Seismic Scale

Tomorrow is Christmas eve, but in Christchurch, New Zealand, strong earthquakes is occurring repeatedly, the news says. According to it, the first and strongest one was of the magnitude 5.8, and then aftershock of around M5 has followed several times.

It seems, since that area is coastal, the (underground) would be unstable, which would cause worse damage corresponding to the magnitude.

But, my expectation is, the aftershock will be soon smaller. In the earlier period, there often be so many aftershock, which might be even as strong as the main quake, and one would feel as if the quake would last forever. But soon the frequency of aftershock will be rapidly decrease.

Every time I hear the news of earthquake in other countries, I wonder how was the intensity of quake itself in that area.
So far as I know, the value magnitude is calculated to be the energy of vibration of the earth crust at the epicenter.

But if the center is apart from any residential area, it would be of less concern.
If it is at the shallow point under the ground, a small magnitude of energy would cause serious damage, and the quake would come abruptly with the max intensity. On the other hand, if the center is offshore, the quake will be intensified gradually and it would last longer. In addition, the quake depends on the toughness of the ground.

And the factor, which effects directly on the life of people, is intensity of the quake, while magnitude will be used to analysis and forecast of quakes.

In Japan, we evaluate every quake by its intensity, in respect of each area. When one occurs, the official bureau of earthquake analysis makes up the map of the intensity, in every city or town, as soon as possible. Then we can find, “Our city had scale 2, don’t worry” or “His area had 5, that’s serious!” Yes, in this march after I myself experienced the scale “5-” I saw the news of the scale 7 in Tohoku, that chilled my blood.

I would not want every country to adopt our scale of intensity. It’s a traditional and specific to us. When I was a child, every intensity was reported on the basis of the physical feeling of analyst(s) in each area. Now the value is calculated from mechanic measurement, but the way of calculation is peculiar, because it is intended to produce the result so close to the traditional value. So the scale 4 of today, for example, is much close to that of decades ago. When I knew it I appreciated the analysts in those days very much, because they had reported the data that went very well with the mathematical calculation afterwords.

I wish there would be a worldwide standard of expression of intensity of quake at each area, which could be converted into our scale, too.
I have heard once, there should be such a scale in Europe. But the world media is still reporting only magnitude every time.


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