1. Introduction to an Earthquake
An Earthquake may be defined as the shaking and trembling of the Earth’s surface caused by sudden releases of stress with the crust of Earth. Thus an earthquake releases the energy that has been gradually stored through the stress of increasing deformation of rocks. The released energy takes the form of seismic waves that radiate in all directions from the epicenter of the earthquake. An earthquake can originate near the surface, deep inside the crust, or even in the layer called the upper mantle. the exact place of the origin of an earthquake is called the focus and the point above the focus is termed an Epicenter. The intensity of the earthquakes has many variations. these range from tremors so small that these are hardly detectable to freat shocks which can play havoc in a region.
Ten causes of an Earthquake (See Detail)
2. Types of Earthquakes:
On the basis of the depth of their origin, there are three types of an earthquake:
(1). Shallow-focus earthquakes (30 miles deep)
(2). Medium-focus earthquakes (30-150 miles deep)
(3). Deep-focus earthquake (150-450 miles deep)
3. Earthquake Zones:
Major Earthquake zones of the world include countries located on the Asian coast of the Pacific, the eastern coast of the Pacific covering the west coast of North America and Latin America. Detail of the major zones is as under:
- The heaviest concentration of earthquakes is along the circum-pacific belt. It includes countries of Japan, Philippines, Indonesia, New Zealand to the west, Aleutian Islands and southern Alaska to the North, and the USA, Mexico, Colombia, Ecuador, Peru, and Chile to the east of the Pacific.
- Trans-Eurasian Belt is another zone of Earthquake. It extends eastward from the Mediterranean Sea through South-west Asia and the Himalayas into Afghanistan, Northern Pakistan, North India and Nepal.
- The third major zone comprises mid-oceanic ridges.
- Moreover, many epicenters also lie scattered in interior Asia, in eastern Africa (Ethiopia to South Africa) , and in the North America east of the West Coast.
4. Earthquake Terminology
Focus: The point of origin of an earthquake within the earth’s crust is called the ”focus”. From the focus, the earthquake waves radiate in all directions.
Epicenter: The point lying vertically above the earth’s surface, directly above the focus, is called the ”epicenter”. In the epicenter, the shaking is most intense. The intensity gradually decreases outwards.
Isoseismal Lines: The imaginary lines connecting points of equal intensity on the ground surface are called ”isoseismal lines”. If the focus is a point, the isoseismal lines will be the circle, but as the focus is commonly a line, the isoseismals are generally elliptical. From the distribution of the lines, the epicenter of an earthquake can be determined
Earthquake intensity: It is a degree of destruction caused by an earthquake. It is expressed by a number as given in the earthquake intensity scale.
Effects of the Earthquakes:
Major earthquakes cause great damage to life and property. Earthquakes caused by movement along faults are very disastrous, especially near cities and areas of congested population. They demolish buildings and damage the infrastructure severely. Vulnerable infrastructure may be railway lines, roads, electricity lines, telephone systems, and bridges. The after-effects of the earthquakes are also very destructive because the tsunami tidal waves play havoc in the coastal areas. In recent history, coastal areas of Java, Sumatra, and Sri Lanka have witnessed colossal damages caused by these waves.
On November 1, 1755, tidal waves created by an earthquake in the ocean floor of the Atlantic West of Lisbon were 35 feet high. These waves not only razed most of the buildings in the coastal areas near Lisbon but also killed 60 thousand people. Similarly, on 1st September 1923, an earthquake destroyed half a million houses in the Japanese cities of Tokyo and Yokohama and killed 250,000 people. The effects of earthquakes are summarized in the following points;
- Buildings are damaged and people get frightened.
- Roads are fissured, railway lines are twisted, and bridges are destroyed.
- In cities, ground waves disrupt underground services and start fires.
- Rivers change their courses. Fissures are opened up in the ground which may cause spring.
- Permanent tilting of the landmasses may occur in certain areas. Landslides may occur in high lands.
7. How to protect from the effect of an earthquake: Read it
8. Causes of Earthquakes
An earthquake is a natural phenomenon, which has many causes behind it. Some of these are natural while others are man-made. Among these causes, the most important are as follow;
(1). Tectonic Movement causes Earthquakes: Earth’s crust has been cracked into large segments (fragments), which are separated along ridges. These segments/parts are known as plate-tectonics (Lithospheric Plates). The term tectonic plates is used to describe the active motile/moveable nature of these plates. There is a continuous process of convection within the lower mantel due to the extremely high temperature and its molten state. The convection causes the lower magma to move upward and the upper magma to move again downward. The type of movement is known as convection. Convection may be divergent as well as convergent. The convection within the mantle put an impact on the solid crust and made the crust crack into many pieces. Each cracked piece of the crust is known as the tectonic/ lithospheric plate. The convection of the mantle moves these plates. The plates under which the convection is divergent, move the plate apart from each other, while the plates, which are above the converging magma, move convergently (one plate overrides the other plate). There is a third type of tectonic movement, which is known as a transverse movement. The transverse movement of the two plates moves them horizontally across each other. The part of the crust where two plates transversely move across each other is known as the transverse boundary. Two transversely moving plates collide with each other and cause stress between the boundaries of the plates. When this stress is (abruptly) released, a huge amount of seismic energy is produced. The seismic energy results in an earthquake by sending strong seismic waves across the earth.
(2). Volcanic Eruption: Volcanic eruption is one of the major causes of earthquakes. These eruptions result in the expulsion of huge amounts of lava, stones, steam, and ashes from the crust of Earth. Thus the surface of Earth near the volcano is shaken. This phenomenon is called an earthquake. In 1883, an earthquake took place in Krakatoa (Indonesia) as a result of the volcanic eruption. It destroyed many villages around Java and Sumatra.
(3). Internal Movement of Earth: Internal movements of Earth comprise of the effects of all the internal forces which build up the features of the crust of our planet. The internal movements of Earth are also called the Tectonic Movements. These include diastrophic forces (faulting, folding, depression, and uplift) and vulcanicity. These forces are the major source of the internal movement of the Earth. Mostly tensional forces and compressional forces are responsible for an earthquake. The best example of an earthquake caused by the internal forces of Earth is the one, which shook Bihar (India) in 1934.
(4). Isostasy: The condition of vertical equilibrium between the floating landmasses and the asthenosphere beneath them is known as “isostasy”. This system of equilibrium is maintained due to a couple of internal forces, which are constantly in working conditions to change the landmasses. The isostasy adjustments cause earthquakes. In 1949, an isostatic Earthquake shook Hindukush mountainous region causing great destruction. Thus isostasy is maintained between the Sialic (Silicon Aluminium) crust and the Simatic (Silicon Magnesium) substratum resulting in earthquakes.
(5). Landslides: In mountainous regions, landslides also cause minor earthquakes. In this case, a large mass of rocks and soil travel downhill very rapidly. The downslope movement of the landmasses causes tremors.
(6). Ejection of Steam: Sometimes a huge volume of hot steam is ejected or moves underneath the surface of Earth. The pressure developed through this movement causes tremors in the region.
(7). Cavern Collapses: Cavern collapses are another source of minor earthquakes.
(8). Displacement of Ice Blocks: In snow-covered mountains, the fall of huge ice blocks into the valley causes minor tremors and shocks.
10. The magnitude of the Earthquake: Read it
11. Types of Earthquake Waves
The Earthquake waves are classified into two main types; (i) Body Waves, and (ii) Surface Waves.
Body waves include primary waves and secondary waves, while the surface waves include ”long waves”, and ”rayleigh waves”. Detail of these waves can be read in this article: Types of Earthquake waves