April 11 quake caused by a strike-slip fault

Besides the subduction zone (where the Indian plate dives under the Burmese plate) that lies to the West of Indonesia, the Indian Ocean has a diffused seismicity zone. This zone lies to the West of the subduction zone.

The East-West trending zone extends from 80 degree East longitude to around 93 degree East longitude (where the April 11, 2012 quake occurred). The length of the zone is around 1,300 km.

There have been seven major earthquakes between 6 and 7.5 magnitude in this region during the last 25 years. The 8.6 magnitude quake of April 11 and the aftershock of 8.2 magnitude should now be added to this list.

The April 11 quake was not in the subduction zone but to the West of it. “It is an intra-plate earthquake,” R.K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad, told The Hindu. An earthquake is called intra-plate when it occurs within the plate and not at the edges of two plates.

Since it is an intra-plate earthquake, no subduction takes place and the fracturing causes a strike-slip fault - where one edge of the plate grinds past the other horizontally. The diffused seismicity zone has an East-West direction. This zone could have formed as a result of westward transfer of stress from the subduction zone. “The April 11 earthquake was about 100 km West of the subduction zone,” he said. “It is within the Indo-Australian plate.” According to him, the movement of this plate (at a rate of 52 mm per year in a NNE direction) in the diffused zone of seismicity is governed by mid-ocean ridges in the Indian Ocean. There are two ridges there - Southeast Indian Ocean ridge and the South Indian Ocean ridge.

Why there was no tsunami

The US Geological Survey (USGS) initially estimated the April 11 earthquake off the west coast of Northern Sumatra, at 8.9 magnitude. But it was subsequently lowered to 8.7 and then to 8.6. Similarly, the focus of the quake was first thought to be 33 km from the surface, but was later changed to 22.9 km.

The signals from a high-magnitude quake flood the nearby earthquake recording station, leading to an initial estimation of high magnitude.

Explaining why tremors were felt in several Indian cities, R.K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad, said: “Tremors are felt at faraway locations due to surface waves produced by an earthquake. Surface waves cause a lateral movement of the particles in the earth’s medium. The earth behaves like an elastic medium when seismic waves are travelling.” Unlike the December 26, 2004 quake caused by a thrust fault, Wednesday’s quake was caused by a strike-slip fault. The fault had moved in a north northwest-south southeast direction. In the case of a strike-slip fault, the fractured crust slides past each other laterally.

“The movement along the fault should be in the order of a few metres,” said Dr. Chadha. “Only a detailed modelling using data from 40-50 stations can reveal the actual amount of displacement.”

The reason why the 8.6-magnitude quake did not cause killer tsunami waves was the nature of the faulting. “To generate giant tsunami waves, there should be great vertical displacement of the water column,” Dr. Chadha said. “This happens only in the case of a thrust fault (the December 2004 quake) or a dip-slip fault. Strike-slip fault will not generate tsunami waves.”

According to him, though the quake was a strike-slip fault, there should have been a small amount of oblique movement along the fault. This is the reason why Wednesday’s quake caused small tsunami waves.

Even the 7.2-magnitude quake of January 10, 2012, at a depth of 20.5 km from the surface off the west coast of northern Sumatra, was due to strike-slip faulting.

Courtesy: The Hindu