Let me take the arguments about the physical damage a bit further.
The depth of the shaft is a function of blowout, not quake damage. At a distance of 6 km, the body wave will be essentially the same whether the shaft is 200m or 600m. You need deeper shaft to prevent blowout.
Arun_S has a valid point. The frequency was indeed higher. But my point is, everything else remaining the same, a bigger blast would have created bigger damage. Or, are you claiming that:
1. A bigger blast would move the frequency further up, reducing the damage?
2. The amplitude response is non-linear, like a clipping circuit?
If neither of the above is true, then a bigger blast would result in a bigger damage.
Sanku also has a valid point about the 0.75 factor. Let us go a bit deeper on that.
What exactly happens when an explosion happens in the shaft? Humongous amounts of energy is released in a very short duration. This will heat up the matter and cause it to expand. This expansion create shock waves. The liberation of the energy itself happens for a period infinitesimally shorter than the period of the mass movement, so the actual mechanism of the explosion is immaterial (FBF or TN)
Some portion of energy appears as heat and the rest gets transferred into the shock wave. The 0.75 power is an empirical figure for this. The actual number depends upon the "coupling", which depends upon the elastic and specific heat factors of the soil. If the soil has infinite specific heat and infinite elasticity, 100% of the energy would appear as shock wave. If it has zero elasticity OR zero specific heat, 100% of energy will appear as heat. Since soil composition is somewhat same all around the world (unless you do the explosion in an iron ore mine), the number will not vary too much IMO.
Things change a bit if you insert vast amounts of other material, like air or water at the blast site. Those materials have remarkably different parameters, so the coupling factor can change. If you blast inside a cavern of air, the air gets heated up, gets compressed, and you end up a big cavern of superheated air, which will dissipate into the soil rather slowly. But you need pretty solid cover for the cavern, because it can blow up much later, like the blanberry test blowup after 3.5 minutes.
Doing it in the water table does the same thing, but it is a terrible idea. The water body is not bound by anything solid, unlike the cavern of air. So, if you burst one in the water table, every well around in a vast area will turn into geysers.
The size of the cavern needed is humongous. You can't really dig it. The only possible way is to do solution mining of salt. You place the device inside, and really seal the cavern. Upon blast, the cavern will hold superheated air for a very long time. If you do not seal it well, it will vent with all kinds of nasty stuff.
As far as the available info goes, POK-2 was conducted in regular shafts in regular soil, so the coupling factor of 0.75 would be a good number IMO.
Now, let us look at the frequency issue.
The frequency is a function of elasticity of the soil. Has someone compared the frequency spectrum of the POK blasts with other blasts? I don't think they are different. So, the same established method of yield calculation should work for POK also.
Does the frequency depend upon the yield? Take a look at the
POK-2 spectral siesmograph. It seems they were. You can see two distinct bands in the spectrum. It looks like the two main blasts had different frequencies
The damage to a structure depends upon the amplitude AND the frequency of the ground wave. The shear force on a wall will be proportional to the acceleration, which is proportional to the amplitude AND the frequency. For a given amplitude, higher the frequency, higher the acceleration and higher the damage.
But the propagation parameters work the opposite. Higher the frequency, higher the attenuation, hence lower the amplitude that reaches the target. So, it becomes a bit complex.
For shorter distances, the selective attenuation should have less of an effect. So, for a place like Khetolai, the primary effect would be, higher the frequency, higher the damage. And I guess if frequency depends upon the yield, it should be higher for higher yield.
Findings:
1. The shaft depth is immaterial for considering siesmic damage. It is relevant only for blowout prevention.
2. The coupling constant 0.75 is pretty reliable.
3. The attenuation constant depends upon the path between the blast site and the monitoring site, as well as the frequency of the signal. It can vary a lot based on the frequency, so you need to use the right constant value for the received frequency.
4. Siesmic damage at a specific location can be scaled using the coupling factor, with one assumption that the frequency is independent of the yield. Even if it is dependent on yield, it should be higher for a higher yield, hence more damaging.
