The concept of a zero- or negative-magnitude quake is easy to visualize if you look at a copy of Richter's scale (link below). The scale comprises three vertical bars - the left one for distance, the middle one for magnitude, and the right one for the amplitude of a specific shear wave as measured on a specific instrument. A ruled line intersecting any two known measurements provides the third (though it would generally only be used for determining magnitude - and that only for moderate quakes in Southern California).

If the distance and amplitudes are small enough, the line just happens to intersect the magnitude scale below the zero mark. I do not know if zero or negative magnitudes are possible with other more modern magnitude measurements.

BTW - while looking for a Richter Scale graphic, I found a stock photo site with a picture of an old drum recorder captioned "Richter Scale"! I've heard that, back when the term Richter Scale was in common [mis]use, the personnel at Pasadena USGS, and presumably elsewhere, would occasionally get calls from people wanting to look at the Richter Scale, or wondering where it was kept . . .

Mike Williams

• Richter Scale