanent Backbone Network component of the EarthScope USArray, complemented by data from the Transportable Array. This work will result in higher resolution constraints on the three-dimensional anisotropic seismic velocity structure of the North American upper mantle, with a particular focus on the local thickness and structural heterogeneity of the continental lithosphere and asthenosphere. Higher resolution models will address questions regarding the evolution of the continental lithosphere and its dynamic relationship to the deeper mantle. How do the properties of the continental lithosphere vary as a function of age? What is the nature of the lithosphere-asthenosphere boundary (LAB) and how does it vary across regions with different tectonic histories? How does seismic velocity anisotropy vary across the LAB and how does it relate to present day flow and/or past tectonic events?
At Berkeley, we have developed methods for inverting long period seismograms to construct regional upper mantle velocity models that include both radial and azimuthal anisotropy and can incorporate other constraints, such as teleseismic shear-wave splitting data, and we have applied this approach to North America. However, the resulting models are laterally and vertically smooth and do not incorporate sharp boundaries such as the LAB. At Brown, we have developed methods that employ teleseismic S to P (Sp) and P to S (Ps) scattered waves to image mantle sharp velocity interfaces in the mantle, and in eastern North America we have detected the presence of a westward dipping discontinuity at depths of ~90 km that we identify as the LAB. However, this approach provides constraints only in the vicinity of stations, and does not provide information about the volumetric structure above and below the discontinuity.
The initial focus is on method development - combining the two approaches - and on testing the approach with data from the permanent Backbone Network; data from the Transportable Array will be incorporated later in the project. This work will bridge the gap between current smooth, large-scale tomographic modeling and the higher resolution modeling which will be possible with the more densely spaced stations of EarthScope Flexible Array experiments.
