Imagine discovering that earthquakes can strike in places where they were thought to be impossible. It’s like finding out your safest bet was never truly safe. This is exactly what scientists are grappling with in regions like Utah (USA), Soultz-sous-Forêts (France), and Groningen (the Netherlands). According to geological textbooks, these areas should be immune to seismic activity because their shallow crust layers theoretically strengthen faults as they move, preventing earthquakes. But here’s where it gets controversial: earthquakes are happening anyway. So, what’s going on? Researchers from Utrecht University dove into this mystery, and their findings, published in Nature Communications, are eye-opening. They reveal that even faults dormant for millions of years can silently accumulate stress, eventually releasing it in a single, unexpected event. This discovery isn’t just fascinating—it’s critical for determining safe locations for technologies like geothermal energy extraction and underground storage.
Dr. Ylona van Dinther, the study’s lead, explains, ‘Faults in the shallow subsurface are typically stable, so seismic activity there is unexpected.’ Yet, surprisingly, earthquakes do occur within the first few kilometers below the surface—the very zone considered most stable. And this is the part most people miss: these shallow quakes are often linked to human activities like drilling or fluid injection. The real puzzle? Why do faults that should grow stronger when they move suddenly weaken and slip, triggering an earthquake?
But here’s where it gets even more intriguing: Many of these quakes occur along ancient, inactive faults that haven’t budged in millions of years. Over time, the rock surfaces along these faults ‘heal,’ becoming stronger and more resistant. When that resistance is finally overcome, it can cause a sudden acceleration along the fault, resulting in an earthquake—even in areas labeled ‘stable’ by geological models. The kicker? These regions often lack a history of seismic activity, leaving communities unprepared. Buildings and infrastructure aren’t designed to withstand the shaking. Plus, these quakes happen at depths where human activities take place—just a few kilometers down—making their impact more noticeable and potentially damaging.
Here’s another twist: The Utrecht team found that these earthquakes are one-time events. Once the built-up stress is released, the fault stabilizes, and seismic activity in that spot ceases. ‘The strength of the earthquakes, including their maximum magnitude, gradually decreases over time,’ says Van Dinther. This is because as faults move, their broken sections slide more easily past one another, acting as natural barriers that prevent larger quakes from forming. So, while the initial event might be alarming, the long-term risk actually diminishes.
But here’s the bigger question: If these earthquakes are rare and self-limiting, does this change how we view ‘stable’ regions? And how should this influence our use of the subsurface for sustainable technologies? The research underscores that even in geologically stable areas, earthquakes can occur—but only once per fault. Understanding fault behavior, their healing process, and what triggers their acceleration is key to minimizing risks for geothermal energy, carbon storage, and more. Utrecht University is already refining computational models to better predict and communicate these one-time risks.
Now, let’s spark some debate: If these earthquakes are essentially one-and-done events, should we reconsider how we classify ‘stable’ regions? And how much should human activities in these areas be restricted, if at all? Share your thoughts below—let’s discuss!
