Picking the Right Earthquake
As I said we picked 30%, but you have to understand this was not the true label. Seismic experience data came from the examination of hundreds of factories and steam plant exposed to earthquakes. None of these had instruments. So basically, they had to use the Shakemap conversion to the Modified Mercalli Scale.
As you can see, 30%g falls right between Intensity 7&8 (forget the dang Roman Numerals!). The description is as follows.
I would put this very close to the epicentre of the Chile earthquake at about a PGV of 30-60 cm/s. This is probably Christchurch2, where old brick buildings are shattered, but that is on poor ground. Cornwall 1944 hit I7 on the soft soil, but barely felt on the hard ground. This is also close to the recent Japanese earthquake where PGV hit 10 to 100 cm/s depending on the ground.
So, when they went to a steam plant, they looked around at the general damage and estimated the MMI. This was used to label the damage (or not) with a %g (for engineers).
In reality, hard ground (rock) cannot physically exceed 10 cm/s (20 at the thrust of a supershear) because the rock cannot carry that stress. So, now I think the PickA review earthquake was overkill for the main plant on piles, but lacking for the soft ground. The repairs they did were so expensive, I only wished for a fraction to go to better science to cut down the PGV in the plant.
At this point, I left the study. However, later, I was extensively trained in the method, and reviewed all the work done at PickA.
The next part will be the actual walkdown, or stress test.