6.9 or 7.9? Where lies the truth?

Now, foreign estimates of the magnitude of Bhuj 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 that IMD still swears by. The question is where lies the truth?

Dr S K Srivastava, the Deputy Director General of IMD, speaking to The Indian Express on Tuesday night, lambasted the mindset of journalists. “If the US says something, they are sacrosanct. If we say something here we are never believed.”

When questioned about the discrepency in the assessment of magnitude, not only Srivastava, but other seismologists from such hallowed institutes as the Indian Institute of Technology also asserted the discrepency is largely because the Japanese and the 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). Srivastava said the 6.9 MB was more realistic as calculations in the MS mode tend to be higher because over long distances the attenuation of the surface waves tend to be zero.

The implications of the statement is that magnitudes of earthquakes differ when the method of calculation alters. Thus magnitudes caculated 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 which are the slowest of all.

Now 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 which incorporates a lot of other inputs like fault parameters. It is universally now accepted to be the most accurate assessment of the quake magnitude.

All that is fine, but what is magnitude of an earthquake? The extent of destruction does not determine magnitude. In layman’s language, magnitude is 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 is an objective entity and cannot be altered or changed just because different methods are used for its calculation. It is axiomatic that whatever the method used the magnitude of an earthquake is unique. The question then arises, why the difference in results when different methods and parameters are used?

The IMD uses the same equations that are used anywhere else in the world by seismo-scientists when calculating the MB or ML of a local quake. Similarly, the distant observatories use the same equations to determine MS, but theoretically the values of MS and MB must be the same. Now, the 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. What does this mean? It means that in terms of energy released, the IMD is saying that the Bhuj quake released 31 times less energy that claimed by the foreign observatories. But the quantum of energy once released by a given quake is fixed. So how does one explain the vast chasm between the two readings?

In science you are permitted a leeway called experimental error. The 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 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 for the Bhuj quake magnitude.

The general consensus of the seismologists this paper talked to, is that the IMD has “grossly underestimated” the Bhuj quake. Srivastava does not agree. He says that 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 wnat to be named pointed out that this could be simply a matter of inconsistency in the calliberation of the recording seismographs.

Srivastava says that he is going to set up a committee consisting of seismo-scientists to go into the magnitude issue. But here is something to chew on: Srivastava says that MS values tend to be higher than MB values, explaining the high MS value of the Bhuj quake. The question is how come the MS values recorded by distant observatories happened to be LESS than the MB values of the Pokharan quake put out by the Indian seismologists who recorded the magnitude of that blast?