Earthquakes

The distributional pattern of earthquakes shows that there are three major belts in the world in which the earthquakes frequently occur. These earthquake belts are:

1. The Circum-Pacific Belt (Convergent Plate Boundaries) — It extends in west from Alaska to Kurile, Japan, Mariana and the Philippine trenches. Its one branch goes towards the Indonesian trench and the other goes towards the Kermadec-Tonga trench to the north-west of New Zealand. On the eastern side of the Pacific the earthquake zone follows the west coast of North America and continues southward along Pem and Chile trench on the west coast of South America.
2. The Mid-Atlantic Belt (Divergent Plate Boundaries) — This belt of earthquakes extends along the mid-oceanic ridges and several islands near the ridges of Atlantic Ocean. Earthquakes of moderate to mild intensity with shallow focus are recorded in this belt. The Rift Valley of East Africa and the Red Sea are considered as an extension of this belt.
3. The Mid-Continental Belt — This belt extends along the Alpine mountain system of Europe, Rocky mountains in North America, through Asia Minor, Caucasia, Iran, Afghanistan and Pakistan to the Himalayan mountain system including Tibet, the Pamir, Tien-Shan, Altai and the mountains of China, Myanmar and eastern Siberia. This zone is characterised by larger earthquakes of shallow origin and some of intermediate origin.

The instrument used to measure an earthquake is Seismograph or Seismometer.

Some examples of earthquakes of the world are:

1. May 31, 1970 in Northern Peru
2. January 26, 2001 in Gujarat, India
3. July 28, 1976 in Tangshan (China)
4. December 26, 2004 in South Asia, off the West Coast of Sumatra, Indonesia

Richter scale is an open-ended, logarithmic scale that estimates earthquake magnitude, designed by Charles Richter in 1935.

This scale can be related to the energy released at the earthquake centre and thus can be used as an estimate of the severity of a particular earthquake.

An earthquake can be defined as a shock or series of shocks due to a sudden movement of crustal rocks generated at a point known as seismic focus within the crust or the mantle. It is a violent tremor or shaking of part of earth crust.

The point of earth's surface directly above the focus of an earthquake is called epicentre. The intensity of earthquake is maximum at epicentre and it goes on decreasing as the distance from epicentre increases.

A seismograph is a device that measures seismic waves of the energy transmitted throughout the earth's interior.

The two main causes of earthquakes are:

1. Volcanic eruptions — Volcanic earthquakes are caused by gas explosions. They occur either simultaneously with eruption or more commonly in the period preceding an eruption. For example - Krakatoa volcano caused a severe earthquake that its impact was experienced at Cape Horn (12,800 km away).
2. Plate Tectonics — Tectonic plates move and slide over each other and their edges produce faults along the line of weakness. This movement of plates causes earthquakes. For example - In 2001, a severe earthquake occurred in Bhuj, Gujarat in India because of the lowering of the Indian plate below the Asiatic plate.

Two destructive effects of earthquakes are:

1. Building Collapse — The earthquakes of high intensity lead to collapse of buildings. People can be trapped under the rubble. There is loss of lives and properties.
2. Landslides — Earthquake causes landslides in hilly areas or liquefaction of soils resulting in damage of properties and loss of lives and disturb the transport system. This occurs due to liquefaction of soils.

Two constructive effects of earthquakes are:

1. Earthquakes may result in fissure opening causing a Geyser or hot spring which are useful from medicinal point of view as it contain sulphur.
2. Sometimes earthquakes result in formation of coastal submergence and changing the coastal forms, forming bays and may prove to be helpful in navigation.

The seismic waves travelling through the ocean resulting in long wavelength shallow water wave caused by rapid displacement of water is called Tsunami. Its velocity can reach 800 km per hour. It causes massive destruction and flood in coastal areas.

A fracture in a rock along which there has been an observable amount of displacement is known as fault. Earthquakes occur when the movement of the Earth takes place along a line of fracture or faults. The San Andreas Fault of California is a typical example which led to earthquakes in 1906.

Seismic Focus refers to the exact point inside the Earth's crust from where the shock or sudden movement of earthquake is generated.

Due to impact of severe earthquake, the dams and embankments develop fissures which become the cause of sudden floods known as flash floods. They cause severe loss of life and property.

A landslide is defined as the movement of a mass of rock, debris, or earth down a slope. Earthquake causes landslides in hilly areas resulting in damage of properties and loss of lives.

Most earthquakes are caused by tectonic movements because the tectonic plates are in continuous slow motion. There is friction on their edges due to which they are stuck. Whenever there is change in the pressure exerted by them, a sudden movement or overlapping of plates occur resulting in earthquakes. Faults & folds are formed when the plates collide with each other or diverge or slide apart, the energy inside the earth crust is released in the form of vibration or earthquake along the fault line.

Vibration or earthquake may happen due to large scale gaseous explosion during the volcanic eruption. Therefore, earthquakes are associated with volcanism.

Sometimes earthquakes result in the formation of coastal submergence and changing the coastal forms, forming bays. This may prove to be helpful for Navigation. Also, by studying the seismic waves generated by earthquakes, scientists have been able to create detailed maps of the ocean floor. These maps help to create accurate navigational charts for ships and submarines.

Japan is situated at the intersection of four tectonic plates, the North American Plate, the Eurasian Plate, the Pacific Plate, and the Philippine Plate. The country sits on the Pacific Ring of Fire which is characterized by high volcanic and seismic activity. Due to these reasons, Japan is an earthquake and Tsunami prone area.

Seismology

Richter scale

It is situated on the surface of the earth & from here the earthquake waves spread horizontally on the earth surface.

Indo- Australian & Eurasian Plate

Tsunami

Taiwan is located near a subduction zone along the boundary of the Pacific Plate, where tectonic plates are constantly colliding and moving. This region is geologically unstable, making it highly prone to frequent earthquakes due to the release of pressure and energy along these active fault lines.

Both A and R are true and R is the correct explanation of A.

Explanation — In the young-fold mountains like Andes, Rockies, Alps and the Himalayas, plates are always in movement causing frequent earthquakes. These earthquakes result into landslides.

Both A and R are true and R is the correct explanation of A.

Explanation — The crust of the earth is made up of a series of movable plates. These plates move and slide over each other (sometimes colliding) and their edges produce faults along the line of weakness. This movement of plates causes earthquakes. Most earthquakes are caused by tectonic movements.