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Evidence of transient increases of fluid pressure in SAFOD phase III cores

Abstract : The San Andreas Fault Observatory at Depth (SAFOD) in Parkfield, central California, has been drilled through a fault segment that is actively deforming through creep and microearthquakes. Creeping is accommodated in two fault strands, the Southwest and Central Deforming Zones, embedded within a damaged zone of deformed shale and siltstone. During drilling, no pressurized fluids have been encountered, even though the fault zone acts as a permeability barrier to fluid circulation between the North American and Pacific plates. Microstructural analysis of sheared shales associated with calcite and anhydrite-bearing veins found in SAFOD cores collected at 1.5m from the Southwest Deforming Zone, suggests that transient increases of pore fluid pressure have occurred during the fault activity, causing mode I fracturing of the rocks. Such build-ups in fluid pressure may be related to permeability reduction during fault creep and pressure-solution processes, resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the SAFOD site.
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Silvia Mittempergher, Giulio Di Toro, Jean-Pierre Gratier, Jafar Hadizadeh, Steven A. F. Smith, et al.. Evidence of transient increases of fluid pressure in SAFOD phase III cores. Geophysical Research Letters, American Geophysical Union, 2011, 38, pp.L03301. ⟨10.1029/2010GL046129⟩. ⟨insu-00681315⟩



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