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 307

reported heard sometimes as frequently as several times a day. The same obser-
vation was made in 1934 (Rothe 1936) when earwitnesses described 'veritables
canonnades' and 'tirs de barrage'.

3.2 The 2006 Conand Aftershocks

The M L = 3.5 earthquake that occurred on the south-western flank of the French
Jura on 11 Jan. 2006 at 11.32 local time is one of the many events that—just like the
Annecy or Laffrey earthquakes—regularly strike the external domain of the Alps
(Fig. 1). The epicentral zone is sited amidst NW-SE-trending ranges where Dogger
(Middle Jurassic) limestone outcrops. The earthquake was felt up to a distance of
∼

20 km, but reached EMS intensity IV in 5 villages only. A maximum intensity of
VI was assigned to Conand (Ain), where more than half of the startled 72 inhabi-
tants left their dwellings. A chimney was knocked down. The church pavement was
cracked on both sides of the aisle, and rock flour was expelled from the fissures.
Drinking water was turbid for 2 days, and a falling in of stones blocked a small road
(Bureau Central Sismologique Fran¸ais 2006).
These effects, unusual for a 3.5 magnitude, were followed by vibrations and ex-
plosions in the next days. Such phenomena were of course reported by the residents
to the prefectoral services, which then addressed the seismological networks. As
the magnitude of the corresponding shocks was much below any detection level,
the obvious answer was that no seismic activity had been observed, hence leaving
the Conand inhabitants in perplexity. It actually took 10 days before we realized
that something unusual was happening. A temporary velocimetric station installed
in the village soon recorded aftershocks which proved very shallow: with S - P =
0.12 s, and by assuming a 5,000 m s − 1 velocity for P waves in Dogger limestone, we
compute a hypocentral distance of 900 m. From the P-wave amplitude recorded on
the vertical and horizontal components, we estimate the station to be sited at ∼ 50 m

from the epicentre, while the focal depth is 900 m.

The largest recorded aftershock occurred on 10 Feb. 2006, 1 month after the
mainshock. This event was heard as a loud explosion. Vibrations were also reported.
It was not recorded by the surrounding monitoring networks although the closest
permanent Sismalp station is only 15 km away. This station, installed in a mushroom
cave bored in Dogger limestone, has a low noise level; however it is only triggered
by an STA/LTA algorithm (no continuous recording).
If we use the seismograms obtained at the Conand local station (Fig. 3) for
computing the M L magnitude of the 10-Feb. earthquake, our routine processing
infers a value of 2.3. This is obviously overestimated because Richter's assumption
of a 15-km focal depth does not apply here with a station at the epicentre and a
shallow focus. To ascertain the seismic moment of this earthquake, we theoretically
modelled the S-wave pulse which has a frequency close to 20 Hz and an amplitude
of 280

∼

ms − 1 . We assumed a 900-m-deep source with a focal mechanism similar
to that of the mainshock (pure normal faulting, N135 ◦ E-trending horizontal tension
axis). We adopted P- and S-wave velocities of 5,000 and 2,900 m s − 1 , and a density