The maps shows phase velocity variations of Rayleigh waves at a period of six seconds. The movie shows the progress of USArray across the conterminous USA, with each frame displaying the tomographic map derived using stations available up to a given month.

The phase-velocity model USANT15 describes fundamental-mode Love and Rayleigh wave propagation across western and central USA in the period range 5 to 40 seconds. The phase-velocity maps were derived from a large number of observed inter-station noise-correlation functions collected from pairs of USArray seismographic stations during the years 2006-2015 using the method of Aki (1957) and Ekstrom et al. (2009).

The model USANT15 is published in Physics of the Earth and Planetary Interiors:
Ekstrom, G., Short-period surface-wave phase velocities across the conterminous United States, Phys. Earth Planet Int. (2017), http://dx.doi.org/10.1016/j.pepi.2017.07.010

Some of the methods were described in the 2013 study:
Ekstrom, G., Love and Rayleigh phase-velocity maps, 5-40 s, of the western and central USA from USArray data, Earth Planet. Sci. Lett. (2013), http://dx.doi.org/10.1016/j.epsl.2013.11.022

This research was made possible by support from the EarthScope Program of the National Science Foundation, grant EAR-1460081.