Role of plate tectonics in earthquake occurrence

THE FACE of the earth with the land mass separated by oceans as we see today is not what it was some 225 million years ago. At that time the land was one single continent called Pangaea (meaning "all lands" in Greek) and the ocean occupied the rest of the earth's surface. So one could actually walk from one end of the land to the other. That was long time ago. Then came the separation of the land mass and its drift in different directions over the surface of the earth. Initially the Pangaea split into two major continents called the Laurasia and Gondwanaland. This was 200 million years ago.

With the passage of time even these two continents split into a number of smaller land masses and these land masses continued their journey in different directions. The drifting continued till such time it attained its present configuration. Today, the land mass is made of several plates moving relative to each other. The "Continental Drift" hypothesis put forth by German geologist A Wegener tried to explain the many unanswered questions that were haunting the geologists, paleontologists and geophysicists. The tectonic force that separated the Pangaea into separate land masses and their drift in many directions did not stop even after the continents came in contact with one another at many places.

Interestingly it still continues at the rate of few centimetres a year and has been the major force in creating mountains, volcanoes and earthquakes on the earth's surface.

Interestingly the volcanoes and earthquakes seem to be concentrated in certain areas or zones of the earth. This does not negate the possibility of some stray incidents of earthquakes occurring in areas other than the specified zones. The movement of the plates and the concentration of the earthquakes in certain zones made the geologists and geophysicists probe for answers.

The hypothesis left many difficult questions to be answered. Particularly, if the hypothesis was indeed right and if the continents still continued moving then what indeed was the driving force. Importantly, what phenomenon produced and confined the earthquakes and volcanoes to small stretches of land?

The puzzle had to be first solved by uncovering the mystery of the driving force which moved the continents across huge distances and which still continued to push them in spite of many plates already touching each other. It was found that unlike the theory that was floating around at that time, the continents did not plough through the oceans.

Instead it was found that the continental crust actually floated above the oceanic crust. This the scientists explained was because the continental crust though several hundreds of kilometres in thickness compared to 25 km of the oceanic crust was lighter due to the presence of lighter minerals like silica and feldspar.

While the heavier oceanic crust helped in explaining how the lighter continental crust could have drifted, it threw up new questions. How could the oceanic crust be lesser in thickness than the continental crust? Searching for this answer also brought about another interesting observation.

The oceanic crust unexpectedly had relatively younger rocks and sediments. Older rocks, a common feature in the continental crust was missing in the oceanic crust. Thus two very interconnected questions had to answered-the smaller thickness of the oceanic crust and the absence of older rocks in the oceanic crust.

Scientists searching for these answers did stumble on a very interesting feature on the ocean floors. Confirming the echo- sounding measurements that demonstrated the ocean floor as not a relatively flat and featureless surface, the scientists did find a very unusual feature-a great mountain range on the ocean floor virtually encircling the entire earth. Called the global mid- oceanic ridge, the submarine oceanic chain more than 50,000 km long and at places more than 800 km across winds between continents. Again this oceanic mountain chain is no small feature as it stood nearly 4,500 metres above the sea floor.

Most prominent amongst this mid-oceanic ridge is the Mid-Atlantic Ridge, which stands majestically taller than even the Himalayas, the highest mountain range seen on land. It was not the size of the global mid-ocean ridge alone that attracted the attention of the scientists but a very unusual and totally unexpected characteristic. The rocks were youngest at the ridge crest and progressively older as one moves away from the ridge crest.

Actually the rocks are in the form of stripes and parallel to the ridge with the age of these stripes progressively increasing away from either side of the ridge crest. The stripe characteristics are symmetrical on either side of the ridge crest. Apart from the age of the stripes, the rock stripes had alternate magnetic polarity (normal-reversal-normal) with the stripe at the ridge crest always having the present day magnetic polarity.

All these evidences did finally help in pointing out the continuous formation of new oceanic crusts as magma flows out of the ridge crest and literally pushes the older sea floor away from the crest. This phenomenon is called the "Sea Floor Spreading." The rate of sea floor spreading along the Mid- Atlantic Ridge averages about 2.5 cms per year. Sea floor spreading over the past 200 million years had resulted in the growth of the Atlantic Ocean from its initial state of a tiny inlet of water between the continents of Europe, Africa and Americas.

