Dear scientists,
On the website of the Royal Observatory I read that on 12.06.2011 an earthquake occurred near La Reid (Spa) with a magnitude of -0.1.
On Wikipedia it is written that magnitudes of earthquakes and brightness of celestial bodies can be compared with each other. Now I understand that a faint celestial body has a high value and on the other hand a very bright one (the sun) has a negative value. Also, it is clear that the magnitude in an earthquake is a measure of its strength or force
But I don’t understand a negative value like in La Reid. Surely an earthquake has a certain strength or force and according to my reasoning it can never become negative?
Answer
The magnitude of an earthquake is a measure of the strength of the earthquake, based on the amplitude of the seismic ground motions (waves) that can be measured by a seismometer.
There are different types of magnitudes, but the Richter scale is the best known scale. This scale expresses the amplitude of the measured earthquake relative to a predefined reference amplitude. The formula is as follows: M = log A (x) – log A0 (X).
Here A is the maximum measured amplitude, x is the distance between the epicenter and the seismic station, and A0 for the predefined maximum amplitude for a magnitude 0 earthquake at a distance x from the seismic station. The latter term depends on the region and Charles Richter chose a reference earthquake of magnitude 0 for Southern California in 1935 with an amplitude of 1 mm on a Wood Anderson seismograph at a distance of 100 km from the epicenter.
Thus, the Richter scale actually has no lower limit and today, with the more modern and sensitive seismometers, amplitudes can be measured that are actually smaller than the predefined reference amplitude for an earthquake with magnitude 0. As a result, the magnitude of such earthquakes will be negative. So these are very small to barely perceptible earthquakes.
Answered by
Dr. Elien De Pelsmaeker
http://www.ugent.be
.