Researchers from Utrecht University have explained why earthquakes occur in geologically stable areas like Utah and Groningen, challenging traditional theories. Their study reveals that ancient, inactive faults heal and strengthen over millions of years before releasing built-up stress in a single event, often triggered by human activities. This finding aids in assessing risks for subsurface projects such as geothermal energy.
Earthquakes in regions considered stable, such as Utah in the USA, Soultz-sous-Forêts in France, and Groningen in the Netherlands, have puzzled scientists because geological models predict that faults in the shallow Earth's crust should strengthen during movement, preventing seismic activity. However, tremors still happen in these areas, particularly within the first few kilometers of the surface, where human activities like drilling, extraction, or fluid injection are common.
A team led by Dr. Ylona van Dinther from Utrecht University investigated this phenomenon. Their research, published in Nature Communications on November 4, 2025, shows that faults inactive for millions of years gradually 'heal,' becoming stronger over time and accumulating extra stress. When this resistance is overcome, it leads to an abrupt acceleration along the fault, causing an earthquake. '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. van Dinther.
These events are typically one-off occurrences. Once the stress is released, the fault enters a more stable state, reducing future seismic risk. 'As a result, there is no more earthquake activity at that spot,' says van Dinther. This stabilization means the maximum expected magnitude decreases over time, as the fault's broken sections act as barriers against larger quakes.
The discovery has implications for sustainable uses of the subsurface, including geothermal energy extraction and underground energy storage. By understanding fault healing and reactivation, scientists can better identify safer sites and communicate one-time earthquake risks, helping to minimize disruptions in areas without historical seismic records.