How can an earthquake have a negative magnitude?
Magnitude calculations are based on a logarithmic scale, so a ten-fold drop in amplitude decreases the magnitude by 1.
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.
An earthquake of negative magnitude is a very small earthquake that is not felt by humans.
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