The Hindu , 19 July 2006

Tsunami more localised this time

CHENNAI: The earthquake that struck 240 km off the Java coast caused a more localised tsunami. Despite meeting all the conditions essential for producing a tsunami the epicentre was on the ocean bed, the magnitude was 7.7, it was a shallow focus earthquake (about 10 km in depth) and it produced a vertical uplift of the ocean crust the killer waves were more localised and in no way comparable with the deadly one that struck on December 26, 2004.

"That is because the vertical uplift of the ocean crust was only about 90 cm and had a fault length of about 150 km," said R.K. Chadha, scientist at the Hyderabad-based National Geophysical Research Institute.

The December 2004 quake had a magnitude of 9.3, lifted the Burma plate by about 15 metres, and ruptured for a length of about 1,200 km. All these provided the killer waves enough energy to travel hundreds of kilometres before reaching India and even the east coast of Africa.

Another possible reason why the killer waves did not reach India this time could be the W-NW direction of the resultant fault. The strike (direction) of the fault has very important implications for India. With maximum energy of the tsunami waves focused in two directions one towards the Java coast and the other towards the open sea in the direction of Christmas Island India was in the least wave energy propagation direction.

"This is definitely a different event and not an aftershock of the quake that struck in May or the December 2004 one," said Dr. Chadha. The fact that about 20 aftershocks were recorded during the 24 hours following Monday's quake underlines that the event that caused a localised tsunami was not an aftershock.

"The number of high energy aftershocks for a 7.7 magnitude earthquake is quite high," he noted. "It means that the energy dissipation has been very fast." This indicates that there is less likelihood of another high magnitude quake striking the region in the immediate future.

The Australia plate diving under the Sunda plate into the Sunda trench caused the quake.

The December 2004 quake was caused when the India plate subducted (dived) under the Burma plate.

Unlike the quake off the coast of Sumatra in 2004, which is more of an oblique subduction, it was a normal subduction on Monday that caused the earthquake.

"Generally, earthquakes occur very deep in the southern part of the Sunda trench as the Australia plate goes down as deep as 300 km," Dr. Chadha said. "But where the brittle crust would rupture [to produce a quake] cannot be predicted."