SAFOD : San Andreas Fault Zone Observatory at Depth

Duration : 2012
ISTerre team involved : Fault mechanics
Contact at ISTerre : Jean-Pierre Gratier

The San Andreas Fault Observatory (SAFOD) aims to better understand the sliding mechanisms associated with large active faults, including the transition from destructive seismic slides that occur episodically to asismic slides that permanently and safely accommodate plate movement.

This observatory was created as part of the Earthscope programme in the USA. Samples of the San Andreas fault, recovered at a depth of 3 km, are available by tender. Several dozen samples were obtained as part of an international consortium of researchers from Grenoble, Padova, Rome, Oslo, Aachen and Louisville.

Research is ongoing, particularly in the context of several theses : S. Mittempergher (2012) and J. Richard (ongoing). They have made it possible to specify the mechanisms of asismic sliding, which turns out to be a complex competition between the processes of friction and dissolution under stress. They also show the important role of fluids both through transient local overpressures and through their chemical interactions with rocks facilitating creep processes. They also explored the possibility of dating earthquakes using thermoluminescence approaches.

Figure : Mécanisme de fluage asismique des failles par dissolution, transfert par diffusion et cristallisation (A,B), avec application à différents minéraux (C), transition entre processus sismique et asismique selon la distance de transfert (D) et exemples de microstructures dans la faille (E, F), Gratier et al. Geology 2011

Some papers from this collaboration

– Spencer, J.Q.G., Hadizadeh, J., Gratier, J-P., Doan, M.L (2012) Dating deep ? Luminescence studies of fault gouge from the San Andreas Fault Zone 2.6 km beneath Earth’s surface, Quaternary Geochronology, in press.
– Hadizadeh, J., S. Mittempergher, J-P. Gratier, F. Renard, G. Di Toro, J. Richard, H.A. Babaie (2012) A microstructural study of fault rocks from the SAFOD ; Implications for the deformation mechanisms and strength of the creeping segment of the San Andreas Fault, J. Struct. Geol. in press.
– Gratier, J-P., J. Richard, F. Renard, S. Mittempergher, M.-L. Doan, G. Di Toro, J. Hadizadeh, and A.-M. Boullier (2011) Aseismic sliding of active faults by pressure solution creep : Evidence from the San Andreas Fault Observatory at Depth, Geology, 39, 12, p1131-1134.
– Mittempergher, S. G. Di Toro, J-P. Gratier, J. Hadizadeh, S. A. F. Smith, R. Spiess (2011) Evidence of transient increases of fluid pressure in SAFOD phase III cores, Geophysical Research Letters, 38, LO3301.