The local (Richter) magnitude scale is open-ended and based on a logarithmic
equation. For most recordable earthquakes the resultant magnitude is a positive
number, but for extremely small earthquakes, the calculated magnitude can be
negative. An equation for determining magnitude is :

ML = log(Amplitude) - 2.48 +2.76*Log(Distance from recorder)

You can see if the amplitude of the maximum seismogram trace is very small and the
distance is short, ML can take on negative values. Richter, who invented the scale
chose as his reference from which other magnitudes were determined that Ml=0
when A = 1X10E-3 at an epicentral distance of 100 km.