Injection-induced seismicity fronts and stress distribution on rough faults

The increasing occurrence of injection-induced earthquakes has raised public concern and highlighted the importance of understanding subsurface processes to assess induced seismic hazards and risks. A feature of natural faults that has not received sufficient attention in induced seismicity modeling is their geometric roughness. We develop a simple physics-based model to investigate how fault roughness can control induced seismicity during fluid injection.

The first approach to modeling along-fault stresses prior to injection is to project the background stress tensor onto the rough fault. In this case, our models and theoretical analysis show that the apparent diffusivity of seismicity fronts can deviate significantly from the hydraulic diffusivity. Faults with realistic roughness generally display slow seismicity migration, producing apparent diffusivities far below the hydraulic values. Thus, seismicity fronts often lag behind the pressure front, especially at low background stresses and small roughness amplitudes. Only in the rare case of very rough faults stressed very close to failure, apparent diffusivity can exceed the hydraulic diffusivity, leading to seismicity fronts that outpace pressure fronts. 

The second approach to modeling along-fault stresses prior to injection is to simulate stress evolution after multiple tectonic rupture cycles. This ongoing work explores the resulting stress heterogeneity after multiple tectonic rupture cycles and examines whether seismicity migration follows the same trend as in the first approach, i.e., whether seismicity migration is generally slower than the pressure front on rough faults.

Apart from seismicity migration, the magnitude-frequency statistics are also analyzed. Along this single rough fault the frequency-magnitude distribution is bimodal. These results demonstrate how fault roughness and stress conditions control the induced seismicity through their influence on the criticality of the fault and stress transfer, and link long-term fault loading processes with short-term seismicity migration patterns in fluid injection scenarios.