Certain earthquakes have a negative magnitude, is this an error?

Certain earthquakes have a negative magnitude, is this an error?
No, it is not an error. As magnitude calculations are based on a logarithmic scale, a ten-fold drop in amplitude decreases the magnitude by 1. Let us assume that on a seismogram: an amplitude of 20 millimetres corresponds to a magnitude 2 earthquake. 10 times less (2 millimetres) corresponds to a magnitude of 1; 100 times less (0.2 millimetres) corresponds to magnitude 0; 1000 times less (0.02 millimetres) corresponds to magnitude -1. Naturally, a negative magnitude is found only for very small events, which are not felt by humans.
From: earthquakescanada.nrcan.gc.ca/negative_magnitude

Info is free, spread it!