Understanding the Causes of Fluid-Induced Seismicity Through Large-Block ~1m³ Laboratory Experiments

Safe geologic sequestration of CO2 is important to decrease the concentration of greenhouse gases in the atmosphere. However, its injection could increase the underground pore pressure and potentially induce sliding of critically stressed faults. We here investigated acoustic emission signals from a laboratory test, where fluid injections close to an artificial interface of ~1m length were conducted with different injection pressures and differences in fluid viscosity. During the injection, we could observe three different types of AE signals: 1) AE with P- and S- wave energy, primarily located on the sliding plane and caused by increasing confining stresses, 2) hydraulic fracture type of AE with predominant P-wave energy signals, and 3) long lasting oscillations or tremor-like signals along with a stick-slip motion along the artificial interface. The pore pressure at the injection point reached up to 6.2 MPa during the hydraulic stimulation and after shut-in, it dropped down to almost zero. However, about 10 minutes later, a sudden sliding of the interface (stick-slip motion) was recorded. The analysis of the spatial distribution of the AE energy was applied to monitor first the evolution of the hydraulic fracture and thereafter the dynamics of stick-slip, indicating a nucleation phase of the sliding, then the rupture propagated through the whole interface with an average rupture velocity of a few m/s. The speed and the energy radiated during this event were approximately 6 orders of magnitude larger than observed during quasi-static sliding preceding the stick-slip. This observed stick-slip motion can be considered a laboratory analogue to earthquakes, and its occurrence can be related to the injection of fluids.