Effects of Pore Fluid Pressure on Extension and Hybrid Fractures in Berea Sandstone and Carrara Marble

Subduction zones are home to a wide range of seismic activity including large earthquakes, low frequency earthquakes, slow slip events, and tremors. Seismological and geodetic observations have captured a spectrum of seismic activity in subduction zones. However, the mechanics of faulting and earthquakes are not fully understood. When drainage is poor, fluids generated from processes such as sediment compaction or mineral dehydration can lead to an increase in pore fluid pressures greater than hydrostatic pressure. Elevated pore fluid pressure affects the strength of the plate boundary faults and deformation behaviors of the surrounding intact rocks, which are evidenced by extensional veins observed in exhumed subduction thrust faults. To better quantify the stress states and fluid pressure at which these extension veins are formed, it is important to determine mechanical strength and the transition from tensile to shear fracture at low effective pressure. However, all previous experimental studies were conducted under dry conditions. Here, we present results of triaxial extension experiments using dog-bone geometry samples of Berea sandstone and Carrara marble under controlled pore fluid pressure. We apply two different loading paths. The first loading path is to apply differential load at constant strain rates of 2.4e-7 to 4.5e-4 s-1 while confining and pore pressure are kept constant. In the second type of experiment, we apply a predetermined level of differential stress and then increase pore pressure at a constant volumetric strain rate of 3.75e-4 s-1 while keeping confining pressure and differential load nearly constant. Preliminary results on both Berea sandstone and Carrara marble indicate that the results of pore-pressure controlled experiments are consistent with those on dry samples at the same effective pressure. We will present the acoustic emission counts recorded during tests and fracture surface morphology of the deformed samples. We plan to run similar experiments on mélange from the Shimanto accretionary complex near the Nobeoka thrust in Japan.