system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is at the bottom of the crack in an earthquake?
gpt
if you drop a layer cake, one or more layers may break, but not always in the same place.  what is happening is the soft layer of filling keeps the breaks seperate.  so where the topping or icing breaks, you may get a deep chasm, you may not, but unless the cake is completely unrecognisable, there's not a single place where you see all the way to the plate. 

 the earth's crust is made of many more layers of all different softnesses.  wherever they get squished or stretched, an earthquake is the vibration of the layers moving around and rubbing.  so there is not always a break, and just like the dropped cake, you might get lucky and the change in shape might not be noticable! 

 when there is a break you will likely see the crack go down through rocks of similar softness, until a layer of softer or loose rocks, perhaps covered by loose rocks fallen in from the sides of the crack. 

 if you were expecting a volcano to burst out, you would have to be next to a geological fault line, where the process is different.  that's more like two cakes being smooshed into each other, and the cracks and earthquakes are usually minimal compared to the slow movement of layers sliding and folding over each other, leaving filling all over the place and very occasionally letting some of the super hot filling of the earth to leak out. 

 please clean up after yourselves and don't waste cake.