How Are Earthquakes Studied?

Seismologists study earthquakes by going out and looking at the damage caused by the earthquakes and by using seismographs. A seismograph is an instrument that records the shaking of the earth's surface caused by seismic waves. The term seismometer is also used to refer to the same device, and the two terms are often used interchangably.

The First Seismograph



Figure 1 - A large-scale model of Cheng Heng's original Earthquake Weathercock.

The first seismograph was invented in 132 A.D. by the Chinese astronomer and mathematician Chang Heng. He called it an "earthquake weathercock."

Each of the eight dragons had a bronze ball in its mouth. Whenever there was even a slight earth tremor, a mechanism inside the seismograph would open the mouth of one dragon. The bronze ball would fall into the open mouth of one of the toads, making enough noise to alert someone that an earthquake had just happened. Imperial watchman could tell which direction the earthquake came from by seeing which dragon's mouth was empty.

In 136 A.D. a Chinese scientist named Choke updated this meter and called it a "seismoscope." Columns of a viscous liquid were used in place of metal balls. The height to which the liquid was washed up the side of the vessel indicated the intensity and a line joining the points of maximum motion also denoted the direction of the tremor.

Modern Seismographs



Figure 2 - Two illustrations of a modern seismograph in action (from Lutgens & Tarbuck, 1989).

Most seismographs today are electronic, but a basic seismograph is made of a drum with paper on it, a bar or spring with a hinge at one or both ends, a weight, and a pen. The one end of the bar or spring is bolted to a pole or metal box that is bolted to the ground. The weight is put on the other end of the bar and the pen is stuck to the weight. The drum with paper on it presses against the pen and turns constantly. When there is an earthquake, everything in the seismograph moves except the weight with the pen on it. As the drum and paper shake next to the pen, the pen makes squiggly lines on the paper, creating a record of the earthquake. This record made by the seismograph is called a seismogram.

By studying the seismogram, the seismologist can tell how far away the earthquake was and how strong it was. This record doesn't tell the seismologist exactly where the epicenter was, just that the earthquake happened so many miles or kilometers away from that seismograph. To find the exact epicenter, you need to know what at least two other seismographs in other parts of the country or world recorded. We'll get to that in a minute. First, you have to learn how to read a seismogram.




Figure 1 used with permission from SEG <http://www.mssu.edu/seg-vm/pict0688.html>. Figure 2 is from Lutgens and Tarbuck, 1989. All other content 2007 Michigan Technological University. Permission granted for reproduction for non-commercial uses.