How Do Seismographs Work?

How Do Seismographs Work?

Seismometers detect and record earthquakes. People sometimes use the word seismograph and seismometer interchangeably, but the seismograph is the output and the seismometer is the machine. Seismographs are the graph that the seismometer produces.

A child’s model of a seismograph

A common model of a seismograph in science classes consists of a box turned upside down. Slits are cut out of the bottom on two sides of the box. A string hangs down in the center of the box, and a pen is attached to the string. Adding machine tape, or any kind of long, continuous paper, is pulled through the box at as constant rate as a kid can manage. Another child gently shakes the table the box sits on. In this way, the “earthquake” is detected.

How the child’s model works

When the table shakes, the pen swings like a pendulum. When the table is still, and the paper is moving at a constant rate, the pen draws a straight line. When the table shakes, the pen moves back and forth. The magnitude of the swing is measured by how far off of the center the pen moves.

Translating the child’s model into a scientific seismometer

Older seismometers used big, heavy objects instead of the child’s cardboard box. The better “boxes” are buildings anchored to the bedrock below, so little bumps like people banging on it wouldn’t cause an “earthquake.”

These days most scientific seismometers don’t use a pen and paper moving at a constant speed. Instead of a pen writing on paper, the “pen” is a weight that is attached to something solid, called a frame, by something like a spring. The attaching device, perhaps a spring depending on the design o the particular seismometer, holds the weight and frame in a fixed position if everything is still. Earthquakes cause the frame to move. Electronics measure the movement between the weight and the frame. That movement differential can then bused to calculate the force required to move the object. The force can then be translated into the strength of the earthquake at that location.

The seismograph

The seismograph is the data. A straight line indicates no earthquake activity. If the line has squiggles, the earth moved. The bigger the squiggles, the bigger the earthquake was. The graph has magnitude on one axis and time on the other axis.