The ASDoc for the Math.LOG10E constant suggests that "The Math.log() method computes the natural logarithm of a number. Multiply the result of Math.log() by Math.LOG10E to obtain the base-10 logarithm."
trace("log10(x)=",Math.log(x) * Math.LOG10E);
There is also a Math.LOG2E constant if you want log base 2. For other bases it might help to remember that logx(y) = ln(y) / ln(x)