Category Science

The word ‘oceans’ implies the large water bodies of the world, namely, the Pacific, Atlantic, Indian, and Arctic, as well as the Antarctic oceans where we find icebergs. Smaller seas are separated from the oceans by island chains or underwater ridges, which are called ‘bordering seas’. These include the North Sea, the Baltic Sea, the Mediterranean Sea, and the Caribbean Sea. The bordering seas and the oceans have some common features: both contain salty water and restless seabeds. But there are also differences in the variety of plants and animals living in them and the storm tides—the tides that lash the coast of the North Sea are hardly found along the Pacific coast. 

The Japanese word ‘tsunami’ means ‘harbour wave’. It is a series of water waves caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes. Tsunamis are caused by undersea landslides or by ‘seaquakes’ that is an earthquake whose epicentre lies below the ocean. If the seabed is displaced up to several metres, the water layer of several kilometres in thickness above it also moves with it. The wave caused by this movement has a lot of energy—much more than a wave of the same height whipped up by the wind. This becomes apparent when the wave moves towards the coast.

Living in an earthquake-prone zone is often unavoidable. For instance, almost entire Japan lies in an earthquake-prone zone. For this reason, buildings here are primarily made ‘earthquake resistant’ so that the loss can be minimized. Buildings are built on foundations made of steel balls or from elastic materials such as bamboo. In skyscrapers, a pendulum weighing several tons is installed in the upper floors, which minimizes the vibrations that start in the upper storeys when an earthquake occurs. If an earthquake occurs, we should take refuge, at best, in the open or seek protection under a table. 

A weak earthquake— up to an intensity of 4.0 on the Richter scale—hardly causes any damage. Earthquakes with stronger intensities can cause cracks in buildings. They move the foundations of buildings, cause houses to collapse and lead to landslides. Supply lines get disrupted and gas leakage causes fires. In large earthquake catastrophes, bridges, dams, and power plants get damaged—and often whole areas are devastated. 

The intensity of an earthquake is measured by recording the tremors of the Earth’s surface with a ‘seismograph’. These tremors appear when the earthquake waves reach the surface of the Earth. Such a seismograph consists of a large, inert weight, which stays at rest even during earthquakes. A pen is attached to this pendulum. When the Earth trembles, the surface of the Earth, together with the paper placed there, moves to and fro below the ‘writing pendulum’ which does not move. We can then detect the different types of waves on the paper, which move at different speeds. The intensity of the earthquake can be calculated from these wave patterns. The intensity is measured on a scale, called ‘Richter scale’, named after its inventor. Earthquakes with an intensity of 2.5 can be felt, but ones with an intensity of 8.0 are very destructive.

Earthquakes occur mainly at the borders of the plates, where two plates slide past each other. Most earthquakes occur in the Himalayan region. The actual seismic centre, the ‘hypocenter’, lies deep below the surface of the Earth. The point on the surface of the Earth lying immediately above this is called the ‘epicentre’. Scientists can determine this point. They measure the time it takes the shock waves to reach the seismic stations and draw circles around the epicentre and two other seismic stations. The radius of these circles corresponds to the measured time. The point of intersection is the epicentre. Right below the epicentre, beneath the Earth’s surface, lies the hypocenter.