Spot checks on North American mantle structure using Earthscope's Transportable Array and preceding PASSCAL arrays

We have measured relative arrival times of teleseismic P and S waves at seismic arrays in North America. These arrays are primarily IRIS-PASSCAL arrays in tectonically stable North America that were not utilized for teleseismic tomography. Teleseismic S-wave delay times averaged for all events measured at a given PASSCAL station illustrate intracratonic structural variations of up to 1 s over several hundreds of km, while the delay-time difference between on-craton stations and station on the Paleozoic margin of North America is more than 2.5 s over less than 500 km. Such measurements can provide integral checks on numerous tomographic models that have been developed since the recent onset of Earthscope. The ratio of S delays to P delays across the various arrays in stable North America is not significantly different from 3, but the best fitting value is 2.5. This is consistent with the relatively cool thermal state of the cratonic lithosphere of stable North America implying a smaller contrast between the temperature sensitivity of S and P velocities than for hotter mantle, such as that beneath tectonically active western North America. The tectonically active part of North America has been sampled spectacularly by Earthscope's Transportable Array (TA). Across this western configuration of the TA teleseismic P-wave arrival time delays differ by up to 4 s to date. A pattern of delays from incidence angles from the southeast shows a region of increased delays beneath northern Arizona, as well as near the Snake River Plain. A thin north-south oriented line of stations along the Cascades shows distinctly early arrivals. Expectedly, but never as beautifully illustrated with smaller arrays, re-measuring the P-wave arrival times after low-pass filtering produces a much smoother and damped pattern of delays, not unlike that found for the S-wave delays, which is consistent with increased averaging of structure within larger Fresnel zones at lower frequencies.