From: Zenaan Harkness <zen AT freedbms.net>
>A 9.0-magnitude earthquake is more than 100 times stronger than a 6.8. A
9.0 should have devastated everything within a 1,000-km radius. There
should have been widespread urban carnage, even worse than what Kobe
The definition of a Magnitude 0 earthquake is one that causes a ground motion of 1 micron (1E(-6) meters) at a horizontal distance of 100 kilometers. (don't recall if this is peak-to-peak or RMS (root-mean-square).)
Each increase of one unit of magnitude amounts to an increase by a factor of 10 of ground motion amplitude.
Each increase of one unit of magnitude amounts to an increase in energy released by a factor of about 31.
So, a magnitude 9.0 should have 10**2.2, or 158x, the ground motion of a magnitude 6.8 earthquake.
A magnitude 9.0 earthquake should have 31**2.2, or 1910, the energy released compared to a magnitude 6.8 earthquake.