Even though the ground under your feet feels very still, it is actually moving really, really slowly. But sometimes, the ground moves so quickly that it feels like it is shaking. When the ground-shakes are big and strong, they cause houses to fall into pieces and make people get hurt and die. Big ground-shakes can make us very afraid. We still don’t know how to tell when they will happen, so they usually hurt a lot of people. But not all ground-shakes are bad! There are small ground-shakes that you can’t feel. We can use these small ground-shakes to learn more about the inside of our world. While you can’t feel the small ground-shakes, we have ground-shake-computers that can feel them. Usually the ground is still, so the ground-shake-computers just draw a straight line. But when a ground-shake happens, the straight line starts to move all over the paper. We can use these not-straight lines from many ground-shake-computers to find out when the ground-shake happened and where it started from. We can then use these facts to tell us how fast the ground-shake moved from one place to another. Because the ground-shakes move through the inside of our world, knowing how fast they move can tell us about the things that are under the ground you stand on. In some places, there are really huge groups of rocks that are so tall that they touch the cold white water in the sky. Most of the time, these groups of rocks are safe. People like to climb them for fun. But sometimes, many fire rocks are thrown from the tops of these climbing rocks. The fire rocks are very hot and hurt people and houses. The good news is that we can use the small ground-shakes to find out where these fire rocks come from, which climbing rocks might throw fire rocks, and when the fire rocks will be thrown out. The fire rocks start out deep inside our world and slowly move toward the top of the climbing rock. When they are high enough, the fire rocks move the smaller rocks that make up the climbing rock. This causes small ground-shakes that we can feel with the ground-shake-computers. If the ground-shake-computers feel enough small and big ground-shakes, we know that the climbing rock might start throwing out fire rocks soon. This gives us enough time to tell people to move so that they can be safe.
So remember: even if ground-shakes sometimes hurt people, we can also use them to keep people safe from fire rocks.”