Category The Earth, Earth Science, Planet Earth

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.

The moving tectonic plates, in which the uppermost layer of the Earth is divided, not only collide at many places but also often slide past each other. This causes friction and leads to problems once in a while. Stress builds up, which then gets discharged very quickly. There is a jerk—the Earth trembles. This gives rise to shock waves that can cause a lot of damage at places, depending on the intensity of the earthquake and where they occur. Luckily, to some extent we can protect ourselves from these waves. A tsunami occurs if the centre of the earthquake lies below the sea. 

Volcanoes are found primarily at the borders of the plates, where the oceanic Earth’s crust submerges below another plate, melts in the depth, and magma rises. This is generally the case all around the Pacific Ocean. Here, the Cocos and the Nazca plates submerge below the middle and South America, and the Pacific plate submerges below the small plates in Southeast Asia. 

Volcanoes are found in areas ranging from Italy to Turkey and Iran upto the Indonesian islands. Very rarely will we find volcanoes in the middle of a plate. These occur only in ‘hot spots’ which are areas with high volcanic activity, such as the Hawaiian Islands. 

Farmers prefer to settle near volcanoes because the soil is very fertile due to the lava and volcanic ash deposits. When the volcano erupts, people leave the place for some time, and return to the risk zone after the eruption because the safer places are already inhabited. Moreover, for many people the area around the volcano is home, which they do not want to leave.

Because volcanoes are highly erratic, it becomes very difficult to predict volcanic eruptions. But there are ways of saving lives. Early tremors and earthquakes are often signs of a volcanic eruption—gases are emitted and the volcano expands by a few centimetres to a metre. The volcanologists carefully observe these signs. In 1991, before the eruption of the Pinatubo in Philippines, these signs were interpreted correctly and over 10,000 people were moved to safer places. 

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