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. Copyright © 2011 by the American Geophysical Union.
Evidence of transient increases of fluid pressure in SAFOD phase III cores / Mittempergher, S.; Di Toro, G.; Gratier, J. P.; Hadizadeh, J.; Smith, S. A. F.; Spiess, R.. - In: GEOPHYSICAL RESEARCH LETTERS. - ISSN 1944-8007. - 38:3(2011), pp. n/a-n/a. [10.1029/2010GL046129]