HR: 0830h
AN: NG41C-0079    [PDF]
TI: Triggered Seismicity in the Hydraulic Stimulation at Soultz (France): Evidence for the Control of the Multiplet Sequences by Major Aseismic Slip
AU: * Bourouis, S
EM: bourouis@ipgp.jussieu.fr
AF: Departement de Sismologie, IPGP, case 89, 4, place Jussieu tour 24- 4eme etage, Paris, 75252 France
AU: Bernard, P
EM: bernard@ipgp.jussieu.fr
AF: Departement de Sismologie, IPGP, case 89, 4, place Jussieu tour 24- 4eme etage, Paris, 75252 France
AU: Cornet, F
EM: cornet@ipgp.jussieu.fr
AF: Departement de Sismologie, IPGP, case 89, 4, place Jussieu tour 24- 4eme etage, Paris, 75252 France
AB: In 1993, an hydraulic stimulation of the natural fracture system of the granitic rock mass of Soultz-sous-Forˆts (Alsace, France), at a depth between 2850 m and 3400 m, and lasting 17 days, has generated more than 11,000 micro-earthquakes. These have been recorded by a network consisting of four seismic stations cemented at the bottom of the wells. First, these data have been classified in multiplets using the SPARS method. Second, in this dataset, only the multiplets (150) containing more than five events have been selected. Third, precise sources relative locations have been determined by a cross-spectrum analysis to compute a mean fracture plane for each multiplet. These planes have been correlated to the fractures observed by diagraphy in the injection well. One of the major fractures observed in this well experienced a slip of 4.3 cm during the injection. We selected the multiplets spatially correlated with the plane of this fracture, and estimated the source parameters of every event in each multiplet. The distance between them is much smaller than their size (as deduced from their corner frequency), showing that each multiplet consists of a repeating slip on the same asperity. The cumulative slip obtained for each multiplet is of the same order as the one observed in the well on the selected fracture. The calculations shows a clear relation in time between the increase of the pore pressure and the cumulative slip of the seismic fractures. There is also a decrease of the seismicity rate with time. We interpret these observations as resulting from the occurence of a mostly aseismic slip on a major fracture (size 100 m), forcing small asperities (size 10 m) to rupture repeatedly for accommodating this creep event, generating the multiplets, and decelerating with time, as pore pressure reaches an equilibrium. The resulting Omori law has therefore little to do with fault patches accelerating their slip towards rupture, such as in Dieterich's models, but would be linked to the contrary to some decelerating aseismic slip on destabilized, creeping faults. We suggest that such a model may apply, at least in part, for sequences of aftershocks from natural earthquakes.
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 1242 Seismic deformations (7205)
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting