 Up here on top is a map that I made with data from the National Earthquake Information Center. It's six months of global seismicity. And down here is a map of plate boundaries in the world. I hope you'll agree with me that earthquake locations are pretty good at telling you where the plate boundaries are. But let's look at North America for a second. Down here on this map on the bottom, you'll see that there's a plate boundary on the western edge of North America. And then here is a plate boundary. This is the Mid-Atlantic Ridge. There's a little plate boundary down here. But if you look up at the top, you'll see a lot of shallow seismicity that locates far away from any of those plate boundaries. Specifically, here's the new added seismic zone right there. Now scientists are interested in intra-player earthquakes because the theory of plate tectonics doesn't really explain them very well. The reading assignment further down this page will allow you to check out three different hypotheses of why these earthquakes happened.