"A negative quake? What's that?"

The magnitude scale is logarithmic and is a measure of energy. It's also offset. You can convert a given magnitude value to energy, and vice versa. So although the magnitude may be 0 (zero), when you run the formula you end up with a non zero number.

If I'm not mistaken, no earthquake would be magnitude minus infinity.

Here's a web page for your perusal,

http://www.seismo.unr.edu/ftp/pub/louie/class/100/magnitude.html

Brian