Folks
Here is my first ever research on this subject. As a common man, it's hard to believe those pillars would not stand 9.0R, but as you can see they failed at 6.7R

http://www.physicalgeography.net/fundamentals/10m.html

And The equation for energy is

E = 1.74*(10^^(5+1.44*M)) Joules, where M= magnitude in RS

So
e7 = 1.74*(10^^(5+1.44*7)) J
e8 = 1.74(10^^(5+1.44*8)) = 1.74(10^^(5+1.44*7)* 10^^(1.44))

e8/e7 = 10^^(1.44) = 26.54

Correct me if I've made a mistake. Somebody said above it was 31.7 times stronger per unit increment of RS? I'd say close.