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 303

At the other end of the gamut of effects produced by earthquakes, the upthrow
of objects was thought for a long time to be an exceptional event encountered in
great earthquakes only. The first such documented account was made by Oldham
from field observations following the great Assam earthquake of 1897. Oldham re-
ported that in some areas stones had been tossed in the air 'like peas on a drum'
(Oldham 1899; Bolt and Hansen 1977).
The magnitude of the great Assam earthquake is estimated to have been close to
8.1 (Ambraseys and Bilham 2003). Reflecting the view that the upthrow of objects
in earthquakes is exceptional, 'Objects thrown in the air' are listed as evidence of
intensity XII on the modified Mercalli intensity scale. In this article, we will dis-
cuss observations of upthrown rocks and boulders produced by earthquakes with
magnitudes much smaller than 8.

2 In Quest of Small Felt Events in South-East France

Since the Sismalp monitoring network run by the Grenoble Observatory was set
up in the 1980s (Thouvenot et al. 1990; Thouvenot and Frechet 2006), the original
procedure proposed by Richter (1935) has been used to compute the local magnitude
M L of earthquakes: the velocity seismogram is first integrated; the magnification
value of the Mark Product L4C or L4C-3D 1-Hz sensors and the field recording gain
are then taken into account to compare the displacement seismogram to the signal
that would have been recorded by a Wood-Anderson torsion pendulum (Frechet
and Thouvenot 2000). In this stage, we use the 2,800 magnification value given
for the Wood-Anderson. Uhrhammer and Collins (1990) found out that this value
had been calculated on the basis of wrong assumptions on the suspension geom-
etry, and a more correct value would be 2,080. We might therefore underestimate
the size of events by 0.13 (Bormann et al. 2002), but we have not introduced this
correction in the present study. We use the same attenuation law as that used by
Richter although this law has been established for California. However Kradolfer
and Mayer-Rosa (1988) analysed a set of earthquakes in and around Switzerland,
and concluded that Richter's law was also suitable for the western Alps. Magnitudes
computed by Sismalp and the Swiss Seismological Service usually differ by less
than 0.2.
A Gutenberg-Richter (1956) analysis of the 11,777 earthquakes located by Sis-
malp in the western Alps between 1989 and 2005 shows that events with a mag-
nitude larger than 1.3 can be confidently located (Marsan et al. 2008). Out of

those 11,777 events, 725 (43 per year) have a magnitude larger than 2. If we follow
Richter in his vague 1958 assumption, these events could be felt. We have checked
this since 1996 by directly appealing to testimonies for most M L >

∼

2 earthquakes
that occurred in the French Alps, instead of letting information reach us. This was
done mainly through telephone calls to gendarmeries, municipal services, and ho-
tels. In recent years, Internet accounts spontaneously sent to us made this quest
dispensable. Out of the 128 M L >

2 earthquakes we checked, 123 (96%) were felt.
The five events that were not reported felt had magnitudes between 2.0 and 2.3; they