Unveiling the Mystery: How 'Impossible' Earthquakes Occur
Scientists have long puzzled over earthquakes in seemingly stable regions, such as Utah, Soultz-sous-Forêts, and Groningen. According to traditional geological theory, these areas should not experience earthquakes due to the strengthening effect of shallow Earth's crust layers. However, tremors persist, prompting researchers from Utrecht University to investigate further. Their groundbreaking findings, published in Nature Communications, reveal a fascinating phenomenon: faults that have been dormant for millions of years can accumulate stress over time, leading to sudden releases of pressure in a single event.
Dr. Ylona van Dinther, the study's lead researcher, explains, "Faults in the shallow subsurface are typically stable, so we don't anticipate shock movements along them." Yet, seismic activity surprisingly occurs within the first few kilometers beneath the surface, where the ground is considered most stable. These shallow earthquakes are often linked to human activities like drilling, extraction, or fluid injection.
The enigma lies in why faults that strengthen with movement can suddenly weaken and slip, releasing energy as an earthquake. The answer lies in the behavior of ancient, inactive faults.
Inactive Faults and Slow Healing
Many human-induced earthquakes occur along ancient, inactive faults that have remained motionless for millions of years. Despite their inactivity, the surfaces where rocks meet slowly "heal" over time, becoming stronger. This gradual strengthening creates additional resistance. When this resistance is overcome, it can cause an abrupt acceleration along the fault, resulting in an earthquake, even in regions labeled as stable by geological models.
The shallowness of these earthquakes, occurring at depths of just a few kilometers, poses challenges for local communities. Buildings and infrastructure are often unprepared for the shaking, and the ground movement can be more noticeable and potentially damaging.
One-Time Events with Long-Term Stability
The Utrecht team discovered that these earthquakes are one-time events. Once the accumulated stress is released, the fault stabilizes into a new, more secure state. Dr. Van Dinther explains, "After the initial event, there is no further earthquake activity at that spot. This means that, although the subsurface won't settle immediately after human operations cease, the strength of earthquakes, including the maximum expected magnitude, will gradually diminish."
This finding has significant implications for sustainable subsurface use. Understanding how faults behave, heal, and accelerate or slow down is crucial for minimizing seismic risks associated with geothermal energy, carbon storage, and similar technologies. Researchers at Utrecht University are developing new computational models to refine these predictions and improve the communication of one-time earthquake risks.
