To make our seismograph we used:
A strong cardboard box
a plastic cup/yogurt pot
weights to put in the cup (we used marbles)
paper to record you tremors
Put the box so the front is open.
In the top of the box make a hole for the wire.
Take the cup, make a hole in the bottom for the pen, secure with tape.
Punch holes in the top of the cup so it can be suspended with the wire.
Weight the cup with the marbles or whatever you are using.
Tie the wire to the cup and feed it through the hole in the top of the box.
Arrange the wire so that the pen at the bottom of the cup just touches the bottom of the box and knot the wire so it stays at this height. This is the tricky bit. The pen will draw on the paper as the box moves, if the pen drags on the paper it won't work so well so take some time over this. Finally put some paper in the bottom of the box under the pen.
There you are, your own seismograph. The only difference between this and the real thing is that the real thing has a roll of paper that moves at a known speed so you can tell how long tremors last and how far apart they are.
How it works. The weighted cup at the end of the string acts like a pendulum. It is free to swing independently of the supporting box; sometimes this is called being "isolated". Because it has been weighted and is heavy it is difficult to start it swinging, it has a large "inertia". When the support moves the pendulum tends to stay still and so you get a movement of one relative to the other which is recorded by the pen.
Unless you are very lucky (or unlucky depending on your perspective) it is unlikey that you will be able to test this with a real earthquake so try placing the box on the floor and then jumping up and down, put it on the back of your bike, walk and then run across the room etc.
For more Science activities check out Science Sunday with Ticia over at Adventures in Mommydom