In order to understand what causes volcanic eruptions and earthquakes, we must first understand the structure of the earth. The formation of the earth lasted for some centuries. At about the same time, mountains were formed and new oceans emerged. All this would not have happened if volcanoes did not erupt and earthquakes did not take place.
Earth quakes occur mainly at the borders of the plates where two plates slide passes each other. Most earth quakes occur in a Himalayan region. The actual seismic centre, the ‘Hypocentre’, lies deep below the surface of the Earth. The point on this surface of the Earth lying immediately above this is called ‘Epicentre’. Scientists can determine this point. They measure the time it takes the shock waves to reach the seismic stations. The radius of these circles corresponds to the measured time. The point of intersection is the epicenter.
Measuring the intensity of earthquakes: 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 different types of waves on the paper, which move at different types of waves on the paper, which move at different speeds.
Damages of earthquakes:
A weak earthquake – Upto 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. In large earthquake catastrophes bridges, dams, and power plants get damaged. Often, whole areas are devasted.