TRIGGERED EARTHQUAKES AS STRESS GAUGE - IMPLICATION FOR THE UPPERCRUST BEHAVIOR IN THE GRENOBLE AREA, FRANCE

Citation
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
Citations number
NO
Categorie Soggetti
Geosciences, Interdisciplinary
Journal title
ISSN journal
00334553
Volume
139
Issue
3-4
Year of publication
1992
Pages
579 - 605
Database
ISI
SICI code
0033-4553(1992)139:3-4<579:TEASG->2.0.ZU;2-4
Abstract
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.