Fault geomechanical properties control the b-value at the Geoven deep geothermal site, France

Between March 2018 and January 2021, fluids were repeatedly injected at the Geoven deep geothermal energy site near the city of Strasbourg, France. These injections caused seismic activity, which ultimately led to the interruption of operations after a M_L 3.6 event. Despite the cessation of fluid injection, seismicity continued in the following months, culminating in a M_L 3.9 earthquake six months after shut-in.

Seismicity occurred in different clusters, which exhibit a wide range of b-values (0.6–1.6) despite their close temporal and spatial proximity. In the context of induced seismicity, b-value variations are often attributed to the activation of faults of different sizes or faults under different stress conditions. However, the clusters observed in this study are of comparable size and, assuming they are subject to the same regional stress field, are likely under similar stress conditions. This raises the possibility that the observed b-value variations are instead linked to differences in the geomechanical properties of the faults. To explore this, we analyzed the spatio-temporal evolution of the clusters.

The clusters were intermittently active through multiple bursts of seismicity, with each burst typically activating a new area. This type of migration is consistent with the process of earthquake interactions and suggests the presence of geomechanical heterogeneities, such as variations in fault strength, along the fault zone.

Despite their similar overall migration behavior, the clusters have different activity characteristics. High b-value clusters are characterized by small burst areas and a stable maximum earthquake magnitude (M_max) over time, indicating that rupture sizes remain limited throughout the cluster evolution. In contrast, low b-value clusters are characterized by larger burst areas and an increasing M_max over time, suggesting less constrained rupture sizes. The difference in b-value between clusters may then be related to differences in rupture behavior, which in turn may be related to variations in the geomechanical properties of the faults, such as different degrees of strength heterogeneity.