In the seismic tomography images why are there red colors under the Southwestern US and blue colors under the eastern US?

Red colors indicate warmer and weaker material, which slows down earthquake waves. Stretching has thinned the tectonic plate in the Southwestern United States, which allows the hotter and weaker mantle beneath to come closer to the surface. Blue colors indicate colder and stronger material, which speeds up earthquake waves. The tectonic plate under the eastern United States is older and thicker than the tectonic plate under the western United States, and thus it is colder and harder.