Source Mechanicms in the Intense SeismicityInduced by Fluids Injection at the Soultz HDRSite, Alsace, in 2003

摘要

We have inverted the peak amplitudes of direct P waves of 45 micro earthquakes with magnitudes ofbetween M = 1.4 and 2.9 which occurred during the 2003 massive fluid injection in the borehole of theSoultz site. The full moment tensor expression of the mechanism was applied. Surprisingly, themechanisms of all are dominantly pure shears. The T-axes are fairly stable, being concentratedsubhorizontally roughly in the EW direction. On the contrary, the P-axes are ill-constrained varying in theNS direction from nearly vertical to nearly horizontal, which points to heterogeneous stress in the Soultzinjected volume. This is in agreement with the stress pattern from in-situ measurements. The time-spacedistribution of the events analysed suggests that the injection activated two segments of the natural faultsexisting in the area showing different source mechanism patterns. The dip-slip regime is characteristic forfault segment I where the seismicity occurred during and also after injection, while the strike-slip regimeprevails in segment II where the seismicity was triggered only after the injection shut in. This indicatesthat the tensile fractures may have occurred on a smaller scale than the pure shear microearthqukesinvestigated.