# Sudden Movements of Earth (MH 9)

## definition

### Earthquake

An earthquake is defined as a tremor below the surface of the earth which causes shaking of the crust. These devastating tremors cannot be regarded as movements as they are caused by the stresses that break the earth's crust.

## definition

### Seismograph

The direction of movement of waves and their passage at a particular point is recorded by an instrument called Seismograph. It has a pen attached to it. The pen vibrates with the earthquake waves, recording their movements on a travelling strip of paper. It also calculates the difference in the arrival of P and S waves.

## shortcut

### Richter Scale

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The Richter scale measures the power of an earthquake on a scale of 1-9. It measures the absolute intensity with mathematical precision. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions.

## shortcut

### Epicentre

An epicentre is a point on the earth's surface directly above where the earth begins to fracture in an earthquake. The focus lies beneath the epicentre. The epicentre is located from records of earthquake waves created by a seismograph.

## definition

### Cause of earthquake

An earthquake is the shaking and trembling of the earth's surface due to the sudden release of earth's energy in the form of waves. Earthquakes are caused to due various reasons. Some of them are:
• Due to the movement of the tectonic plates.
• Colliding of the tectonic plates.
• When the tectonic plates slides one below the other earthquake occurs.
• Forming of fractures in the rock layers.
• Volcanic eruptions.

## definition

### Extinct volcano

Those volcanoes which have not erupted in the past since long and are not likely to erupt in the future are called extinct volcanoes. For example, Mt. Kilimanjaro volcano in Tanzania.

## definition

### Folding and faulting

Folding and faulting are both diastrophic process which changes the earth's surface from time to time.

Folding is a type of earth movement resulting from the horizontal compression of rock layers by internal forces of the earth. If the pressure is very high, large scale folds are formed and results in the formation of fold mountains. The Himalayas, the Aravalis, the Andes are major fold mountains of the world.

Faulting occurs when Earths crust cracks under tension causing layers of rocks to stretch & crack. This leads to formation of fractures in the rocks. A normal fault develops when one block moves down relative to other block in direction of the fault. Due to faults block mountains and rift valleys are formed.

## definition

### Sudden movements of the earth

The earth's crust is continuously changing due to various geomorphic processes. These process includes the endogenetic and exogenetic processes. The endogenetic process mainly includes the internal process like plate tectonics, earthquake, volcanic eruptions etc. Earthquake and volcanic eruptions are called as the sudden movements, since they occur suddenly and causes considerable deformation over a short span of time.

## definition

### Active volcano

If the volcanic eruptions are regular even in the present times, then such volcanoes are called active volcanoes. For example, Mt. Fujiyama in Japan, Mt. Stromboli in Mediterranean Sea.

## definition

### Dormant volcano

When a volcano has not erupted since long, but may become actively suddenly it is called dormant volcano. For example, Mt. Vesuvius in Italy, Mt. Katmai in Alaska, Barren Island, India.

## definition

### Surface or L-waves

The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves.
Surface or L-waves are the highly destructive waves among all the three waves. These waves are generated after the primary and secondary waves reach the epicenter.

## definition

### Secondary or S-waves

The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves.
Secondary or S-waves: The waves which reach the earth's surface after the primary waves are called secondary waves. The velocity of these waves  are lesser than the primary waves and they scatter in all direction from the focus. These waves can travel only through the solid medium.

## definition

### Primary or P-waves

The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves.
Primary or P-waves: These waves are the first waves to reach the earth after the energy is transmitted in the interior. They travel at a very fast speed and can travel through all the medium, solid, liquid and gaseous but while travelling through the liquid medium, its direction gets changed.