 Several thousand feet of bishop tuff flowed out and ash buried into the sky and left about six inches thick over Nebraska. Good Lord. And it would have put Mount St. Helens and Pinatubo and everything else to shame. But just the quakes themselves aren't enough to really alarm anyone. It's that the ground begins to swell and begins to build. And the earthquake foci, the center where the earthquakes begin, gets progressively shallower.