Jr. Grasso et al., TRIGGERED EARTHQUAKES AS STRESS GAUGE - IMPLICATION FOR THE UPPERCRUST BEHAVIOR IN THE GRENOBLE AREA, FRANCE, Pure and Applied Geophysics, 139(3-4), 1992, pp. 579-605
A sequence of moderate shallow earthquakes (3.5 less-than-or-equal-to
M(L) less-than-or-equal-to 5.3) was located within the Vercors massif
(France) in the period 1961 1984. This subalpine massif has been a low
seismic area for at least 5 centuries. During the period 1962 1963, 1
2 shallow earthquakes occurred in the neighborhood (DELTA less-than-or
-equal-to 10 km) of the Monteynard reservoir, 30 km south of the city
of Grenoble. The latest four M(L) greater-than-or-equal-to 4.0 earthqu
akes occurred in 1979 1984 either at larger distance (35 km) or greate
r depth (greater-than-or-equal-to 10 km) from the reservoir. Two trigg
ering mechanisms are suggested for this sequence: (i) the direct effec
t of elastic loading through either increased shear stress or strength
reducing by increased pore pressure at depth; (ii) the pore pressure
diffusion induced by poroelastic stress change due to the reservoir fi
lling. The weekly water levels, local balanced geological cross sectio
ns, and focal mechanisms argue for two types of mechanical connection
between the earthquake sequence and the filling cycles of the Monteyna
rd reservoir. The seismic sequence started with the 1962 - 1963 shallo
w earthquakes that occurred during the first filling of the reservoir
and are typical of the direct effect of elastic loading. The 1979 deep
er earthquake is located at a 10 km depth below the reservoir. This ev
ent occurred 16 years after the initial reservoir impoundment, but one
month after the previous 1963 maximum water level was exceeded. Moreo
ver the yearly reservoir level increased gradually in the period 1962
1979 and has decreased since 1980. Accordingly we suggest that the gra
dual diffusion of water from reservoir to hypocentral depths decreases
the strength of the rock matrices through increased pore pressure. Th
e transition between the two types of seismic response is supported by
the analysis of M(L) greater-than-or-equal-to 3.5 earthquakes which a
ll occurred in the period 1964 1971, ranging between 10 and 30 km dist
ance from the reservoir. The three other delayed earthquakes of the 19
61 1984 seismic sequence (M(L) greater-than-or-equal-to 4 during the 1
979- 1984 period) are all located 35 km away from the reservoir. Based
on the seismic activity, the estimates for the hydraulic diffusivitie
s range between 0.2-10 m2/s, except for the first event that occurred
30 km nor-th of the reservoir, the filling just started. The lack of i
n situ measurements of crustal hydrological properties in the area, sh
ared by most of the Reservoir-Induced-Seismicity cases, prevents us fr
om obtaining absolute evidence for the triggering processes. These obs
ervations and conceptual models attest that previous recurrence times
for moderate natural shocks (4.5 less-than-or-equal-to M(L) less-than-
or-equal-to 5.5) estimated within this area using historical data, cou
ld be modified by 0.1-1 MPa stress changes. These small changes in dev
iatoric stress suggest that the upper crust is in this area nearly eve
rywhere at a state of stress near failure. Although the paucity of bot
h number and size of earthquakes in the French subalpine massif shows
that aseismic displacements prevail, our study demonstrates that trigg
ered earthquakes are important tools for assessing local seismic risk
through mapping fault zones and identifying their possible seismic beh
avior.