10.10 A Seismometer

10.10 A Seismometer

A seismometer is an instrument that responds to any movement of the rocks under the ground or vibration caused by earthquakes, volcano eruption and explosion. A seismometer detects earthquakes by using electromagnetic induction to convert ground motion into electrical signals. Typically, a seismometer includes a weight suspended by a spring. When an earthquake occurs, the ground moves but the weight tends to stay stationary due to inertia. This results in relative motion between the weight and the frame of the seismometer which is attached to the ground. The weight is often attached to a magnet which moves inside a coil of wire (Fig. 10.19). This setup works according to Faraday's law of electromagnetic induction, that is, the changing magnetic flux through the coil induces an emf in the coil. This gives rise to an induced electric current.

The induced current is proportional to the velocity of the ground motion. The electrical signals generated are then amplified and recorded. Thus, data is provided on the amplitude, frequency and the duration of the earthquake waves.

The data is analyzed to determine various characteristics of the earthquake, such as its location, magnitude and depth.

Usually, a seismometer is buried under the ground at a depth of $50$–$1000$ metres. It is placed in a protective case called a vault. This is a cylindrical steel tank that is approximately $1$ metre wide and $2$ metres deep with a concrete pad at the bottom (Fig. 10.20).

For your Information

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There are two main types of seismic waves that generate earthquake i.e., P-waves are primary waves which are longitudinal and S-waves are secondary waves which are transverse in nature.