EARTHQUAKE
Introduction
An earthquake is a natural hazard and disaster that takes place due to sudden movement and disturbance in the rocks in the interior of the earth.
It is one of the most dangerous and destructive natural calamities that cause great damage to men and materials on the surface of the earth.
An earthquake is a movement within the earth caused by natural or man-made stresses.
Earthquake
An earthquake is a sudden vibration or oscillation in the crust of the earth.
It is a form of energy wave motion transmitted through the wave surface of the earth.
The place of origin of an earthquake in the earth’s crust is called ‘Focus’
The point of the earth’s surface which is perpendicular to the focus, receiving seismic waves is called ‘Epicentre’.
Points to be remember
The Scientific study of earthquakes is called ‘Seismology’
The Place, time, velocity, and direction of seismic waves are recorded by an instrument known as a seismograph.
Causes of earthquake
On the basis of their occurrence earthquakes can be classified into three types,
1. Tectonic Earthquake:
These are caused due to folding, faulting, and displacement of blocks of rocks in the earth’s crust. They are highly intensive and destructive seismic activities.
2. Volcanic Earthquake:
These are associated with volcanic activities and are usually caused at great depths. They are of low intensity and magnitude. The destruction and damage are slightly lesser than in earthquakes caused by tectonic forces.
3. Earthquake caused by man-made factors:
Over interaction of man with nature such as the construction of huge dams, reservoirs, deep mining, underground nuclear explosion, etc.
Earthquake waves
Earthquake waves are classified into three main types based on the mode of travelling and velocity.
Primary waves
Secondary waves
Surface waves
1. Primary waves
These are the first and the fastest earthquake waves from the seismic focus to the epicentre.
These waves travel horizontally in the land, gaseous and water bodies and make the materials move forward and backward.
These are also called Push or longitudinal or compressional waves.
The speed of the waves is around 4 to 13 km per second.
These waves reach the epicenter first.
2. Secondary waves
These waves are also called Transverse or Distortional waves.
Secondary waves reach the epicentre after the primary waves.
These waves make the particles move at right angles
Secondary waves cannot pass through liquid materials.
The speed of these waves is around 4 to 6 km per second.
P WAVES | S WAVES |
These are also called primary waves. | These are also called secondary waves. |
These are similar to sound waves (longitudinal waves). | These are transverse waves |
They travel through gaseous, liquid and solid materials. | They travel only through solid materials. |
These are the first waves to arrive at the surface. | These waves are arriving at the surface with some time lag. |
3. Surface waves
These are the slowest waves and reach the epicentre at the last stage.
Surface waves are also called long waves or long period waves.
The speed of these waves is around 3 to 4 km per second and are confined to the upper layer of the crust.
These waves cause heavy destruction and damage to the crust of the earth. Therefore, they are called destructive earthquake waves.
Tsunami
A large sea wave is occasionally experienced along the sea coast caused by an underground or sub-marine earthquake is called Tsunami.
It is quite common in Japan and locally called Harbour waves.
These waves have become common in recent years in seismic zones of the Pacific and the Indian ocean and are causing great havoc.
A tsunami causes sudden high tidal waves leading to problems for navigation, fishing, and coastal activities.
To measure the intensity and magnitude of the earthquake two scales are used:
Mercalli Scale:
commonly used to measure the intensity of the earthquake.
Richter Scale:
The most common scale used to measure the magnitude of an earthquake. But in recent years it is used to measure the intensity of the earthquake.
Seismic recording centers in India
Gowribidanur- Karnataka
Kodaikanal- Tamil Nadu
Kolaba – Maharashtra
Hyderabad – Telangana
Dehradun – Uttarakhand etc.,
Earthquake Zones
Earthquakes are closely connected with certain seismic zones of the world. Most of the earthquakes occur in the areas of folded regions, zones of faulting, active volcanic areas and zones of continental and oceanic margin.
1. The Circum- pacific belt
This belt is found all along the coastal margins of the Pacific Ocean.
The important areas in this belt are the Rockies of North America, Andes of South America, Philippines, Japan, Alaska, Mexico, and Indonesia.
This belt has a high frequency of earthquakes of very high magnitude.
2. The extensive ocean basin of the Pacific Ocean is seismically active.
3. The Alps-Himalayan fold mountain region. It comprises Southern Europe, North Africa, Central Asia, Northern Mountain regions of India
Earthquake Zones of India
This is the most important seismic zone of India in the foothills of the Himalayas: Jammu and Kashmir, Uttarakhand, Himachal Pradesh, Assam and parts of West Bengal record the earth earthquake frequently.
Effects of earthquake
Loss of life
Loss of property
Land slides
Rock slides
Tsunamis
Flood
Facts to be remember
The magnitude of an earthquake is conventionally reported using the Richter scale or a related Moment scale.
The location below the earth’s surface where the earthquake starts is called the hypocenter (focus).
The location directly above it on the surface of the earth is called the epicenter
Earthquake waves are basically of two types — body waves and surface waves.
Tsunami is a Japanese word, which means “harbour wave”.
Most tsunamis–about 80 percent–happen within the Pacific Ocean’s “Ring of Fire,”