Why the tremor lacked vigour

Horizontal quakes spare lives, ensure less devastation

The 8.5 magnitude earthquake and five strong aftershocks within three hours of it did not generate any tsunami in Indian Ocean because all quakes were triggered by horizontal motion in seismic faults within the plate.

The motion in the seismic faults this time was unlike the one that had occured in 2004 - at the junction of two tectonic plates - gigantic chunk of earth’s crust on which continents and oceans rest.

The 2004 tsunami was caused by a massive 9.1 undersea mega quake at the boundary of two tectonic plates – known as subduction zone – where one plate slips under another displacing a huge amount of ocean bottom and triggering the monstrous tsunami.

On the contrary, half-a-dozen quakes off Indonesia coast took place on Wednesday in parallel seismic faults within the Indo-Australian plate as a result of which there was no tsunami.

Though the Indian National Centre for Ocean Information Sciences in Hyderabad that runs the Indian Ocean tsunami warning centre, initially issued tsunami alert for Andaman and Nicober islands and eastern coast, it was later withdrawn. “These quakes did not happen at the subduction zone but in parallel faults off Java coast.

The motion of the earth’s crust was horizontal. Since there is no vertical motion (of the ocean bottom) there is no tsunami,” explained C P Rajendran, a professor at Centre for Earth Sciences in Indian Institute of Science in Bangalore.

The first quake with a magnitude of 8.5 (the US Geological Survey puts the magnitude at 8.6) happened just after 2 pm Indian time. It was followed by five major aftershocks of 7, 6.8, 8.2, 6.8 magnitudes and 6.6 within the first three hours. Large aftershocks are still continuing in that area, which is known as an highly active seismic zone.

“After the 2004, we witnessed close to 300 aftershocks in that area, many of which had magnitudes greater than 6. Its not unusual,” Harsh Gupta, former director of Hyderabad-based National Geophysical Research Institute, who was secretary in the department of ocean development during the 2004 tsunami, told Deccan Herald.

“For the first few days, there will be too many quakes with gradually reducing intensity. The aftershock effects are likely to continue for few years,” said P C Rao, a geophysicist at NGRI.

Rao said these earthquakes are examples of typical “slide-slip quakes” in which two blocks of earth’s crust slide past each other. The sliding does not cause any vertical movement and hence no tsunami. The researchers have ruled out the possibility of these earthquakes adding to the stress build up in India’s quake areas.



 – the Himalayas and North East. “There is no plausible model,” Rao said. “Much more research is required to understand if stress build up in the North East or in the Himalayas is influenced by these events,” said Gupta.

Within the Indo-Australian plate, quakes are generated in an area where there are three plates – Indian plate, Australian plate and Burma plate. But since the boundary separation between the Indian and Australian plate is not so stark and distinct unlike other tectonic plate boundaries – known as diffused plate – many scholars prefer to describe it as Indo-Australian plate.

“The region is extremely active since 2004. Some of Wednesday’s quakes happened in fault lines running parallel to Ninety East Ridge – a 5000 km long mountain rising from the ocean bed,” said Rajendran, who studied the area extensively.