Ans.

A seismometer is a device that is sensitive to vibrations. It works on the principle of a pendulum: a heavy, inert mass with a certain resistance to movement (i.e. inertia) due to its weight is suspended from a frame by a spring that allows movement. The energy from any seismic activity excites this “proof mass” as it is called by geophysicists, making it vibrate.

When a quake occurs, or a tremor is produced by any kind of shock, it is actually the ground—and therefore the frame attached to it—that moves!

If you change your viewpoint and look at the mass, we may consider that when a tremor occurs, the mass—which has inertia because of its weight—will only move after a certain time, whereas the frame will move in keeping with the ground motion.

In addition to the mass, the spring and the frame, a seismometer needs a device to constantly record the motion of the mass relative to the frame. This is a central part of the seismic sensor and the difference between seismometer technologies, as some measure the speed of the mass and other its displacement. In both cases, the recording that shows ground motion over time is known as a seismogram.

An even simpler technique to represent this signal consists in attaching a pen to the pendulum. The pen touches a roll of paper wound around a rotating drum. This is known as a seismograph, an instrument which directly plots the signal rather than recording it in digital form.

Seismograph use to measure Earth quake intensity on Richter scale 0-9 . Seismographs are designed so that slight earth vibrations move the instrument.