From the link I sent you which you didn't bother to read:
If an amplitude of 20 millimetres as measured on a seismic signal corresponds to a magnitude 2 earthquake, then:
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.
