## Big Earthquakes on the Computer

We're asking a more particular question though. Why does the earth crackle? That is, why aren't all earthquakes small (individual boulders rubbing against one another like popcorn popping)? Or why aren't they all big (all of California snapping like a pencil in one big earthquake?)

We don't have a simple answer. Maybe we don't really understand it completely yet. But if we take the simple theory we just saw, and make it 50 green blocks instead of eight, we get something that looks a lot like real earthquakes!

Although this is exactly the same as the last simulation we're drawing it completely differently. The red line is the edge of one of the tectonic plates. The other plate (the marble tabletop in the small version) isn't sliding sideways any more: we're drawing it sliding downward. When the blocks are moving fast, we fill in yellow: when they are completely stuck to the downward-moving plate, we draw a black line. The regions between the black lines are the earthquakes. A big region is a big earthquake: a little region is a small earthquake.

We should mention that we've sped up the parts when no blocks are moving. Otherwise, you'd spend all your time waiting for the next earthquake (just like living in California!) That's why it looks a little jerky.