Recorder: How Does a Seismograph Record the Magnitude of an Earthquake?
How does a seismograph record the magnitude of an earthquake?
- Cardboard box, measuring about 12 inches (30 cm) on each side
- Adding-machine paper
- Sharpened pencil
- 5-oz (150-ml) paper cup
- Masking tape
- Black marking pen
- 5 ounces (150 ml) small rocks
- Modeling clay
- Adult helper
- Ask an adult to prepare the box as follows:
- Cut the lid off the box, then turn the box on its side so that the opening faces outward.
- Cut a 2-inch (5-cm)diameter hole in the center of the top of the box.
- Cut two 1/2by-4-inch (1.25-by-10-cm) slits in the box. Make the first slit in the center of the bottom, near the opening. Make the second slit, in line with the first slit, in the back of the box.
- Cut a 24-inch (60-cm) section of addingmachine paper.
- Thread the strip of paper through the slits in the box so that about 2 inches (5 cm) of paper extends past the front edge of the box.
- Cut two 18-inch (45-cm) pieces of string.
- Ask an adult to use the point of the pencil to punch two opposite holes below the rim of the cup.
- Attach the two pieces of string to the cup. Do this by inserting one end of a string through one of the holes, then looping the string over the rim and tying it to itself. Tie the other string to the other hole in the same way.
- Set the cup in the box and insert the free end of each string through the hole in the top of the box.
- Lay the pencil across the hole and tape the ends of the strings together to the center of the pencil. The bottom of the cup should be about 1 inch (2.5 cm) above the floor of the box.
- Push the tip of the marking pen through the inside bottom of the cup.
- Fill the cup with small rocks to surround the pen.
- Wind the string around the pencil until the tip of the pen barely touches the addingmachine paper.
- To prevent the string from unwinding, secure the pencil to the box with a small piece of modeling clay near each end of the pencil.
- Pull the adding-machine paper toward you with one hand as you gently shake the box with your other hand.
- Observe the markings made by the pen on the paper.
The pen draws a zigzag line on the paper as the paper moves underneath the pen.
The inertia (resistance to a change in motion) of the heavy cup keeps it steady while the box vibrates (shakes back and forth repeatedly). As the box vibrates, it moves the paper from side to side underneath the pen; thus, a zigzag line is drawn as the paper is pulled. The shaking energy for an earthquake (a violent shaking of the earth's crust caused by a sudden movement of rock beneath its surface) is measured and recorded by an instrument called a seismograph. like your model, a seismograph uses a very heavy suspended object that remains steady, while the frame to which it is attached moves when the earth vibrates. A recording pen attached to the suspended object records the vibrations on the moving paper. The width of the zigzag produced increases with the magnitude (measurement of the amount of shaking energy released) of the earthquake being recorded. The written record is called a seismogram.