April 11 quake: Distance from epicentre protects Indian cities

Published in The Hindu on April 13, 2012

If the amount of energy released during an earthquake is measured in terms of magnitude, its effects on the earth’s surface are expressed in terms of intensity.

Unlike magnitude that is determined using an instrument, intensity is measured only by effects felt by people. The one widely used today is the Modified Mercalli (MM) intensity scale.

The intensity of Wednesday’s tremor was felt indoors by many people in several cities (MM intensity scale III). But reports of cracks developing in buildings increase the intensity scale to IV. Intensity of an earthquake varies from place to place. It will be the maximum at the epicentre and will keep reducing with distance from the epicentre.

Acceleration, on the other hand, is a measure of shaking of the ground at a particular point due to an earthquake.

“If an object has to be lifted from the ground, the force required has to exceed 1 G,” Dr. R.K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad, told The Hindu.

The seismically active zone off the west coast of Indonesia is about 1,500 km away from the east coast of India. “The acceleration value will be very low at a place which is at this distance from the epicentre. Acceleration falls drastically beyond 200 km from the epicentre,” Dr. Chadha said.