What is the Lowest Magnitude Threshold at Which an Earthquake can be Felt or Heard, or Objects Thrown into the Air?
Résumé
This article is a reflection on effects produced by earthquakes at both ends of intensity scales: II (‘Scarcely felt') and XII (‘Completely devastating'). Now that most seismic regions—at least in developed countries—are monitored by seismic networks with magnitude thresholds close to magnitude 1, less attention is paid to reports of abnormal phenomena such as vibrations or noises. The alleged reason is that, if the event has not been detected by monitoring networks, there was no event at all. This point of view is discussed in the light of recent examples in South-East France, where tectonic earthquakes with a very shallow focus (sometimes only 300-m deep) can be heard and felt, whereas the nearby (less than 20 km) seismic stations could not record the events. Our study concludes that events with a magnitude smaller than 1, and even negative magnitudes, can be felt, thus making the human being an instrument eventually much more sensitive than monitoring networks. Another type of remarkable observation which has been reported during earthquakes is the upthrow of objects into the air. Such observations are evidence of ground acceleration exceeding gravity. Although this type of observation is associated with an intensity of XII on the modified Mercalli intensity scale, we show that earthquakes of magnitude as low as 6 can produce such effects.