CBSE Book Class 8 Disaster Management Earthquakes

EARTHQUAKES

An earthquake in simple terms is a sudden trembling or shaking movement of the earth’s surface, called the crust. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined by the use of scales, such as the Richter scale and the Modified Mercalli scale.

Why do earthquakes happen?

The earth’s crust is made of massive interlocking blocks of rock called tectonic plates, which resemble a jigsaw puzzle. These blocks float on a layer of semi-liquid rock called the mantle. This flowing semi-liquid rock causes the blocks in the crust to move against or relative to each other. As tectonic plates collide or move, pressure is built up.

This pressure is released when these plates slip or slide relative to each other, resulting in a fault. These movements cause vibrations to pass through and around the earth in waveform, just as ripples are generated when a pebble is dropped into water. Earthquakes are natural phenomena. They cause the most sudden of all disasters, without any warning time. Floods and cyclones, the hazards that cause the other major sudden disasters have a warning period, and hence can be anticipated to some extent, allowing for evacuation and protection. Earthquakes happen without notice, and are not predictable or preventable. We can however reduce the damage that they can cause. Most of the damage is caused not by the earthquake itself, but by the buildings that we live in, especially in urban or semi urban areas with concrete structures. The picture shows a high-rise building succumbing to the Gujarat earthquake in 2001 because the required structural engineering norms were not adhered to during construction and design of this building.

How are people affected by earthquakes?

The effects of earthquakes are diverse. People are more likely to die or be injured where large numbers of people live close together, and where local buildings are not designed to resist earthquakes. About 95 per cent of people killed in an earthquake are killed by falling buildings. Earthquakes are most dangerous when they happen at night. This is because people may sleep through the first tremors (foreshocks), and so have less time to prepare. Also, lying flat in bed means that you are more likely to be hit by falling objects than if you are standing up. The effects of an earthquake are strongest in a broad zone surrounding the epicentre.

An earthquake can cause other dangerous events, such as landslides, floods, fires, and huge ocean waves called tsunamis. Why are some people more at risk from disasters than others? Simply, Some people are more at risk because of where they live. Others may be more at risk because of their socio-economic situation. Certain types of disasters are specific to certain geographical areas. Earthquakes commonly happen along a geological fault (a place where two sections of the earth’s crust join). Other types of natural hazards, such as flooding, are widespread across the globe.

Earthquake vibrations last longer and are of greater impact in unconsolidated surface material, such as poorly compacted fill (loose soil) or river deposits; bedrock areas receive fewer effects. Hence, flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. When disaster strikes, the poor are usually the worst affected. This is because they are likely to have less choice about where they live, and many poor communities are forced to settle on dangerous land – on steep hillsides, or in river floodplains for example. In poorer regions, people often build their own houses, and may not have access to information about how to make their homes safer. Even when people do have information, they may not be able to afford stronger, better-quality building materials.

After a disaster, poorer people find it hard to recover. The physical injuries and destruction, the loss of livelihoods can have serious long-term consequences. If people lose vital tools, or means of transport, then they will have no way to support themselves and their families for the future. They are usually people with no insurance, and will probably have to make do with whatever they can find. They may find it very difficult to regain the position they were in before the disaster struck.

Please refer to the link below for - CBSE Class 8 Disaster Management - Earthquakes