The truth’s somewhere at 6.9 or 7.9

The truth’s somewhere at 6.9 or 7.9
D.N. MOORTY

It may seem now academic at this stage to discuss what the exact magnitude of the earthquake in Gujarat that occurred at 0846 hrs on Republic Day. Yet, the debate is one which has serious implications for the science of seismology in India.

The first time the debate occurred was when the seismic yield of the Pokharan blasts came in question. Foreign seismic observatories put the magnitude of blast to around 4 when India claimed 5.2 on the day of the blast, subsequently revised to 5.4 by the Bhabha Atomic Research Centre after ‘‘studying the various parameters and geology in-depth’’.

Now, foreign estimates of the magnitude of the Gujarat earthquake initially put the Richter Scale reading at 7.9, compared to 6.9 put out by the India Meteorological Department (IMD) headquartered in Delhi, a figure which IMD still swears by.

Dr S.K. Srivastava, the Deputy Director General of IMD, blamed it all on journalists. ‘‘If the US says something, they are sacrosanct. If we say something here, we are never believed.’’

When questioned about the discrepancy in the assessment of magnitude, not only Srivastava, but other seismologists from institutes like the Indian Institute of Technology asserted the discrepancy is largely because Japanese and American assessments were calculated with surface waves as the basic input. Whereas, the IMD figure was arrived at by using P waves, i.e. the calculation was done on Local Magnitude (ML) and the Body Wave Magnitude (MB).

The 6.9 MB was more realistic, as calculations in the MS mode tend to be higher because over long distances, the attenuation of surface waves tend to be zero, informed Srivastav. Meaning: magnitudes of earthquakes differ when the method of calculation alters. Magnitudes calculated in the ML mode would be different than those calculated in the MS mode or the MW mode (moment magnitude).For the layman, different types of waves are produced when a seismic event takes place. These waves are P (primary) waves also called compressional or longitudinal waves S (secondary or shear) waves which are 1.7 times slower than P waves and oscillate the ground perpendicular to the direction of the wave travel, and surface waves, the slowest of all.

The amplitude of surface waves diminishes less rapidly than P or S waves over long distances. They form the main input for calculating the magnitude in seismic observatories that are at great distances from the epicentre of the quake. The S and P waves form the basis for calculating the MB or ML.

A more reliable and accurate measurement of the quake magnitude recently developed is the moment magnitude. It incorporates other inputs like fault parameters, and is universally now accepted as the most accurate assessment of quake magnitude.

What about the magnitude of an earthquake? The extent of destruction does not determine magnitude. In layman’s language, magnitude is the size of the quake. In technical terms, it is a quantity characteristic of the total energy released by an earthquake. This is not to be confused with the ‘‘intensity’’ of the quake.

The magnitude, thus, is independent of the method of calculation or the distance of the seismic observatory from the epicentre of the quake, because energy released by a quake cannot be altered just because different methods are used for its calculation. Why then is there a difference in results when different methods and parameters are used?

The IMD uses the same equations used the world over when calculating the MB or ML of a local quake. Similarly, distant observatories use the same equations to determine MS, but theoretically the values of MS and MB must be the same.

Now, foreign observatories placed the MS of Bhuj at 7.9 and the IMD’s MB value is 6.9, i.e. the IMD’s value is ten times lesser than that of the American or the Japanese value. This means that in terms of energy released, the IMD is saying the Bhuj quake released 31 times less energy than claimed by foreign observatories. But then, the quantum of energy once released by a given quake is fixed. So how does one explain the vast chasm?

In science, you are permitted a leeway called experimental error. Different readings could occur because of differences in recorded observations. Internationally, the range of permitted error for different mode of calculations cannot exceed plus or minus 0.2.

Thus, the latest figures put out by the US Government’s seismology department sticks to an MS of 7.9, but says the MW value is 7.7. The IMD has not yet announced its MW value.

The general consensus among seismologists this paper talked to is that the IMD has ‘‘grossly underestimated’’ the Bhuj quake.

Srivastava disagrees with this. He says on the very first day itself the IMD had come out with two values the MS and MB, the first being 7.6 and the second being 6.9. The MB figure was announced because the custom is to peg the local quakes in MB.

The point is, IMD’s own calculations expose a differential of 0.7, a difference beyond the boundaries of experimental error. A scientist who did not want to be named pointed out that this could be simply a matter of inconsistency in the calliberation of the recording seismographs.

Srivastava says he’s going to set up a committee comprising seismo-scientists to go into the magnitude issue. But here is something to chew on: Srivastava says MS values tend to be higher than MB values. How then were MS values recorded by distant observatories less than the MB values of the Pokharan quake put out by Indian seismologists?