What that means is that a movement of +1 on the scale (say from 5.5 to 6.5) indicates that the earthquake is ten times larger.

So, to work out how many times bigger one earthquake is than another (say 1940 and 1986), you get yourself a scientific calculator (online version here) and calculate the difference between the numbers:

7.7-7.1 = 0.6

(OK, you didn't need a calculator for that bit)

and then press "INV" followed by "log".  This converts the answer back into "regular" numbers, and in this case, gives an answer of 3.98.  So the 1940 quake was about four times more powerful than the 1986 one.

For discussion, I'll just throw in the thought that whether or not an earthquake is a catastrophe is measured by loss of life (and property). Is it possible that the buildings that replaced those destroyed in the 1940 quake were built to a higher standard, leading to a lower rate of collapse next time?