Earthquakes can sometimes occur in non-seismic areas due to various factors, such as distant tectonic plate movements, hidden faults, or local geological processes that are still poorly understood.
Regions considered as non-seismic are usually areas where earthquakes are not truly expected. There, the lithosphere (the Earth's crust and the upper part of the mantle) is often stable and quiet, as it is far from the turbulent boundaries between tectonic plates. These places, typically located within the plates, usually show very little to no signs of notable seismic activity. Yet, even here, tremors can catch us off guard. Deep, discreet, or ancient faults, sometimes never previously identified, can suddenly move and seriously shake an entire region that was deemed calm and serene. These events remind us that even in seemingly stable and tranquil zones, our lithosphere sometimes hides a few unexpected surprises.
Beneath the surface, even in so-called quiet seismic regions, the lithosphere hides surprises. The internal forces related to plate tectonics do not stop abruptly at the boundary of a non-seismic area: sometimes, the pressure accumulated elsewhere migrates and suddenly triggers an unexpected earthquake. Another common phenomenon is that ancient faults, once thought to be in permanent dormancy, can sometimes gently awaken due to slow adjustments or a rebalancing of deep stresses. It also happens that the Earth's crust experiences slight subsidence or uplift due to isostatic variations, for example, caused by the melting of thick ice, which temporarily creates unusual tensions, hence these unforeseen tremors where no one really expected them.
Humans can frankly shake the Earth, even where it seems calm. For example, fracking involves injecting water under high pressure to extract gas and oil: this increases the pressure on underground rocks, which can suddenly shift, causing tremors. The same goes for underground gas storage or the construction of large dams: the enormous mass of water held back deforms the ground, sometimes creating unexpected shocks in regions that are not known for being at risk. Intensive mining also plays a role, digging deeper and disturbing the local geological balance, leading to sudden ground movements and tremors in areas that were previously quiet. In short, it is often said that nature decides, but sometimes humans really give a nudge to lithospheric tremors.
In the eastern United States, a region far from the boundaries of major tectonic plates, a surprising earthquake shook the city of Charleston, South Carolina, in 1886. An unusual and intense tremor, felt far and wide, caused by the sudden reactivation of ancient faults buried deep beneath the Earth's crust.
Australia, typically considered seismically quiet, has also experienced several unexpected shocks, such as the one in Newcastle in 1989. Here again, old faults hidden in the depths temporarily resumed activity.
And then, in France, in La Rochelle in 2016, a minor but completely unexpected earthquake reminded us that even places known for their tranquility can sometimes shake. This small tremor likely resulted from a sudden and localized reorganization of accumulated stresses deep underground.
Some unusual earthquakes can be triggered by the immense weight accumulated by artificial water reservoirs created by giant hydroelectric dams, altering the local stress exerted on the Earth's crust.
The phenomenon of hydraulic fracturing used to extract shale gas can induce small earthquakes in regions previously considered stable and free from seismic activity.
Some rare but surprising earthquakes are linked to atypical natural phenomena, such as the sudden collapse of caverns or the dissolution of soluble rock layers like limestone, causing sudden ground subsidence.
The lithosphere, often perceived as rigid, can sometimes behave flexibly over long geological periods, which may lead to very gradual adjustments resulting in occasional tremors in unexpected locations.
The accurate prediction of such earthquakes remains very complex and difficult due to the poorly understood internal mechanisms at work in these typically non-seismic regions. However, regularly monitoring local geological and human activity allows for better preparedness.
Among these signs, one can find unexplained changes in well water levels, unusual dull underground noises, or abnormal movements detected by sensitive instruments, such as inclinometers or accelerometers.
Several activities can induce these events. For example: deep mining, hydraulic dams, hydraulic fracturing for shale gas extraction, or the underground reinjection of used fluids.
Yes, although rare, these tremors can pose risks due to their unexpected nature and the infrastructure not designed to withstand such events. They are generally of low magnitude, but it is important to remain vigilant.
Sure! Here’s the translation: "No, their frequency varies considerably from one region to another, depending on geological characteristics and the type of human activities conducted in the area. They are often isolated and unpredictable, but no continent is completely immune to these phenomena."
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