How Does a Seismograph Work?

In 132 C.E., Chinese astronomer and mathematician Zhang Heng created the first known earthquake detector. Zhang Heng's seismoscope took the shape of a large vase surrounded by dragons, with frogs at the base. When the ground moved, a ball would fall out of a dragon's mouth into a frog, making a noise.

The first seismograph was built in 1875 by Filippo Cecchi, an Italian physicist, and featured pendulums that activated a clock and a recording device to measure the duration of an earthquake.

A seismograph is a recording device that scientists use to measure earthquakes. The goal of a seismograph is to accurately record seismic waves during a quake.

If you live in a city, you may have noticed the ground shaking when a big truck or a subway train rolls by. Good seismographs are therefore isolated in seismograph stations and connected to bedrock to prevent "data pollution" from other types of ground motion.

The main problem that must be solved in creating such an instrument is that when the ground shakes, so does the instrument. Therefore, most seismographs involve a pendulum of some sort.

You could make a very simple seismograph by hanging a large weight hanging from a rope over a table. By attaching a pen to the weight and taping a piece of paper to the table so that the pen attached to the weight can draw on the paper, you could record tremors in the Earth's crust (earthquakes).

If you used a roll of paper (a simple rotating drum) and a motor that slowly pulled the paper across the table, you would be able to record tremors over time. However, it would take a pretty large tremor for you to see anything.

In a real seismograph, levers or electronics are used to magnify the signal so that very small tremors are detectable. A big mechanical seismograph may have a weight attached that weighs 1,000 pounds (450 kg) or more, and it drives a set of levers that significantly magnify the pen's motion.

Short-period seismographs are designed to pick up the high-frequency seismic waves of local earthquakes, while long-period seismographs measure the lower-frequency waves of more distant earthquakes. Very large earthquakes may trigger strong-motion seismographs, which magnify seismic activity to a much lesser degree.

There are two types of seismic waves: body waves and surface waves. Body waves include P and S waves, which interact with the surface of the Earth to create surface waves.

P waves, or primary waves, are longitudinal waves. P waves move back and forth, like a coiled spring that has been compressed and then released. S waves (secondary waves) are transverse waves, moving up and down like ripples of water.

Both seismographs and seismometers measure seismic activity, but a seismograph has the ability to record the seismic data. (The word "graph" comes from the Greek "graphos," to write.) This recording is called a seismograph.

The first seismographs recorded ground oscillations on an analog recording surface like photographic paper or magnetic tape, but today's seismographs are often digital.

The Richter scale is a standard scale used to compare earthquakes. It is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0.

On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a microquake. Microquakes occur constantly. Moderate earthquakes measure less than 6.0 or so. Earthquakes measuring more than 6.0 can cause significant damage.

The biggest quake in the world since 1900 scored a 9.5. It rocked Chile on May 22, 1960.