And then the earth shook
By Naresh Kochar

EARTHQUAKES are vibrations of the earth caused by the rupture and sudden movement of rocks that have been stretched beyond their elastic limit. Three major types of seismic waves are generated by an earthquake shock. Each type travels at a different speed and each is recorded by an instrument called seismograph. The first waves to arrive are called primary waves (P waves). These waves are identical to sound waves passing through a liquid or gas. The particles involved in these waves move forward and backward in the direction of wave travel, causing relatively small displacement. The next waves to arrive are secondary waves (S waves). In these particles oscillate back and forth at right angles to the direction of wave travel. The S waves cause strong movement to be recorded by the seismograph. The last waves to arrive are surface waves which rather travel slowly over the earth’s surface. Particles involved in surface waves move in an orbit similar to that of particles in water waves.

The point within the earth where the initial slippage generates earthquake energy is called the focus. The point on the earth’s surface directly above the focus is called the epicentre. Shallow-focus earthquakes occur from the surface to a depth of 70 km. The deep focus earthquakes occur between 300 and 700 km. The intermediate focus earthquakes occur between 70 and 300 km. The shallow-focus earthquakes cause the maximum damage. The Chamoli quake is of this kind.

How to locate the epicentre? The P waves travel faster than the S waves and are, therefore, recorded first at the seismic centre. The time interval between the arrival of the P wave and the S wave is the function of recording the station’s distance from the epicentre.

The magnitude of an earthquake is a measure of the amount of energy released. The magnitude is based on direct measurement of the size (amplitude) of seismic waves made with the recording instrument. The total energy released by an earthquake can be calculated from the amplitude of waves and the distance from the epicenter. The magnitude is expressed by using the Richter scale. This is a log scale. So the vibrations of an earthquake with a magnitude of two are 10 times greater than inamplitude of an earthquake with a magnitude of one. The largest earthquake ever recorded had a magnitude of approximately 8.8 on the Richter scale. Significantly, larger earthquakes are not likely to occur because rocks are not strong enough to accommodate more energy.

The intensity is the destruction power of an earthquake. It is an evaluation of severity of ground motion at a given location. The intensity of an earthquake at a specific location depends on a number of factors: (a) total amount of energy released; (b) distances from the epicentre; (c) type of rock and degree of consolidation. In fact, the wave amplitude and destruction are greater in soft and unconsolidated material than in dense crystalline rocks such as granite and basalt.

Earthquakes demonstrate that earth continues to be a dynamic planet changing each day by internal tectonic forces. The primary effect of an earthquake is ground motion. Secondary effects include landslides, tsunami and regional or local uplift or sunsidence. Tsunami are seismic sea waves which can go as high as 60 ft above the normal sea level and are caused by earthquakes and volcanic activity. Most of the damages to the buildings in Chamoli are due to landslides and sinking of ground.

The theory of ‘elastic rebound’ can be applied to all the earthquakes. This theory can be illustrated by a simple experiment. Bend a stick until it snaps. Energy is stored in the elastic bending and is released if rupture occurs, causing the fractured ends to vibrate, and send out sound waves. A detailed study of active faults shows that this model applied to all the earthquakes.

According to the theory of plate tectonics, the earth’s surface consists of six major and many small plates in relative motion. The plate regarded as rigid comprises 100 km thick lithosphere. The plate motion is caused by convection currents in the underlying ‘plastic layer’ of asthenosphere. There are three kinds of plate boundaries: (a) convergent; (b) divergent; (c) transform faults. The Himalayas are an example of convergent plate boundaries. The Indian plate has moved from south to north and collided with Asia about 65 million years ago. It travelled some 7000 km in about 30 to 20 million years, breaking away from Africa and Madagascar. The Indian plate still continues to move 5.4 cm per year, The Himalayas and the Tibetan plateu define a wide belt of shallow earthquakes.

Two fault lines pass through the Himalayan region — the main central thrust (MCT) and the main boundary fault (MBF). Chamoli and Uttarkashi fall on the MCT, whereas the Shimla-Dehra Dun-Almora belt lie on the MBF. The entire region falling between the MCT and the MBF is sesimically active and lies in seismic Zone V.

The rate at which the Indian plate is moving northward can be ascertained by palaeomagnetic studies as well as by satellite. The MCT and the MBF are characterised by tectonically disturbed and fractured rocks and loose regoliths which help in wave propagation, thereby causing damage to buildings and roads.

The seismic waves generated by nuclear explosions are of compression (P wave) type with more rapid risetime but of shortened duration. That is how the earthquake waves are distinguished from the nuclear explosion disturbances.

The peninsular India comprises a shield, which is supposed to be a geologically stable area. But in the recent past the area has been witnessing seismic activity — Latur earthquake and the one in the Jabalpur.

There is no cause of major earthquakes in and around Delhi and Chandigarh as the tremors felt here in the recent past have helped the earth to release its stresses. However, there is an urgent need to open many new seismic centres/stations to monitor the shock waves.

Earthquakes are also caused by volcanic eruptions. But fortunately there are no active volcanos in India. Tosham in Haryana has the site of an ancient extinct volcano which erupted some 750 million years ago.

Effective earthquake prediction is an elusive goal. However, Chinese scientists claim to have been successful in predicting about 15 earthquakes in the recent past. They rely heavily on the centuries old idea that animals sense various underground changes prior to an earthquake and behave abnormally. Most of the bizarre behaviour is simply increased restlessness. Cattle, sheep and horses refuse to enter their corrals. Rats leave their hideouts, ants pick up their eggs and migrate enmass.

Much of the work on earthquake prediction in the USA has shown that rocks are subjected to stress swells just before an earthquake erupts. As the rock dilates, changes in electrical resistance, seismic wave velocities, and magnetic properties occur. There is also emanation of gases. But all these properties need to be closely monitored for a correct prediction.

Some do’s and don’ts;

Do not panic. Stay calm and let others stay calm.

During an earthquake, the safe place is under the bed or table.

Don’t move. Don’t switch on a light or any other electrical appliances.

Keep the gas cylinder off. Don’t light it.

If the earthquake occurs during day time, move out of the house. Stay in a park or open place.

In an earthquake-prone areas, store food, drinking water and medicines, including first aid box.

Keep some chlorine tablets to disinfect water, as the water supply is likely to be affected. It is advisable to keep the fire extinguisher handy.