Making the jelly
Mix up the jelly (it should be 6-10x the normal strength - I found this easiest in a saucepan, in a sink full of hot water so it oesn't cool down too fast.) poar it into the box and add cotton wool in smallish puffed ot lumps and try and distribute it throughout the jelly. Leave overnight to set.
If we have corrigated plastic - integrating it into the mould would be a really good idea.
Making the plates (do this before the visitors are present)
Cut the lump of jelly in two. Stow knife away somewhere safe. Then get some corrugated card and strip the paper off one side, so the corrugations are visable. cover the back with something waterproof like gaffer tape (or the card will go soggy), then attach this onto the faces of the fault...
The two lumps of jelly should be placed on pieces of card to stop them sticking to the table
Making earthquakes.
Push the two lumps of jelly past each other slowly and it should stick and slip making the jelly wobble - forming earthquakes.
Other things to talk about.
Drag fingernails across a blackboard/ steel across steel/ bow across violin strings - all cases of slip stop frictionas well as squeaking beaks and may others