Unveiling the Mystery of 'Impossible' Earthquakes: A New Perspective on Fault Behavior
The occurrence of earthquakes in seemingly stable regions, such as Utah, Soultz-sous-Forêts, and Groningen, has long puzzled scientists. According to traditional geological theory, these areas should not experience earthquakes due to the strengthening effect of shallow Earth's crust layers. However, recent research from Utrecht University challenges this notion, shedding light on a fascinating phenomenon.
Dr. Ylona van Dinther and her team discovered that faults, even those that have been inactive for millions of years, can accumulate stress over time. This stress buildup can lead to a single, powerful release, resulting in earthquakes. This finding is particularly significant for assessing the safety of areas where technologies like geothermal energy extraction and underground energy storage are employed.
The study highlights a surprising aspect: seismic activity occurs in the shallow subsurface, where the ground is considered most stable. These shallow earthquakes are often linked to human activities such as drilling and fluid injection. The question arises: why do faults, which typically strengthen with movement, suddenly weaken and slip, releasing energy as an earthquake?
The answer lies in the behavior of ancient, inactive faults. These faults, though seemingly dormant, experience a slow 'healing' process where the surfaces of rocks gradually strengthen over time. This 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 poses a challenge. Since these areas have no long-term record of seismic activity, local communities are often unprepared. Buildings and infrastructure are not designed to withstand the shaking. Moreover, these earthquakes occur at a depth where human activities are common, making them more noticeable and potentially damaging.
Interestingly, the Utrecht team found that these earthquakes are one-time events. After the stress is released, the fault stabilizes, and there is no further earthquake activity at that spot. This means that, although the subsurface may not immediately settle after human operations cease, the strength of earthquakes will gradually decrease. The risk of stronger quakes diminishes once the fault has slipped, allowing for a reevaluation of the overall risk.
The research has profound implications for subsurface management. It emphasizes that even geologically stable regions can experience earthquakes under specific conditions, but only once per fault. Understanding fault behavior, their 'healing' process, and the factors influencing acceleration or slowing down is crucial for minimizing seismic risks associated with various technologies. Utrecht University researchers are now refining computational models to better predict and communicate these one-time earthquake risks.
