[Janssen] Fault zone damage and chemical reactions at depth in the San Andreas Fault Zone: A study of SAFOD drill core samples
German Title: Fault zone damage and chemical reactions at depth in the San Andreas Fault Zone: A study of SAFOD drill core samples
Current Status: completed
Main Applicant:Dr. Christoph Janssen
Prof. Dr. Georg Dresen
Our research project was structured around a series of questions addressing some basic issues in fault dynamics. The central scientific question was “Why are the San Andreas and some other major faults absolutely or relatively weak?" In response to this question several deformation mechanisms (e.g. amorphous material, nano-scale pore spaces, dissolutionprecipitation processes, intracrystalline plasticity), operating in the fault gouge of SAFOD core samples, were successfully analyzed using TEM, SEM and microprobe analyses. Amorphous material and nano-scale pore spaces were proofed as possible weakening mechanisms. Furthermore and in agreement with previous studies we also suggest that the high amount of weak clay minerals in general and the growth of Mg-rich phyllosilicates in particular may play a key role in the mechanical behavior of the San Andreas fault and can be directly related to fault weakening. The presence of abundant clay minerals is often related to the occurrence of nano-scale porosity that may indicates high fluid pressure. However, the SAFOD borehole was drilled into a creeping (aseismic) portion of the San Andreas Fault. The mechanisms relating earthquakes and aseismic creep remain still elusive due to the absence of microstructures that enable the evaluation of coseismic slip in SAFOD core samples. So, the inclusion of TCDP and Nojima fault drilling samples enabled us to identify key similarities and differences between characteristic microstructures of slip zones recently formed by co-seismic processes in earthquake faults (TCDP; Nojima fault) and active but aseismic creep that is evident in the SAFOD borehole by pronounced and ongoing casing deformation.