~kujala~ said:
Ground shaking is amplified in soft sediments and dampened in hard rock...
Not to defend TEPCO, but we must keep in mind that the safe design criteria for buildings of this type are diferent from those of ordinary apartment/office buildings. I recall reding the words 'floating foundation' somewhere, perhaps in a TEPCO document. Instead of anchoring the building to the underlying rock, or driving deep piles into the ground, the idea is to build the basement as a big stiff box that 'floats' on the soil/rock below. That way, if the ground cracks, deforms, liquefies, or shakes too much, the building may move or sink a bit but is less likely to crack or collapse.
In any case, building on bedrock (granite or other volcanic rock) will not help if the rock cracks and shears during a big earthquake. (Recall that the cause of the earthquake was a 500 km long crack that caused two pieces of that 'solid bedrock' to slide tens of meters past each other.) Anchoring the building to the bedrock seems a rather bad idea in that case.
According to the blueprints, the thickness of the reactor building walls increases from top to bottom. The service room wall panels are perhaps 30--50 cm thick and thinly reinforced; but 2nd floor walls seem to be 1.2 meters thick, basement walls are at least 1.5 meters thick, and the basement floor slab is 4 meters thick. (That by itself would explain why the explosions did very little visible damage below the 3rd floor.)
Thus I am willing to believe that the
buildings of the reactors are pretty safe against earthquakes, and suffered no damage on 11/mar. (That does not apply to the reactors proper and other equipment, of course) I have yet to see a picture of any crack or other damage to those buildings that was caused by the earthquaque itself.
In particular, I do not see any possibility of their lower levels sinking, tilting or collapsing to any significant extent, even if they were resting on top of soil rather than rock. Thus the tilting of #4 that is supposedly visible in the webcam ---
and only there --- can only be an optical illusion or artifact.
The type of the underlying rock
could matter for the safety of the equipment inside (mudstone results in stronger shaking than granite, it seems) and for the water leakage problem (mudstone probably has more cracks and pores).
Of course, some parts of the upper floors that were damaged by the explosions are now visibly unsafe, even against minor quakes or storms. (Could a tornado blow away the loosely-hanging wall panels of #3 and #4?)
