Shock waves:
Six kinds of shock waves are generated in the process. Two are classified as body waves—that is, they travel through the earth's interior—and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are travelling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and be recorded at geophysical research stations throughout the world.

Seismologists have devised two scales of measurement to enable them to describe earthquakes quantitatively. One is the Richter scale—named after the American seismologist Charles Francis Richter—which measures the energy released at the focus of a quake. It is a logarithmic scale that runs from 1 to 9; a magnitude 7 quake is 10 times more powerful than a magnitude 6 quake, 100 times more powerful than a magnitude 5 quake, 1,000 times more powerful than a magnitude 4 quake, and so on.
An estimated 800 quakes of magnitudes 5 to 6 occur annually throughout the world, in comparison with about 50,000 quakes of magnitudes 3 to 4, and only about one earthquake of magnitudes 8 to 9. Theoretically, the Richter scale is an open-ended one, but until 1979 an earthquake of magnitude 8.5 was thought to be the most powerful possible. Since then, however, improvements in seismic measuring techniques have enabled seismologists to refine the scale, and 9.5 is now considered to be the practical limit. On the basis of the newly refined scale, the magnitude of the 1906 San Francisco earthquake has been revised from 8.3 to 7.9.
The other scale, introduced at the turn of the 20th century by the Italian seismologist Giuseppe Mercalli, measures the intensity of shaking with gradations from I to XII. Because seismic surface effects diminish with distance from the focus of the quake, the Mercalli rating assigned to the quake depends on the site of the measurement. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale can be correlated with magnitudes 8 to 9 on the Richter scale.

Earthquake Prediction:
Attempts at predicting when and where earthquakes will occur have met with some success in recent years. At present, China, Japan, Russia, and the United States are the countries most actively supporting such research. In 1975 the Chinese predicted the magnitude 7.3 quake at Haicheng, evacuating 90,000 residents only two days before the quake destroyed or damaged 90 per cent of the city's buildings. One of the clues that led to this prediction was a chain of low-magnitude tremors, called foreshocks, that had begun about five years earlier in the area. Other potential clues being investigated are tilting or bulging of the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behaviour.

 

Broken road after earhtquake