The Hindu : National : Why Monday's quake did not produce big tsunami?

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Wednesday, Mar 30, 2005

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National

Why Monday's quake did not produce big tsunami?

By R. Prasad

CHENNAI, MARCH 29. The 8.7 magnitude earthquake that struck off the Sumatra coast on Monday has raised several questions.

Was it an aftershock of the December 26 quake that killed hundreds of thousands or was it an earthquake in its own right? Aftershocks of lesser intensity generally occur after a major quake. Several such aftershocks have been recorded in the Indonesian and Andaman regions since the last earthquake. According to scientists, these aftershocks would continue for a couple of more months.

So can the Monday quake be called an aftershock as its magnitude is 8.7, which is lesser than the main quake, and occurred in the same region? "It is not an aftershock but an earthquake in its own right," said R.K. Chadha, scientist at the Hyderabad-based National Geophysical Research Institute.

According to Dr. Chadha, the epicentre of the Monday's quake was 120-130 km south of the December 26 quake epicentre. The quake that struck last year-end ruptured the ocean floor for a distance of more than 1,000 km in a northerly direction from the epicentre.

"Since the epicentre of Monday's earthquake is south of December 26 epicentre, it should not be considered as an aftershock but a separate earthquake," Dr. Chadha said.

Intriguing question

But a more intriguing question is why the Monday quake did not produce a tsunami like last year's event even though its magnitude was 8.7. "A tsunami will be caused only when the rupturing of the plate produces a thrust fault, has a significant intensity and has a shallow focus," he explained.

In the case of thrust fault, a portion of the earth's crust is moved up relative to the other and this vertical displacement of the crust produces huge displacement of the water column. In the December 26 event, the vertical displacement of the ocean floor at the fault site was 15 metres. The plate boundary had also ruptured for a distance of 1,000 km producing, what is called, the megathrust earthquake. Megathrust earthquakes by default can cause highly destructive tsunamis.

Shallow focus

The March 28 quake was indeed a thrust fault and had a high intensity of 8.7. "But the third factor — shallow focus — that is essential to produce killer waves was not present," Dr. Chadha said. The focus of the earthquake was 30 km below the ocean floor. The December 26 event had a focus of just 10 km. Quakes having a focus of 10 km are called shallow focus earthquakes.

Even deep focus earthquakes produce tsunamis, but they do not attain wave heights of 30 metres and above when they reach the coast. The Monday quake has been found to produce smaller tsunamis that can be detected only by buoys and pressure gauges. A buoy monitored by Australian officials indeed detected a small tsunami.

The deep focussed nature of the earthquake is one reason why the tremors were not felt in India unlike the previous quake.

"Two large earthquakes with very close epicentres are unusual," Dr. Chadha said. "And this earthquake will produce aftershocks the same way as the December 26 quake." Already more than a dozen aftershocks have been recorded since Monday. According to him, aftershocks would continue to strike the Indonesian and Andaman regions for the next few months. "These aftershocks will be the result of the December 26 and Monday's events."

The short interval for the second major earthquake to strike the same region is seen as a pressure releasing mechanism. According to a paper published recently in the journal Nature, the possibility of a high intensity earthquake of 7.5 was to be expected. The scientists found the pressure on the Sumatra fault on the plate's eastern margin had increased as a result of the December event. They reported that the pressure on the 50-km stretch of the Sunda trench where the India plate is pushed under the Burma plate had increased by 5 bars and a 300-km segment of the Sumatra fault was under an extra 9 bars of strain.

While some of their predictions have more or less come true, some have not. For one, the intensity estimated was 7.5, way below the actual intensity of 8.7. And it was also predicted that the resultant quake would wash up on the southern African shores too. This was not the case though.

"A prediction has to be correct on three counts — location, timing and magnitude," Dr. Chadha said. "While the location and timing have been right, the magnitude has not." An intensity difference between 7.5 and 8.7 is more than ten times.

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