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. 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 of the liquid was washed up the side of the vessel indicated the intensity and a line joining the points of max motion also told the direction of the tremor. Most seismographs today run on electricity, 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. The P wave will be the first wiggle that is bigger than the rest of the little ones. Because P waves are the fastest seismic waves, they will usually be the first ones that your seismograph records. The next set of seismic waves on the seismogram will be the S waves. These are sometimes bigger than the P waves.
For earthquakes in the ocean, hydrophones can be used to detect and measure submarine earthquakes. Seismic energy from submarine earthquakes is converted into acoustic energy at the seafloor-water boundary. A Tertiary wave is the acoustic signal from these earthquakes. A T-wave typically has frequencies ranging from 4 to 50Hz. T-waves spread efficiently in the ocean compared to seismic waves through the earth and can be detected at great distances. Hydrophones can detect an earthquake that is 1.5 to 2.0 orders of magnitude lower than those earthquakes that seismometers typically detect on land. Location of earthquakes guessed using hydrophones may not be as accurate as locations guessed by seismometers. The T-wave is created where the seismic waves face the seafloor-water interface, which might not be at the epicenter and give a bit information about the depth of the earthquake within the Earth’s crust.
Hydrophone arrays such as those in the Sound Surveillance Systems and the Comprehensive Nuclear-Test-Ban Treaty hydroacoustic monitoring system have been successfully used to detect and monitor earthquakes. For 20 years, researchers have used arrays in the northeast Pacific Ocean. About 47,934 seismic events were detected and located in the northeast Pacific.