Earthquakes in places like Utah (USA), Soultz-sous-Forêts (France), and Groningen (the Netherlands) seem puzzling to scientists because, according to geological theory, they shouldn't be possible. In these regions, the shallow layers of the Earth's crust are thought to behave in a way that strengthens faults when they begin to move. Textbooks suggest that this strengthening effect should prevent earthquakes from happening at all. Yet, tremors still occur in these supposedly stable zones. Researchers from Utrecht University set out to understand why. Their findings, recently published in Nature Communications, reveal that faults which have remained inactive for millions of years can accumulate extra stress over time. Eventually, that built-up pressure may be released in a single event. This insight is crucial for identifying safer areas for technologies such as geothermal energy extraction and underground energy storage.
"Faults can be found almost everywhere. Faults in the shallow subsurface are usually stable, so we do not expect shock movements to occur along them," explains Dr. Ylona van Dinther, who led the study. Yet, surprisingly, seismic activity does take place within the first few kilometers beneath the surface -- precisely where the ground is considered most stable. These shallow earthquakes are often linked to human activities such as drilling, extraction, or fluid injection. The question, then, is why faults that normally grow stronger when they move can suddenly weaken and slip, releasing energy as an earthquake.
Inactive faults and slow healing
Many human-induced earthquakes occur along ancient, inactive faults that have not shifted for millions of years. Although these faults remain still, the surfaces where the rocks meet slowly "heal" over time, becoming stronger. This gradual strengthening creates additional resistance. When that resistance is finally overcome, it can cause an abrupt acceleration along the fault. That acceleration produces an earthquake, even in regions that geological models label as stable.
Because areas like these have no long-term record of seismic activity, local communities are often unprepared. Buildings and infrastructure are not designed to handle the shaking. "Furthermore, these earthquakes take place at a depth where human activities occur, in other words, no more than several kilometres deep. That is considerably less deep than the majority of natural earthquakes." This shallowness means that such quakes can cause more noticeable and potentially damaging ground movement.
One-time events that stabilize over time
Interestingly, the Utrecht team found that these earthquakes are one-off events. Once the accumulated stress is released, the fault settles into a new, more stable state. "As a result, there is no more earthquake activity at that spot," says Van Dinther. "This means that, although the subsurface in such areas will not settle immediately after human operations stop, the strength of the earthquakes -- including the maximum expected magnitude -- will gradually decrease." When a fault strengthens as it moves, its broken sections can slide more easily past one another afterward, acting as natural barriers that prevent larger earthquakes from forming. This means the overall risk can be revised downward, since the potential for stronger quakes diminishes once the fault has slipped.
Implications for sustainable subsurface use
The research has significant consequences for how we use and manage the Earth's subsurface. It shows that even in regions considered geologically stable, earthquakes can occur under certain conditions -- but only once per fault. After the initial event, the area tends to become more secure. Understanding how faults behave, how they "heal," and what causes them to accelerate or slow down is essential for minimizing seismic risks associated with geothermal energy, carbon storage, and similar technologies. With new computational models, Utrecht University researchers are already working to refine these predictions and improve how one-time earthquake risks are communicated.