Understanding fluid injection-induced earthquakes: From causal mechanisms to fault frictional slip

The global energy transition increasingly relies on the sustainable use of the subsurface, which commonly involves fluid injection. Such injection can induce earthquakes, posing significant challenges to the safety and operability of geo-energy applications. Addressing these challenges requires a geomechanical understanding of induced seismicity and the coupled subsurface processes that govern it. This Award Lecture introduces recent research on fluid injection-induced earthquakes, spanning the evaluation of causal mechanisms to an in-depth understanding of the fault-slip processes that control earthquake magnitude and frequency.

The first part of the presentation focuses on identifying and evaluating the causal mechanisms for injection-induced earthquakes. The problem is formulated as assessing the susceptibility of fracture and fault slip driven by coupled thermo-hydro-mechanical (THM) processes in fractured porous media. Through several geo-energy case studies, it is demonstrated that induced seismicity commonly results from fracture and fault reactivation through multiple, co-occurring mechanisms. The relative contribution of these mechanisms largely depends on regional geology, fracture and fault properties, ambient stress conditions, and operational parameters. Fluid overpressure typically develops rapidly following injection and may influence a large area, depending on hydraulic connectivity and fault permeability. Poroelastic stressing accompanies fluid pressurisation, with its contributions controlled by the distance to susceptible faults and fault orientation relative to the ambient stress field. Thermal stressing is generally more spatially localised around injection wells but can become dominant over longer timescales. In addition, fault slip-induced stress transfer can explain seismicity beyond the region affected by fluid pressure and poroelastic stress changes. Understanding these mechanisms enables the development of physics-based approaches for induced seismicity hazard assessment that explicitly account for both geological conditions and operational strategies.

The second part of the presentation addresses fault frictional slip processes that ultimately control the earthquake magnitude and frequency. Three key governing processes are identified for injection-induced fault slip: fluid pressurisation, hydraulic diffusion, and frictional nucleation, each characterised by a distinct timescale. Their interactions give rise to a wide range of induced earthquake behaviours. To disentangle their combined effects, a coupled hydro-mechanical-frictional modelling framework was developed that integrates frictional contact models for faults with poroelastic models for surrounding rocks. The results have shown that frictional properties exert first-order control on fault slip regimes and the maximum earthquake magnitude, whilst fluid pressurisation primarily governs earthquake frequency and also influences the maximum magnitude through poroelastic stressing. These effects are further modulated by hydraulic diffusion, highlighting the role of reservoir hydraulic conductivity in controlling how injected fluids interact with distant faults. Building upon this understanding, this contribution illustrates how fluid pressurisation rate influences induced earthquake magnitude and frequency, and discusses the implications for designing injection strategies that minimise seismic risk while maintaining operational efficiency.

Acknowledgement: I gratefully acknowledge the support and nomination by Prof. Sevket Durucan, Dr. Suzanne Hangx, Prof. Chris Spiers, Prof. Paul Glover, and Prof. Keita Yoshioka, and the many collaborators who contributed to the research presented.