Triggering processes of seismicity and changes in crack shape during the water penetration into a stressed rock

We studied hydro-mechanical effects on earthquake generation processes in the laboratory. The changes in the pore pressure or penetration of water sometimes cause the deformation of rock samples and trigger the seismicity. We focused on the microcrack activities caused by hydrological effects inside rock samples. We estimated crack characterizations such as the aspect ratio and the degree of water saturation and their changes during the water penetration into a granitic rock subjected to the differential stress under the confining pressure. Water penetration induced microcrack activities. The concentration of acoustic emission (AE) events and formation of water channels marks a positive feedback process in which the local strength of the rock is reduced by the decrease of the effective confining pressure caused by increasing pore pressure which, in turn, triggers more microfracturing. During the water penetration, P- and S-wave velocities of elastic waves that propagated along the paths parallel to the top and bottom surfaces of the sample were measured. We also measured strains of the sample surface and monitored AE. We estimated the aspect ratio and the degree of water saturation by applying the cracked solid model to the data on P- and S-wave velocities and the crack density. We found (1) the aspect ratio of dry cracks before the water injection was 1/400, (2) during the water migration, the aspect ratio and degree of water saturation increased.