What's under the long line of heavy rocks? Waves can tell us. (Seismic tomography of the lithosphere below the Midcontinent Rift)

Abstract:

A long long time ago, the middle of my home land started to tear. But it didn't tear all the way, and actually the torn part ended up getting pushed back together. This left a long line of heavy rocks in the top part of the ground. We don't know why the tearing stopped instead of forming a new big water place in the middle of the land. We also didn't know know if the deeper ground still looked like it was torn too, because we couldn't see it very well. We hoped by looking at the deep ground, we could figure out why the tearing stopped. 

I make pictures of the deep under ground places using waves that are made when the ground breaks quickly. I look at waves that go through the torn area and try to match them. I match the real waves by making my own pretend waves and changing how fast they move through the ground. After I match a lot of waves, I put them all together to make a picture that shows what's deep under ground.