Nucleation and rupture of induced earthquakes in Groningen confined to the gas reservoir

To assess seismic hazard in the Groningen gas field, it is crucial to understand earthquake source processes, including the locations of nucleation and possible arrest. These fundamental characteristics, however, remain poorly constrained by seismological observations due to limited resolution. Interpretations of seismological observations are often inconsistent because the focal depth inversion uncertainty (~300 m) is comparable to reservoir thickness (50-300 m). Two fault segments, the velocity-weakening anhydrite layer within the caprock sequence and the velocity-strengthening sandstone reservoir experiencing substantial healing, are suggested to be seismogenic [1]. However, their respective roles in nucleation and rupture remain unclear. Additionally, whether ruptures can propagate into the over- and underburden layers is also debated, yet this is a key constraint for the maximum possible earthquake magnitude (Mmax).

 

Here, we use physics-based earthquake sequence simulations to investigate how stratigraphic layering, lithology-dependent elastic and frictional properties, and long-term fault healing govern rupture behavior. We find that earthquake nucleation consistently occurs within the sandstone reservoir, even when velocity-weakening friction is assigned to the overlying anhydrite caprock. Rupture propagation is predominantly confined to the reservoir thickness, with only limited penetration into adjacent formations. The anhydrite can only be activated, in rare cases, through rupture propagation. Introducing mechanical heterogeneity exerts a dominant control on rupture behavior by substantially suppressing slip rates and limiting rupture extent, whereas frictional heterogeneity has a comparatively minor effect in the opposite sense. Fully runaway rupture into the underburden is exceedingly rare. It only occurs in one out of 2,000 simulations and requires an extreme and unlikely combination of geometric, mechanical, and frictional conditions. Statistical mapping of simulation outcomes onto the Groningen fault network indicates that most fault segments have 5% or less likelihood of rupture propagating over a distance larger than the reservoir thickness. The likelihood of fully runaway rupture is 0.3%–1% only in a few peripheral regions beyond the locus of recorded earthquake occurrence and below 0.3% elsewhere. Together, these results demonstrate that lithological heterogeneity imposes strong physical constraints on rupture extent, providing robust, physics-based limits on Mmax and improving seismic hazard assessment for Groningen and other energy-producing regions.

 

[1] Li, M., Niemeijer, A., Van Dinther, Y. (2025, Nat. Comm.) https://doi.org/10.1038/s41467-025-63482-3.