On the Potential Role of Viscoelasticity in Fluid-Induced Seismicity

Fluid-induced earthquakes adversely affect industrial operations like hydraulic fracturing (e.g., 4.6 Mw in Alberta, Canada) and enhanced geothermal systems (e.g., 5.5 Mw in Pohang, South Korea). Identifying all underlying physical processes contributing to fluid-induced seismicity presents an open challenge. Recent work reports signatures of event-event triggering or aftershocks-common for tectonic settings-within the context of fluid-induced seismicity. Here, we investigate the underlying potential cause of these field observations from a modeling perspective. We extend a novel conceptual model to simulate the characteristics of crustal rheology and stress interactions in a porous medium by combining viscoelastic effects with fluid diffusion and invasion percolation associated with a point source. Our model successfully reproduces realistic aftershock behavior and statistical properties similar to those resulting from tectonic loading indicating that the statistical properties of aftershocks are unaffected by the fluid injection rate. At the same time, the Gutenberg-Richter relation, the spatial footprint of fluid-induced events and their dependence on the permeability field are largely unaltered by the viscoelasticity of the medium and the aftershocks it causes. Furthermore, we investigate the impact of varying fluid injection rates on detecting aftershocks and event-event triggering sequences during viscoelastic stress redistribution. We find that when the injection rate is sufficiently high, aftershock detection and recovery of their statistical properties are only feasible when the underlying internal stress redistribution is directly accessible. This could explain why aftershocks have not been reported in some field studies of fluid-induced seismicity. Fluid-induced earthquakes are an unintended consequence of industrial activities such as enhanced geothermal systems and hydraulic fracturing. These operations involve injecting high-pressure fluids into the Earth's crust to improve the permeability of deep geothermal reservoirs or enhance oil and gas extraction from rock formations, respectively. The resulting seismic activity has raised concerns among the industry and residents. Similar to tectonic earthquakes, some cases of fluid-induced earthquakes have shown the presence of aftershocks during or after field operations, which need to be taken into account for seismic hazard assessments. In particular, understanding the fundamental physical mechanisms that connect fluid injection and earthquakes is crucial to assess seismic hazards correctly. Here, we extend a conceptual model of fluid-induced earthquakes by incorporating a slow mode of stress transfer. In this case, aftershocks occur and our results show that the aftershock behavior in fluid-induced settings resembles that of tectonic aftershocks. We also find that the fluid injection rate does not affect the characteristics of aftershocks. However, detecting aftershocks and recovering their characteristics depends on the accessibility of the internal stress dynamics. Our findings shed light on the behavior of fluid-induced earthquakes and why aftershocks may not be detected in some field cases. Incorporating viscoelasticity in models of fluid-induced seismicity leads to aftershock triggering, independent of the injection rateGutenberg-Richter relation and the spatial footprint of fluid-induced events and its dependence on the permeability field remain largely unalteredWhen fluid injection rates are high, aftershock detection is only feasible when the internal stress field is accessible