The first instrument to measure an earthquake was invented in AD 132 by Zhang Heng, a Chinese philosopher.
The first instrument to measure an earthquake was invented in AD 132 by Zhang Heng, a Chinese philosopher. It was a 2 m in diameter bronze jar with a central pendulum inside and decorated outside with a series of dragon heads connected to a pendulum, each with a ball in a hinged mouth. Directly below each dragon’s head on the surface of the stand was a bronze frog head with its mouth open to receive the ball from the dragon’s mouth. During the earthquake the ground motion would move the balls to fall from the dragon’s mouth into the frog’s mouth. The direction of earthquake was indicated by
which of the dragon heads dropped a ball. By the end of the nineteenth century several European inventors had constructed different seismographs. Most used electromagnetics operated by suspending magnetic mass or pendulum within an electric coil as shown in Fig. 16.7.
A seismometer is an instrument that senses the Earth’s motion and a seismograph combines a seismometer with recording equipment to obtain permanent record of motions. Scientists can compute how much energy is released during an earthquake. Calculations are made in various seismographs near to and far from the epicentre to determine the intensity of the earthquake.
From three different locations the place of epicentre can be located exactly, as shown in Fig. 16.8. One such instrument is required in each of the two orthogonal horizontal directions. Of course for measuring vertical oscillations the string pendulum is replaced with a spring pendulum oscillating about a
fulcrum. Some instruments do not have a timer device (i.e., the drum holding the chart paper does not rotate). Such instruments proved only the maximum extent of the motion during the earthquake and for that reason they are called seismoscopes.
A number of smaller size earthquakes take place before and after a big earthquake (main shock). Those occurring before the big one are called fore shocks and the ones after are called after shocks.