Seismometer (Seismograph)Seismometer (Seismograph)

The devices used by seismologists for the measurement and recording of the vibrations of earthquakes is known as seismometer or a seismograph.

Principle of seismographsPrinciple of seismographs

All seismometers work on two basic All seismometers work on two basic principles:principles:

First oneFirst one is the principle of inertia is the principle of inertia which is a characteristic of a body which is a characteristic of a body considered due to its mass. considered due to its mass.

Second is the principle of simple Second is the principle of simple harmonic motion.harmonic motion.

Components of SeismographsComponents of Seismographs

A simple seismometer consists of A simple seismometer consists of following following components:components:

11. A spring. A spring 22. Confiner. Confiner 3. 3. WeightWeight 4. 4. Pen or pencilPen or pencil 5. 5. Rotating DrumRotating Drum 6. 6. Rigid FrameRigid Frame

Working of SeismographWorking of Seismograph To understand the working of a simple To understand the working of a simple

seismometer consider a seismometer that seismometer consider a seismometer that is sensitive to up-down motions (vertical is sensitive to up-down motions (vertical seismometer) of the earth.seismometer) of the earth.

As the earth moves, the relative motion As the earth moves, the relative motion between the weight and the earth provides between the weight and the earth provides a measure of the vertical ground motion. If a measure of the vertical ground motion. If a recording system is installed, such as a a recording system is installed, such as a rotating drum attached to the frame, and a rotating drum attached to the frame, and a pen attached to the mass, this relative pen attached to the mass, this relative motion between the weight and earth can motion between the weight and earth can be recorded to produce a history of ground be recorded to produce a history of ground motion, called a seismogram motion, called a seismogram

SeismogramSeismogram

The graph plotted by the seismograph is known The graph plotted by the seismograph is known as seismogram. as seismogram.

It shows the amplitude of seismic waves.It shows the amplitude of seismic waves. Those one having lower amplitude are high Those one having lower amplitude are high

speed waves known as body waves speed waves known as body waves The waves which have slow speed have high The waves which have slow speed have high

amplitude and are known as surface waves.amplitude and are known as surface waves.

The pattern of body waves depict that The pattern of body waves depict that since they are high speed waves they since they are high speed waves they quickly passes by causing less damage.quickly passes by causing less damage.

But surface waves are slow speed waves. But surface waves are slow speed waves. They shakes the building very badly. They shakes the building very badly.

Thus surface waves are more responsible Thus surface waves are more responsible for the destruction caused by an for the destruction caused by an earthquake.earthquake.

Types of seismometersTypes of seismometers On the basis of configuration On the basis of configuration

seismometers have following two types:seismometers have following two types:1. Horizontal Seismometers1. Horizontal Seismometers2. Vertical Seismometers2. Vertical Seismometers Other seismometers include:Other seismometers include:1. 1. Tele-seismometers Tele-seismometers 2. Strong Motion Seismometers2. Strong Motion Seismometers3. Long period seismometers3. Long period seismometers4. Short period seismometers4. Short period seismometers5. Broadband seismometers5. Broadband seismometers

Figure shows a typical Tele-seismometer showing the sophisticated electronics used to hold the mass

Scales used in SeismographyScales used in Seismography

Following major scales are used in Following major scales are used in seismography:seismography:

1.1. Richter Magnitude ScaleRichter Magnitude Scale2.2. Mercalli Intensity ScaleMercalli Intensity Scale Other scales are:Other scales are:1.1. Moment Magnitude ScaleMoment Magnitude Scale2.2. Peak Ground AccelerationPeak Ground Acceleration3.3. Ground AccelerationGround Acceleration

Richter Magnitude Scale Richter Magnitude Scale

Richter scale measures magnitude of an Richter scale measures magnitude of an earthquake and it is related to the energy earthquake and it is related to the energy released during an earthquake.released during an earthquake.

Its advantage is that it is an open scale i.e. Its advantage is that it is an open scale i.e. it has no minimum and maximum value.it has no minimum and maximum value.

Energy released by an earthquake of 6 Energy released by an earthquake of 6 richter scale magnitude is compared with richter scale magnitude is compared with the energy released by 1 Megaton Nuclear the energy released by 1 Megaton Nuclear bomb.bomb.

Mercalli Intensity ScaleMercalli Intensity Scale

The Mercalli scale has 12 levels and it is a The Mercalli scale has 12 levels and it is a subjective scale. When an earthquake hits an subjective scale. When an earthquake hits an area then its intensity will be measured by the area then its intensity will be measured by the damage it has done.damage it has done.

Its disadvantage is that it is perception based Its disadvantage is that it is perception based and its result can vary from person to person.and its result can vary from person to person.

Another limitation of this scale is that it is of no Another limitation of this scale is that it is of no use to the seismologists for research work.use to the seismologists for research work.

This can is useful to describe an earthquake to This can is useful to describe an earthquake to a common person a common person