- An earthquake is a sudden movements, vibration or oscillation in the earth crust caused by sudden release of heat and enormous pressure.
- There are three types of earthquakes namely: shallow, intermediate and deep earthquakes.
- The point at which the earthquakes originates is called focus while the point on the earth surface immediately above the focus is called Epicentre.
- An instrument used measuring the intensity of earthquake is known as seismography.
- One of the causes of earthquake is the sudden tectonic actions in the faultline or along the cracks.
- The energy of earthquake means the amount of strength which is scattered from the earthquake centre in all directions.
- Earthquakes can cause the raising and lowering of parts of the sea floor.
Earthquake which is popularly known as a natural disaster is one of the major causes of economic damage, physical injury and death in the world. Other natural disasters include, tsunamis, volcanoes, bushfires, drought, flooding, lightning and high winds. Earthquakes have differing impacts depending on their magnitude and the character of the affected area (For example, a heavily populated area is more affected than a sparsely settled area). One of the greatest earthquakes known was the great Lisbon earthquake of November 1st(1755) which claimed more than 60,000 lives and several buildings. The earthquakes of September 1st (1923) that struck Yokohama and Tokyo City was shocking; half a million buildings as recorded were collapsed in both cities and a quarter of a million people were killed. Other disastrous earthquakes that have ever occurred include that of San Francisco in 1906, San Fernando (California) in 1971, Nicaragua (Central America) in December 1972 and that of Mexico in 1973. The Accra region in Ghana and Ibadan area in Nigeria are the only two countries in West Africa that have experienced earths tremor in the past years.
Definition of Earthquake
An earthquake is a vibration or oscillation of the surface of the earth caused by sudden release of enormous pressure. Earthquake occurs as a result of disturbance within and below the earth crust. For example, if a stone is thrown in the still water of a pond, it will create ripples in the water which will scatter all around the pond. However, the sudden release of enormous pressure from the interior of the earth, creates waves in the surrounding rock like the waves in water which also scatter all around. Earthquakes are often concentrated in the belts making the boundaries of plate which include the mid-ocean ridge, the ocean deeps and volcanic islands and regions of crystal compression. The point where an earthquake originates or the energy is released is known as the focus or centre. The point on the earth surface immediately above the point of origin is known as the Epicentre, it is the point where the shock waves first hit the surface.
Seismography is an instrument used in measuring the intensity of the earthquake waves, this instrument records the data using a special pen and paper rolled on a moving cylinder. As the earthquake is recorded, the pen moves back and forth on the paper showing theintensityof the earthquake. It can record tiny earthtremorsthousands of miles away. The magnitude or gravity of shock is measured according to the Richters Scale. Charles F. R. Richter was the first seismologist to introduced an arithmetic scale for measuring the intensity of seismic waves, The scale is commonly called the Richter scale.
The gentle waves of vibrations within the earths crust results in earth tremors and they are not as disastrous as earthquakes. There are two types of shock waves, namely body waves and surface waves.
Body waves are the type of waves that travel through the crust while the surface waves travel through the surface rocks. The waves that travels through the crust set up vibration while those that travel through the surface waves produce most of the damages. Earthquakes often cause parts of the earths crust to be displaced vertically and horizontally.
Types of Earthquakes
Normally, an earthquake originates between 5 to 1,126 kilometres deep into the interior of the earth. The earthquake centre has been grouped into three classes according to their depth. These are:
- Shallow– The earthquake that originates at a depth of 60 kilometres is referred to as shallow earthquake.
- Intermediate– An earthquake that originates at a depth of 60 to 300 kilometres is termed as an Intermediate earthquake.
- Deep– An earthquake that originates at a depth of 300 kilometres and above is known as deep earthquake. About 90 per cent of the earthquakes that occur throughout the year over the world originate within less than 100 kilometres of depth. The terrible earthquakes which occurred so far over the world are mostly of shallow type.
Causes of Earthquake
Some geologists opine that sudden tectonic actions in the faultline or along the cracks are responsible for earthquake occurrence. Besides, sudden ejection of lava with great force can also produce earthquakes. Radiation of heat from the earths surface causes shrinkage which in turn to maintain equilibrium, develops cracks along the fault line or folds and the earthquake is caused. The rocks if dislodged due to earth movement or development of faults will also cause the earthquake to occur.
Effects of Earthquake
An earthquakes is one of most occurring natural disasters in the world. Most of the times, it causes great loss of lives, property and wealth in some parts of the world. Generally, it may be mentioned that the devastative nature of earthquakes brings the following damages and changes in the earths crust.
- Faults, cracks or landslides occur in the earths crust due to the earthquake. The course of the rivers is also changed. For example, the bed of the Brahmaputra river was raised and the river changed its original course and started flowing through the channel of the Jamuna due to the earthquake of Assam in 1787.
- Earthquake, if it occurs in a thickly populated area, then causes great devastation to houses, roads and railways, water and gas supply line and other properties of the area. Additionally, if the area is a hilly one, then the landslidesand mud flow create devastation to the settlements located at the foot of the mountains.
- Earthquake that originates in the ocean floor creates tremendous waves on the surface of water. Waves that are created by earthquakes are known as Tsunamis. Tsunamis bring sudden flood in towns and other localities especially in coastal areas.
- Earthquake can cause the raising and lowering of coastal regions as in the case of Alaska in 1899.
- It can cause landslides as in the case of Loess country of North China in 1920 and 1927.
- Earthquake can cause vertical and lateral displacement of parts of the earth crust.
- Earthquake can cause the devastation of cities and can bring about outbreak of fires and diseases.
Earthquake Prone Areas in the World
The severity of earthquake is not equal throughout the world. Earthquakes mostly occur in the comparatively smaller areas which are long and narrow in size. Among them, the most noteworthy areas are the circular islands (Philippines and Japan), young fold mountains and ocean ridges. the earthquake prone areas can be divided into three principal zones :
- The Pacific Zone : Maximum number of earthquakes occur along the outer border of the Pacific Ocean. Japan, the Philippines, Chile, Aleutian islands and Alaska haves been identified as the most earthquake prone areas.
- The Mediterranean and the Himalayan Zone: This region extends from the Alps to New Zealand covering Caucasus, Iran, the Himalaya, Indo-China and East Indies following the northern coast of the Mediterranean Sea.
- Ridges of the Mid-Atlantic and the Indian Ocean : The north-south ridges of the Mid-Atlantic Ocean meeting with the ridges of the Indian Ocean extends up to the Mediterranean zone through the Red Sea. Besides these three specified belts, there are some areas located separately within the continents and in the deeps of the oceans where the occurrence of the earthquake is felt.