What is the Lowest Magnitude Threshold at Which an Earthquake can be Felt or Heard, or Objects Thrown into the Air? - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 301

What is the Lowest Magnitude Threshold at
Which an Earthquake can be Felt or Heard,
or Objects Thrown into the Air?
F. Thouvenot and M. Bouchon
Foreword 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 (some-
times 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 earth-
quakes is the upthrow of objects into the air. Such observations are evidence of
ground acceleration exceeding gravity. Although this type of observation is associ-
ated 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.
1 Introduction
The question of the lowest magnitude threshold at which an earthquake can be
felt or heard is of particular importance when small historical events are used
for delineating active zones in moderately seismic areas. The answer provided
by most encyclopaedias and earth-science primers is that earthquakes are usu-
ally felt for shocks with magnitudes 3 and above. Actually, most authors of seis-
mology textbooks are reluctant to tackle the question. Although Richter (1958)