Repeated Rupture of Asperities on a Single Fault: Aseismic and Seismic Interactions. A Case Study From Induced Seismicity at Soultz (France)

HR: 09:30h
AN: S31D-07 [Abstracts]
TI: Repeated Rupture of Asperities on a Single Fault: Aseismic and Seismic Interactions. A Case Study From Induced Seismicity at Soultz (France)
AU: Bourouis, S
EM: bourouis@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris, 75252 France
AU: Bourouis, S
EM: bourouis@ipgp.jussieu.fr
AF: Centre de Recherches Astronomiques, Astrophysique, et GＰphysiques, Route de l'Observaotire BP63, Alger, 16340 Algeria
AU: * Bernard, P
EM: bernard@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris, 75252 France
AU: Cornet, F
EM: cornet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 Place Jussieu, Paris, 75252 France
AB: The hydraulic stimulation of the granite rock mass around 3 km in depth at the geothermal site of Soultz-sous-Forets (Alsace, France), in 1993, has produced more than 11,000 earthquakes in 17 days, recorded on surface and borehole seismometers. Among them, more than 150 multiplets consist of at least five events, which were studied for constraining the related fracture orientations. The injection borehole is cut by several major fractures, among which a few have slipped by up to a few centimetres during the injection, as evidenced by the logging repeated at the end of the experiment. We present the results concerning the fracture which showed the largest slip in the borehole, 4.3 cm, with a normal faulting mechanism constrained by in situ measurement of the dip, strike, and slip angle. We selected the multiplets located close to the extrapolated plane, and showing a similar mechanism, assuming that they belong to the same fault surface. For each multiplet, we calculated the source radius from corner frequency. This dimension (typically 10 m) appears larger than the hypocenter distances (a few meters), which implies that they consist of the repetition of the rupture of a single asperity. We calculate the seismic moment to get the slip for each event, and, for each multiplet, we are thus able to deduce the cumulative slip of the related asperity. The latter shows that the slip rate progressively slows down after the first events, following an Omori law. A more detailed space-time analysis shows that the multiplet sequences are thus the repeated slip of asperities forced by this transient slip. Their Omori-type decay are thus not related to the standard model of accelerating slip deduced from rate-and-state velocity weakening friction law (Dieterich,1994), but to a decelerated slip due to velocity strengthening friction law. The asperities also interact between themselves, when their distances are smaller than about twice the source dimension, as shown by the statistics of interevent times. Some asperities also happen to trigger repeatedly within a few seconds, which we interpret as a retro-action process related to co-seismic fault compaction around the asperity involving delayed fluid pressure increase within the asperity.
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting