b‐Value, aseismic deformation and brittle failure within an isolated geological object: Evidences from a dome structure loaded by fluid extraction

Studies of b-values are usually performed either at rock sample scale (laboratory experiments) or at crustal scale (earthquakes). But interpretations at crustal scale are extrapolated from small scale experimental laws with well defined boundary conditions, to a larger object with no clear boundary conditions. We examine variations of the b-value in time and space at spatial (lOxlOx10 km3) and temporal (20 years) scales intermediate between laboratory analyses and tectonic processes, in a dome structure that is an isolated geological object with defined boundaries and known geomechanical properties. Seismic activity (about 1000 events with magnitude 4 4.5) and aseismic displacements (6 -7 cm of cumulative subsidence) have been induced by gas extraction in an area where no displacement had previously been reported. We find no agreement between temporal variations in b-values and results from laboratory experiments : there is no correlation between b-values and stress histories, nor between b-values and the spatial migration of seismicity. Aseismic slips introduce anomalies in b-value behaviour when seismic instabilities are a second order process compared to the whole deformation. These observations imply that when changes in b values have been used for earthquake forecasting, false alarms can be explained by the Occurrence of aseismic displacement.