Real magnitude of Bhuj earthquake

By Our Special Correspondent

CHENNAI, JAN. 27. The confusion over the magnitude of the earthquake that struck Bhuj yesterday remains as the media had reported two figures. While the India Meteorological Department (IMD) estimated the magnitude to be 6.9 on the Richter scale, the U.S. Geological Survey estimated the magnitude to be 7.9. So what exactly is the correct magnitude.

``The magnitude reported by us is 6.9 on the Richter scale while the figure reported by the U.S. Geological Survey is based on a different mode of measuring the magnitude,'' clarified Dr. A. K. Shukla, Director of IMD, Delhi.

Earthquakes produce three different types of seismic waves. Of these it is the slow moving S-wave, also called the secondary wave, which causes much more damage than the fast moving P-Wave (primary wave). The third one is the Surface wave. ``The 7.9 measure given by the U.S. Geological Survey is based on the S- wave characteristic and non on P- wave,'' he clarified. ``They have reported a magnitude based on P-wave 7.1 whichis close to our Richter measurement.'' Magnitude based on the S-wave according to the IMD is 7.6, comparable to the U.S. Geological Survey's estimate 7.9.

Maximum damage in any earthquake is primarily caused by the S- wave, also called shear wave due to its shearing action. S-wave causes elastic up and down vibrations perpendicular to its direction of travel. The P-wave, on the other hand causes a train of compressions and dilatations parallel to the direction of propagation. This characteristic of the P-wave causes smaller damage to buildings and human life.

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