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 302

clearly states that 'the smallest shocks reported felt by persons are near magni-
tude 2', he does not expatiate on key parameters such as focal depth or population
density.
Samuel Johnson (Boswell 1791) had a poor opinion on the accuracy and use-
fulness of popular reactions after an earthquake. Upon Boswell's reporting him a
small earthquake which had just happened in Staffordshire (England), he replied:
'Sir, it will be much exaggerated in popular talk: for, in the first place, the common
people do not accurately adapt their thoughts to the objects; nor, secondly, do they
accurately adapt their words to their thoughts: they do not mean to lie; but, taking no
pains to be exact, they give you very false accounts. A great part of their language
is proverbial. If any thing rocks at all, they say it rocks like a cradle ; and in this way
they go on.' This peremptory, extreme, although clever statement is an early (14
Sept. 1777) critical analysis of earthquake descriptions by lay persons. Fortunately,
seismologists have long since reconsidered this viewpoint and, using appropriate
precautions, now value such accounts.
Browsing Web pages can supply a wealth of information on felt earthquakes
as shown for instance by the Community Internet Intensity Map developed by
Wald (2007) at USGS, but low-magnitude events are rarely included in such lists
because persons experiencing a faint rattle seldom bother to report it. If they ever
do, the information is often judged insignificant and not deserving publication.
However, out of the many Web sites providing information on felt earthquakes, the
Australian Seismology Research Centre (http://www.seis.com.au) is one of the few
to list carefully small events felt in Australia. Over the last seven years, the smallest
magnitude value they report is an M L (Richter local magnitude) 1.3 earthquake felt
in 2000 in the suburbs of Melbourne.
There are good reasons to believe that this magnitude threshold can be still lower.
Feeling small-magnitude shocks is perhaps not that unusual, the main problem be-
ing only how to collect this kind of information. Small earthquakes which occur
in mines when the upper soil layers are depleted are often reported heard because
they emit acoustic energy in the 200-1,000-Hz frequency range. Audible acoustic
waves in the 50-70-Hz range have also been reported for many tectonic earthquakes
(e.g. Hill et al. 1976; Tosi et al. 2000). Sylvander and Mogos (2005) analysed a
macroseismic regional database which contains detailed reports of sounds heard
for M L < 4 earthquakes. They demonstrate that, in the Pyrenees, 'events with M L

as low as 1.0 (and perhaps even smaller) may be perceived under very favourable
conditions'.
We will not discuss here the now-recognized audibility of small shocks, but
rather address the question of repetitive occurrence of earthquakes, another factor
which increases the sensitivity of the population. Long aftershock series or swarm
earthquakes often further a flow of information, even though the phenomena are
faintly felt or heard. We present two cases of low-energy, unusually-shallow seismic
activity reported felt in 2002-3 and 2006 in South-East France. Records obtained
at temporary stations only tens of metres from epicentres demonstrate that, under
particular circumstances, even negative magnitude values can be associated with
felt events.