Metre-scale stress heterogeneities and stress redistribution drive complex fracture slip and fracture growth during a hydraulic stimulation experiment

SUMMARY We investigated the induced seismicity, source mechanisms and mechanical responses of a decametre-scale hydraulic stimulation pre-existing shear zone in crystalline rock, at Grimsel Test Site, Switzerland. The analysis reveals metre-scale complexity stimulation, which remains hidden reservoir-scale. High earthquake location accuracy allowed separation four distinct clusters, three were attributed to fractures damage zone. mechanism larger-magnitude seismicity varied by cluster, suggests heterogeneous stress field already prevailing before is further modified during stimulation. In course experiment, redistribution led aseismic initiation tensile-dominated fracture, fourth identified seismic clusters. streaky pattern separated zones without fluid flow conduits along existing fracture planes. observed submetre scale questions forecasting ability hazard reservoir from small-scale experiments.