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 304

either occurred in remote mountainous areas or had a focus deeper than 10 km.

Although common farther east in Italy, such 'deep' earthquakes seldom occur in the
French Alps, where the seismogenic zone is mostly restricted to the first 10 km of
the crust (Thouvenot and Frechet 2006).
There is also fair evidence that protracted aftershock series favour the perception
of still smaller magnitudes. We have in mind two recent destructive earthquakes,
viz. the M L = 5.3 1996 Annecy earthquake, and the M L = 3.5 1999 Laffrey earth-
quake (Fig. 1). The Annecy earthquake (maximum MSK intensity VII-VIII) had its
epicentre in the NW suburbs of the prefecture town of Haute-Savoie. Its focus was
shallow (

∼

2 km), within the Mesozoic sedimentary cover. The densely-inhabited
epicentral zone was formerly a marsh area whose loose sediments amplified ground
acceleration by a factor close to 10 in the 1-10-Hz frequency range (Thouvenot
et al. 1998). The strike-slip mainshock generated aftershocks for more than 3 years,
a much longer span than what could be anticipated for a 5.3 magnitude. Many after-
shocks were locally felt that were recorded only by a temporary station maintained

∼

8˚

6˚ 7˚

Geneva
Conand

Annecy

Lyons

Grenoble

Turin

Laffrey

Tricastin

Nice

France

Fig. 1 Map of South-East France, with the 4 earthquakes discussed in the text: Annecy (15 Jul.
1996, M L = 5.3), Laffrey (11 Jan. 1999, M L = 3.5), Tricastin earthquake swarm (Dec. 2002-Mar.
2003), and Conand (11 Jan. 2006, M L =3.5)