A shallow quake but high magnitude

Published in The Hindu on September 1, 2012

The 7.6-magnitude quake that occurred at a depth of nearly 34 km below the ocean floor was due to the Pacific plate subducting (sinking) under the Philippines plate on the eastern side of Philippines.

“From the seismologic point of view the earthquake is a shallow one,” said R.K. Chadha, Chief Scientist, National Geophysical Research Institute (NGRI), Hyderabad. “Shallow earthquakes extend till a depth of about 40 km, and intermediate quakes is from a depth of 40 km to 70 km.” Quakes with a focus of more than 70 km depth are called deep-seated quakes. Generally, the magnitude is not high when the quake occurs at greater depths.

The reason why the quake had a magnitude of 7.6 despite the focus being at a depth of about 35 km, is the angle at which the Pacific plate is subducting below the Philippines plate. “The Pacific plate is subducting at a steep angle of more than 45 degrees. At such steep angles, it is very easy for the plate to break. Imagine when a stick is being bent at more than 45 degrees, the chances of the stick being broken is very high,” he explained. “The plate can fracture at shallow or deeper parts of the subducting plate.”

At such a steep angle, the effect of gravitational forces gets all the more pronounced as gravity tends to pull down the subducting plate. “As a result of this pulling, the Pacific plate gets thinner and thinner. And in turn the plate becomes weaker at some point. Fracturing finally happens at this weak point,” said Dr. Chadha.