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.
Effects of the Earthquakes:
Major earthquakes cause great damage to life and property. Earthquakes caused by movement along faults are very disastrous, especially near the 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.
Ten causes of an Earthquake (See Detail)
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)
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.