Pore-fluid pressure pulses from rapid, localized compaction of a porous rock

Changes in pore fluid pressure are often implicated in producing a range a fault slip behaviour and in the triggering of earthquakes. For fluid pressure increases to influence fault mechanics, either low permeability materials must be present to inhibit fluid loss, or a mechanism for rapid production of pore fluid pressure must operate. In this work, we present observations of rapid generation of pulses of pore fluid pressure, by the unstable propagation of localized low porosity bands formed during compaction of porous rock. We perform a series of high pressure compaction experiments on intact cores of porous α and β bassanite that are hydrostatically loaded at different compaction rates. We find that at hydrostatic loading rates of ≥ 0.01 MPa/s, pulses of pore fluid pressure are observed only for α -bassanite, associated with expulsion of fluid during the rapid formation of low porosity bands. At slower hydrostatic loading rates ( < 0.01 MPa/s) no pore fluid pulses are observed and compaction proceeds smoothly, despite the development of a similar network of low porosity bands in α -bassanite, suggesting that the bands propagate in a slower and more stable fashion at these conditions. We attribute the absence of pore-pressure pulses and low porosity bands in β -bassanite to differences in bassanite grain size and shape. We conclude that at high hydrostatic loading rates pore fluid pulses are produced during the rapid formation of low porosity bands when stress concentrations at the band tip reach the critical stress intensity factor K C causing unstable propagation. • Pore pressure pulses are produced during bassanite compaction. • Low porosity bands are formed during slow and fast compaction. • Slow to fast low porosity band growth is controlled by stress concentration.