How do seismographs record the magnitude of an earthquake?
Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location and magnitude of an earthquake can be determined from the data recorded by seismograph stations. Around 132 AD, Chinese scientist Chang Heng invented the first seismoscope, an instrument that could register the occurrence of an earthquake. Heng’s invention was called the dragon jar (see picture above). The dragon jar was a cylindrical jar with eight dragonheads arranged around its brim; each dragon had a ball in its mouth. Around the foot of the jar were eight frogs, each directly under a dragonhead. When an earthquake happened a ball dropped from a dragon’s mouth and was caught by the frog