But this did not explain why the oceanic crust was thinner than expected and was very much younger compared to continental crust. But it did seem to explain the driving force required to push the continents every year to its present location and still continue pushing the plates. So the driving force behind the continent's motion was explained. This especially since the mid-oceanic ridge was not a local phenomenon but was a global feature with its 50,000 km chain and encircling the continents.

There were also other questions. Was the earth's crust continuously growing in size and the area of the earth's crust ever increasing? Was the earth expanding in size? Many scientists believed that the earth was of the same size since its formation 4.6 billion years ago. Hence some other phenomenon was operational to explain these missing links.

Harry H. Hess, a geologists at the Princeton University, and Robert S. Dietz, a scientist with the U.S. Coast and Geodetic Survey, understood the implications of sea floor spreading. Hess reasoned that if new oceanic crust was being formed continuously around the globe and if the earth was not expanding then the oceanic crust must be consumed by the earth at other locations.

It was found that the oceanic crust moving in a conveyor belt- like motion finally descends into the earth along the oceanic trenches-very deep (several hundreds of kilometers into the earth), narrow canyons along the rim of the Pacific Ocean basin. Thus new oceanic crust was being formed at the mid-oceanic ridges and consumed at the trenches. In short, the oceanic crust was continuously being recycled. This explained why oceanic rocks were younger in age, thinner and the earth remained without expanding in size.

Just like how new oceanic crust was being formed along specific locations and consumed along definite places, the concentration of earthquakes along specific zones became clear. There indeed seemed to be a connection between earthquakes and trenches and ridges, and along certain plate margins where indications of plate collision in the form of mountains were present.

Plates and oceanic crust that are in constant motion are consumed or adjusted to maintain the same size of the earth. Oceanic crust get consumed or destroyed when it comes in contact with a large continental crust. This happens as a rule as the oceanic plate being heavier than the lighter continental crust always sinks into the earth. Sinking or subduction also happens when two oceanic plates or two continental plates collide.

Along the Peru-Chile trench, the oceanic Nazca plate subducts under the South American plate. The South American plate called the overriding plate is being lifted up due to subduction and this has created the Andes Mountains. Though the Nazca plate is subducting smoothly into the trench, the deepest part of the subducted plate breaks into smaller pieces which become locked up. These locked up plates suddenly move to generate large earthquakes.

Similarly, when two plates collide neither plate subducts or sinks as both are light and resist downward motion. Instead the plate margin gets bucked, folded and thrust upward or sideways. In the case of the Indian plate about 80 million years ago it moved in a northward direction at a rate of 9 metres a century and rammed into Asia about 40 to 50 million years ago. The collision was marked by the formation of the Himalayas. Towering as high as 8854 metres above sea level, most of its growth had occurred during the past 10 million years. And the Indian plate still continues to move at the rate of 4.5 metres a century.

This continuous movement builds up stress in the rocks. Similar to snapping of a stick which is bent beyond its elastic limit, the rocks that are pushed against another plate accumulate strain. It does not snap till such time its elastic limit has been reached. After a point of time the elastic limit is reached and the rocks snap releasing enormous amount of energy. This sudden release of energy causes earthquakes. In the process the earth's surface gets ruptured causing huge faults.

Once formed, faults become areas of weakness and any further release of energy as earthquakes is mostly confined to the existing faults. New faults may appear when the strain is released at places away from the existing ones. As a rule, the stress build up is mostly at the plate margin where the plate is constantly impinging or moving against another plate. Hence high magnitude earthquakes are mostly confined to the plate margins. Intra-plate earthquakes (within the plate) cannot be ruled out either but these are generally lesser in magnitude.

Stress build up takes place over a long period of time and the time of release cannot be accurately predicted. High intensity earthquakes (over a 6 on the Richter scale) do not take place frequently. However regular release of energy produces low magnitude earthquakes.

Earthquakes would continue to ravage the earth's surface till such time new oceanic plates are formed, plates move as a result of this and stress builds up in the rock. The only way then to tackle this nature's fury is to ensure that all structures are built using the latest quake proof engineering techniques. That would mean the difference between death and slight damage to buildings with very less damage to human life.

This at least in earthquake prone areas which come under zone 4 and 5 and other places that have been registering regular though less magnitude earthquakes.

R. Prasad in Chennai

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