Sumatra quake: Why there was no tsunami

Published in The Hindu on April 12, 2012

The U.S. Geological Survey (USGS) initially estimated Wednesday’s 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.