Thanks for that illuminating discussion Don. Think about your argument for
a moment, however. Is it really the energy or the amplitude of ground motion
that causes damage to buildings?
I would contend that while the energy is vastly greater in a 9.7 than in a 6.8
earthquake, it is NOT the energy we should be concerned with, but the
amplitude. Buildings shake because the ground is moving underneath them,
that is related to the magnitude of the earthquake not the energy (as you said
the magnitude is measured by the amplitude of shaking). The energy get
lost through heat, sound, movement etc. most of which have little effect
on structures.
The other thing to remember is that it is not so much the amplitude of
shaking but the acceleration from rest to movement that really causes
damage. Smaller quakes can see accelerations near or greater than g,
just as larger quakes can. Poorly built structures in Iran come down in
a magnitude 5+ quake just as readily as well built structures do in San
Francisco in a magnitude 8 earthquake. The answer to your question
really depends on where you are.