Published: April 2006
Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

Seismographs Through Time

Throughout human history we have tried to learn more about the workings of earthquakes through various types of tools and gadgets. The first person to create a seismoscope, an earthquake recording instrument, was a Chinese philosopher named Chang Heng. In a.d. 132, Heng invented what he called an "earthquake weathercock," which could measure the occurrence of an earthquake and from which direction it came. The instrument—technically a seismoscope—looked like a ceramic urn with eight dragons attached to its sides, each representing one of the eight compass directions. Every dragon held a small bronze ball in its mouth. When an earthquake occurred, one of the balls would fall out and into one of eight toads sitting below. The dragon with the empty mouth would be located in the direction opposite from which the earthquake came. Although no one knows what was inside the jar, most scientists assume it held a pendulum that would trigger a specific dragon with its motion.

The 17th and 18th centuries saw the invention of more seismoscopes. In one invention by Luigi Palmieri in 1855, an earthquake would cause mercury to spill out of a bowl and into a particular container, depending on the direction of the quake. Contact with this container would stop a clock, indicating the exact time, and start recording ground motion onto a drum.

In the late 1800s the earliest seismograph (also known as a seismometer) was invented. A seismograph can provide more information about the intensity and other details of an earthquake than a seismoscope can. This first seismograph was later improved by British researchers in Japan, among them a man named John Milne, who invented the horizontal pendulum seismograph. Pendulum designs became more advanced throughout the 19th and 20th centuries.

The typical seismograph we see today uses pendulum technology and is made by securing a bar deep into the ground on one end and adding a weight to the other end. A pen is attached to the weight and is held against a rotating drum that has paper wrapped around it. In periods of inactivity, the pen draws straight lines with little ticks that indicate each passing minute. Any small wiggles in the lines are usually caused by outside noise, such as a truck rumbling by. When an earthquake happens, everything moves except the weight and the pen. The spiky lines recorded by the pen show the ground motion made by the earthquake. The paper containing these lines is called a seismogram and can be analyzed by a seismologist to find the epicenter, time, focal depth, cause, and even magnitude of the earthquake.

Although these types of seismographs are still used today, most scientists have switched to digital means of recording earthquakes. Computers that are linked together at different sites quickly process the information collected by digital seismographs during an earthquake and send it out over the Internet. What used to take days, weeks, or even months to analyze can now be done in a matter of minutes, allowing the media and rescue workers to react as quickly as possible.

—Emily MacDowell