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Drilling Into the San Andreas Fault
The San Andreas Fault Observatory at Depth (SAFOD) is a borehole observatory drilled into seismogenic depths to investigate the physical and chemical processes associated with faulting and earthquake generation. The hydromechanical properties of drill cuttings and cores retrieved during SAFOD Phases 1 and 2 were systematically investigated. A review is presented on the rock mechanics data and their earthquake mechanics implications, with focus on the coupling between stress state, heat flow and pore pressure. In light of the findings that the SAFOD cores contain hydrous platy minerals and can be relatively weak, we extended Rice's (1992) model on development of pore pressure excess in a fault zone to a scenario with relatively low frictional strength and derived explicit expressions for the stress state and pore pressure distribution. To connect the theoretical predictions to field observations, it is necessary to prescribe the friction coefficient of the SAF gouge as a function of depth and accordingly, experiments were also conducted to characterize the frictional properties of SAFOD cores under hydrothermal conditions. Since the gouge is weaker than the country rock, a lower pore pressure excess is required for failure to occur along a fault zone under near normal compression. Nevertheless, the predicted pore pressure required is still high, well in excess of the lithostatic pressure. This is in apparent discrepancy with preliminary borehole observations at SAFOD, which have yet to detect such pressure excesses.
Drilling Into the San Andreas Fault
The San Andreas Fault Observatory at Depth (SAFOD) is a borehole observatory drilled into seismogenic depths to investigate the physical and chemical processes associated with faulting and earthquake generation. The hydromechanical properties of drill cuttings and cores retrieved during SAFOD Phases 1 and 2 were systematically investigated. A review is presented on the rock mechanics data and their earthquake mechanics implications, with focus on the coupling between stress state, heat flow and pore pressure. In light of the findings that the SAFOD cores contain hydrous platy minerals and can be relatively weak, we extended Rice's (1992) model on development of pore pressure excess in a fault zone to a scenario with relatively low frictional strength and derived explicit expressions for the stress state and pore pressure distribution. To connect the theoretical predictions to field observations, it is necessary to prescribe the friction coefficient of the SAF gouge as a function of depth and accordingly, experiments were also conducted to characterize the frictional properties of SAFOD cores under hydrothermal conditions. Since the gouge is weaker than the country rock, a lower pore pressure excess is required for failure to occur along a fault zone under near normal compression. Nevertheless, the predicted pore pressure required is still high, well in excess of the lithostatic pressure. This is in apparent discrepancy with preliminary borehole observations at SAFOD, which have yet to detect such pressure excesses.
Drilling Into the San Andreas Fault
Shao, Jian‐Fu (editor) / Burlion, Nicolas (editor) / Wong, Teng‐Fong (author)
2013-02-19
15 pages
Article/Chapter (Book)
Electronic Resource
English
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