Seismic and aseismic motions generated by fluid injections

• Identify the signature of hydromechanical coupling on microseismic activity development.
• Outline the role of aseismic motion generated by deep fluid injection on the development of induced microseismicity.
• Discuss the role of aseismic monitoring for mitigating ground surface disturbances.

Pore pressure increase associated with the injection of fluids in rock masses often generates some microseismicity. But pore pressure variations depend on fluid diffusion, which itself depends on hydromechanical coupling. We identify in the present review paper four different pore pressure levels that control hydromechanical coupling and therefore the development of fluid induced microseismicity. But more importantly, fluid injections have been shown to generate also non seismic motions, i.e. motions that are too slow to be detected by classical monitoring networks. Such aseismic motions have been identified both through direct observations and through their indirect effects. They have been found to affect volumes equivalent to those associated with magnitude 5 earthquakes, when no such large seismic event has been observed. These aseismic slips generate large stress perturbations that have been found in some occasions to develop long after fluid injection has stopped. It is recommended that specific attention be given to these aseismic motions in order to keep nuisances observed on ground surface at acceptable levels.