Useful information on the Distribution of Earthquakes

According to a moderate estimate about 30,000 earthquakes occur every year. But most of these are so slight that we cannot feel them. There is no visible damage from them. But every year there are some earthquakes of great intensity and magnitude.

If one of these occurs in a densely populated region, there is damage and destruction enough to draw people’s attention all the world over.

Every year hundreds of earthquakes pass unnoticed because they occur in areas where there is no possibility of any loss of human life and damage to property.

Earthquakes have a definite distribution pattern. There are three major belts in the world which are frequented by earthquakes of varying intensities. These belts are as under:

1. The Circum-Pacific Belt

2. The Mid-Atlantic Belt

3. The Mid-Continental Belt

1. The Circum-Pacific Belt:

This belt is located around the coast of the Pacific Ocean. In this belt the earthquakes originate mostly beneath the ocean floor near the coast. The Circum- Pacific Belt represents the convergent plate boundaries where the most widespread and intense earthquakes occur.

This belt runs from Alaska to Kurile, Japan, Mariana and the Philippine trenches. Beyond this, it bifurcates into two branches. One branch going towards the Indonesian trench and the other towards the Kermac-Tonga trench to the northwest of New Zealand.

This belt is located on the western side of the Pacific Ocean. On the eastern side of the Pacific Ocean, the earthquake belt runs parallel to the west coast of North America and moves on towards the South along the Peru and Chile trench lying on the west coast of South America.

This belt has about 66 percent of the total earthquake that are recorded in the world. Most of the earthquakes occurring in this belt are shallow ones with their focus about 25 km deep.

It may be pointed out that these belts being the zones of convergent plate boundaries (the subduction zones) are isostatically very unstable. Japan alone experience about 1500 earthquakes per year.

2. The Mid-Atlantic Belt:

This belt is characterised by the sea floor spreading which is the main cause of the occurrence of earthquakes in it. This earthquake belt runs along the mid- oceanic ridges and the other ridges in the Atlantic Ocean.

In this belt most of the earthquakes are of moderate to mild intensity. Their foci are generally less than 70 km deep. Since the divergent plates in this belt move in opposite directions and there is splitting as well, transform faults and fractures are created.

All this becomes the causative factor for the occurrence of shallow focus earthquakes of moderate intensity. The sea floor spreading is the main cause for the occurrence of earthquakes in this belt.

3. The Mid-Continental Belt:

This belt extends along the young folded Alpine mountain system of Europe, North Africa, through Asia Minor, Caucasia, Iran, Afghanistan and Pakistan to the Himalayan mountain system. This belt continues further to include Tibet, the Pamirs and the mountains of Tien Shan etc.

The young folded mountain systems of Myanmar, China and eastern Siberia fall in this belt. This belt happens to be the subduction zone of continental plates. It is in this belt that the African as well as Indian plates sub-duct below the Eurasian plate.

This Mid- Continental belt is characterised by experiencing about 20 per cent of the earthquakes in the world. This belt records earthquakes of shallow and intermediate origin. However, it is true that sometimes earthquakes of great violence occur in this belt.

This belt forms a great circle approximately east and west around the earth, through the Mediterranean, Southern Asia, Indonesia and the East Indies, where the great majority of recorded shocks occur.

It may be pointed out that more than 50 percent of all earthquakes are associated with the young folded mountains which are said to be still growing.

The Andes, Himalayas and Coast Ranges of the United States are the specific examples. It is worthwhile to remember that this girdle of young fold mountains has no correspondence with the line of active volcanoes like the Circum-Pacific earthquake zone.

There are some regions on the earth’s surface which are relatively immune from violent and vigorous earthquakes. This is so because diastrophism and volcanism are either absent or only moderately active. But the infrequent occurrence of minor shocks in such regions is not ruled out. Such shocks may occur due to local causes like the subterranean movement of imprisoned gases or liquids.

The most glaring example of the occurrence of minor earthquakes in quite unexpected places is the Koyna earthquake which shook Koynanagar on September 13 and 14, 1967.

It is believed that the earthquake in this stable area was caused due to the building of a 103 m-high concrete dam across the Koyna River which impounded a huge volume of water to form an artificial lake.

The magnitude of the earthquake was 6.5 on the Richter scale. Then again on December 11, 1967, the most disastrous earthquake occurred in the same area which affected the whole of western Maharashtra. The zone of maximum intensity of the shock and its epicentre was in the vicinity of Koynanagar.

The death toll rose to 1000 people, and a large number of people were injured. Its impact was felt as far north as Ujjain and as far South as Bangalore. Other towns like Surat, Ahmedabad, Broach and Hyderabad also felt the shock.

Since there was no record of earthquakes in this particular region before 1962, it was thought that Koyna earthquake was caused due to the hydrostatic pressure exerted by the reservoir. But the recent investigation does not support this view.

Now, the geologists are of the opinion that the shock in this region was the result of tectonic movement along a north-south axis of weakness in the underlying rocks buried below the Deccan trap.