The huge ranges of seismic wave amplitudes, seismic moments, atmospheric energies, and other physical quantities is a problem. It would be much easier to communicate the energies or moments of earthquakes to the public on a simple scale that just ranges about from one to ten.
To turn these astronomical ranges into a simple scale, we can use the mathematical device of the logarithm. If we raise ten to some power x to make a big number N, then the log of that big number N is the power x:
10x = N log10 x = N
Now let's review the log (base 10) of some example numbers:
log10(10) = 1
log10(1) = 0
log10(100) = 2
log10(1e25) = 25
log10(25) is between 1 and 2, about 1.4
log10(0.01) = -2
log10(-1) is undefined
Note that while you can take the log of a very small number, which will be negative, you can't take the log of any negative number.