Of quakes and killer waves
IF MAN has a way of demarcating and classifying the various regions of earth into several countries and continents, nature has its own way too. The earth's surface is not one continuous piece of land mass. On the other hand, it is broken up into several large and small plates. A plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
These plates, each about 50 miles thick, are not anchored to a particular place; they move relative to one another at an average speed of a few inches a year. And earthquakes and volcanic eruptions occur when these plates collide at their boundaries.
Three types of movement
But not all plate movements lead to collision. Three types of movement are recognised at the boundaries between plates: convergent, divergent and transform. At the divergent boundaries new crust is formed when lava flows out pushing the plates away from each other; convergent boundaries, on the other hand, lead to plate destruction as one plate dives (subducts) under another. Crust is neither created nor destroyed at the transform boundaries as the plates just slide past each other horizontally.
The formation of new crust, called sea floor, spreading at the divergent boundaries and destruction at convergent plate boundaries happen at the same rate globally. And this is the essence of plate tectonics, which goes to explain how volcanic eruptions, and earthquakes that ravage the earth's surface happen.
Diving Indian plate
The earthquakes of December 26 that occurred off the west coast of northern Sumatra took place at the interface between the Indian and Burma plates. It is a typical convergent plate boundary where the Indian plate moving at a rate of 5 cm a year relative to the Burma plate came together, collided and the Indian plate dived (subducted) under the Burma plate.
Making it all the more complicated is the interaction of the Australian, Sunda and Eurasian plates in addition to the Indian and Burma plates.
In addition to the classification based on the nature of plate-plate interaction at the boundaries, plate tectonics is also built on the premise that plates are of two types — continental and oceanic. And the plate interaction could thus be continental-continental convergence, oceanic-continental convergence and finally oceanic-oceanic convergence.
Continental-continental convergence is best seen when the Indian plate collided and subducted under the Eurasian plate and in the process, produced huge mountains like the Himalayas.
It is because both the plates have the same density that plates get pushed up to produce mountains. In the December 26 episode, it was an oceanic-oceanic plate convergence. Volcanic eruptions are commonly seen at such converging boundaries.
Proximity to sea bed
What made the Sumatra earthquake very unique is that the focus (where the plate rupture actually takes place inside the earth) of the earthquake was very near the sea bed — only 10 km deep. Shallow earthquakes by default cause more damage than deep focus earthquakes. And the magnitude was 9 on 1-10 point scale Richter.
"But more important was the fact that the subducting Indian plate pushed up the Burma plate by 15 metres," said R.K. Chadha, Scientist at the Hyderabad based National Geophysical Research Institute. "Vertical displacement of a plate of such magnitude is unheard of."
The aftershocks within two hours at the Andaman islands following the main earthquake in the Burma plate have gone further to fracture and move the Burma plate boundary by 1000 km. That in essence is the power of the earthquake that struck off the Sumatra coast. The U.S. Geological Survey has called this event a megathrust earthquake referring to the large cracking of the plate boundary.
According to them, megathrust earthquakes often generate large tsunamis that can cause damage over a much wider area than is directly effected by ground shaking near the earthquake's rupture.
Lethal combination
A lethal combination of huge magnitude and shallow depth focus led to high vertical displacement of the Burma plate that acted like a great piston deforming the sea. Large aftershocks following the megathrust show that approximately 1000 km of the plate boundary has slipped following the quake. The gravity and incompressibility of water force the seas above the subducting plate to react immediately. Huge quantities of water get displaced. The water at the surface starts to shift downhill and that makes a tsunami.
If plate interactions on land cause earthquakes, those that occur in the sea pose the biggest risk of generating tsunamis. Earthquakes occurring on land do not cause death directly; buildings that collapse do. Similarly, earthquakes that occur in the sea do not kill life; tsunamis that are produced by them do.
"Shallow focus earthquakes measuring 6.5 can also cause tsunamis," Dr. Chadha said. "But such tsunamis will die out after some distance." The vast expanse of the Indian Ocean posed little challenge to the movement of the killer tsunami. Reaching a distance of 2000 km to hit the Indian coast was not difficult. "Giant tsunamis can travel as far as 5000 km," Dr. Chadha pointed out.
Though this was the first time that a tsunami of this magnitude had struck the Indian coast, tsunamis of lesser magnitude cannot be ruled out in the future. Though a second tsunami may be many years away, there is no room for complacency.
Surface expression
The Sunda trench which lies to the west of the earthquake's epicentre is the surface expression of the India-Burma plate interface.
And if a trench marking subduction is on the Burma plate's west margin, the east margin is marked by an oceanic rift where new ocean floor is being produced. And what makes this very unique is the fact that the Burma plate is a very narrow strip.
"The Burma plate is an elongated plate with a width of 700-800 km and a length of nearly 2000 km," Dr. Chadha said. "Two major plate tectonic features on either side of a narrow strip show how seismically active the region is."
That is not all. Scientists have shown that the Indo-Burma plate margin is a region of compression as the Australian plate is rotating counter clockwise in to the Indian plate. All these highlight the fact that the Northeast Indian Ocean has become a region of heightened seismic activity.
Since a large amount of pent-up energy in the compression zones along the plate boundaries has been released, it will take years for another incident of the same magnitude to recur. But countries in the Indian Ocean should pay more attention to earthquakes and tsunamis in the future.
R. Prasad in Chennai
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