Major earthquakes and the majority of smaller ones are generated by sudden decrease or release, in the volume of rock, of elastic strain previously accumulated over an interval of time varying from a minimum of about one year in regions of great activity to many centuries in others. The accumulation of strain is produced by differential movements of contiguous portions of the earth's crust down to about 700 km. The release of strain results in the generation of a series of elastic waves (which should be distinguished from the continuous vibrations known as microseisms). These waves may: create landslides in hilly regions; change the direction of flow of rivers; disrupt man-prepared land areas; cause severe damage to buildings; or give rise to tidal waves, tsunamis. They are the most severe of natural disasters.
Between 1980 and 1985, earthquakes killed 28,404 persons across scattered parts of the world and caused damage estimated at US$ 36,439 million. Probably no area of the earth is entirely free of earthquakes if a sufficiently long time interval is considered. At present, however, the great majority are concentrated in a belt around the Pacific Ocean and in a wedge running from SE Asia through the Mediterranean to Portugal. About 50,000 earthquakes occur annually which are of sufficient size to be noticed without the aid of instruments. Of these, about 100 are large enough to produce significant damage if they occur in built up areas. The largest earthquakes occur at a rate of approximately one per year. During the last three centuries it is estimated that earthquakes have resulted in the loss of 2.5 million lives. Earthquakes of the magnitude that destroyed Maharashatra in India (killing 10,000) in 1993 and which shook Los Angeles in 1994 occur approximately 100 times per year. The earthquake of June 1990 in Iran caused estimated human losses of 40,000 dead. 500,000 people were rendered homeless. Vulnerability to earthquakes is however increasing because of increases in world population, especially in urban areas with high rise buildings, often in unsafe locations.