The earth's crust is thinnest wherever the lava is coming up and where there are hot springs. Declivities are the thinner spots. Any place that is below sea level is thinner. California has quite a few geothermal plants and many houses that are producing energy from geothermal. I have seen several articles about houses that are heated by these geothermal springs. I could not find the spot in North America where it is thinnest but I would have a guess at Yellowstone Park. I do know that it is thicker wherever there is a mountain.