Earthquake

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The earth is constantly undergoing change. Some changes are fast and some are slow. Earthquakes and volcanoes are endogenic forces which cause sudden changes on the earth. Weathering and agents of sculpture are exogenic forces which bring about slow changes. In the following pages a description has been given about the earthquakes and volcanoes.

Earthquake is the trembling of the earth initiated by sudden shock. Sometimes it is so violent that it damages or destroys strong buildings. Most of the shocks that cause the earth to tremble are associated with diastrophic movements. The sudden slippage of rocks along the faults and also in some cases faults reaching the ground surface, displace the objects on the ground. In other cases, crustal rocks bend elastically and set waves in motion.

The place where the earthquake originates inside the earth is called its focus. The point on the earth’s surface vertically above the focus is called the epicentre. The waves travel away through rocks in all the directions. Shaking is the strongest near the epicentre. Great accumulation of elastic strain is built up constantly along active lithospheric plate boundaries, particularly along converging (where two plates meet) boundaries. Slippage of opposed plates in these converging zones release enormous quantities of energy in the form of great earthquakes. The relationship between earthquake activity or seismicity and lithospheric plate boundaries is remarkably strong.


TYPES OF SEISMIC WAVES

There are three types of seismic waves generated by earthquake called Primary, Secondary and Long waves designated as P, S, and L respectively. P waves are fast travelling waves; they arrive first. S waves are slower; they take almost twice as long to travel. In fast travelling P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves the particles vibrate up and down at right angles to the direction of propagation.

The P waves travel through the body of the earth at an average rate of 6 km/sec. They are faster at depth (about 8 km/sec to 11 km/sec) than near the surface. They travel through both solid and liquid medium. The S waves also go through the body of the earth but at a slower rate of 3.5 km/sec. L waves follow the circumference of the earth and travel at more or less constant rate. They are also known as surface waves. Thus, P and S waves travel by a direct path through the earth while L waves follow its circumference.

The Seismograph

The seismograph is a sensitive instrument that records earthquakes thousands of kilometres distant and so weak that their vibrations could not possibly be recognised by the human senses. The seismograph design is based on the principle of inertia — the tendency of any mass to resist a change in a state of rest or of uniform motion in a straight line is greater, the greater the mass of the object. The first indication that a severe earthquake has occurred at a distant point is the sudden beginning of the Primary waves (P waves). These waves then die down somewhat for a few minutes; then a second burst of activity sets in with the arrival of Secondary waves (S waves) which are somewhat larger in height than the P waves. Following it, smooth waves that increase greatly in amplitude to a maximum and then die slowly are recorded. These are the surface waves.

Two measurements are made directly from the seismograph:
1. Amplitude in mm of the largest (highest) wave recorded; and
2. Time in seconds elapsed between arrivals of the P and S waves.

Earthquake Prediction

Two approaches have been adopted in earthquake prediction. One approach is the measurement of several kinds of significant physical changes that take place immediately prior to an earthquake

The second approach is historical i.e. long-term seismic history of the affected region. The physical changes are:
P Wave Velocity

Numerous small earthquakes are expected to cause change in the velocity of P waves that return to normal immediately before the large earthquake. This change is monitored on seismograph.
Ground Uplift



Slow movement of crustal masses before the earthquake causes a large volume of rock to become riddled with countless minute cracks or microcracks. The ground water moves into the newly created microcracks. The presence of water acts like hydraulic jack; the rock mass swells and the ground surfaces experience doming or uplift before major earthquakes. This change is referred to as dilatancy.
Radon Emission



Emission of radon gas increases just before a major earthquake takes place. Monitoring of the emission of radon gas can serve as forewarn to the major destruction.
Animal Behaviour



It has been observed that prior to the major earthquake, animals especially burrowing animals, behave unusually. The ants, termites and other burrowing animals shift their hiding places. The birds chirp loudly and animals such as dogs howl and bark very regularly.
Induced Earthquake



In a number of cases human activities have set off earthquake. The pumping of fluids into the oil fields to raise the hydrostatic pressure and to increase oil recovery is responsible for setting off minor earthquakes. Human beings are responsible for setting off earthquakes by building large dams on major rivers. The load of water from new lakes impounded behind the large dams is responsible for triggering earthquakes.

DISTRIBUTION OF EARTHQUAKE ZONES

Abundance of large earthquakes in a ring surrounding the Pacific Ocean, known as Circum-Pacific Belt is a line of intense volcanic activity as well as seismic activity. The second belt of intense seismicity stretches from Mediterranean Sea to the Indonesian region, corresponding to tectonically active boundary between Eurasian Plate and the African and Australian Plates. A third belt of high seismicity runs down the middle of the Atlantic Ocean basin through the Indian Ocean and across the southern Pacific Ocean.