Earthquakes can occur even far from the boundaries of tectonic plates due to the presence of active geological faults within the plates. These faults can break and generate earthquakes, called intraplate earthquakes.
Tectonic plates constantly accumulate forces within their very surface. Even far from the edges, these internal stresses can slowly build up over thousands or even millions of years until they exceed the strength of the rocks. At that point, it breaks, it shifts, and there is an unexpected earthquake in the middle of a plate. Generally, this kind of intraplate earthquake is rare but can be quite intense because the stresses involved have been stored for a long time. This phenomenon mainly affects regions located on ancient faults or deeply buried fractures, often unknown to local populations.
Sometimes, far from the boundaries between plates, ancient geological faults, formed a long time ago and considered dormant, can suddenly awaken. These faults, already present but almost invisible to the naked eye, slowly accumulate stress due to the internal constraints of the plates, subtle variations in rock weight, or even changes in underground pressure. One day, these accumulated tensions exceed the fault's resistance, and there is a sudden rupture: this gives rise to an intraplate earthquake, at a location where almost no one expected it. This phenomenon can explain why a region that has been quiet for thousands or even millions of years can suddenly experience a real earthquake, catching everyone by surprise.
Over time, layers of sediments gradually accumulate, creating an enormous weight on the Earth's crust. Imagine it like placing an increasingly heavier backpack on your shoulders: eventually, something will crack. This additional weight generates mechanical stresses that alter the balance of the underground, leading to deformations. As a result, even far from the usual boundaries of plates, pre-existing faults can move and cause unexpected earthquakes. This is typically the kind of phenomenon observed in certain areas like the Mississippi basin or near large deltas. The gradual piling up of sediments thus merges geology and physics to create earthquakes where no one really expects them.
Some earthquakes far from plate boundaries are linked to human activities, particularly hydraulic fracturing. This technique, used to extract deeply buried gas or oil, creates significant underground pressures by injecting large quantities of water or fluid at high speed. This injection can trigger ground movements by increasing the pressure in existing geological faults. Other human activities, such as massive mining or the underground storage of industrial fluids (for example, wastewater), can have similar effects, sometimes causing unexpected earthquakes far from the usual tectonically active areas. These tremors are generally not very powerful, but occasionally, they can reach an intensity sufficient to be felt, or even to cause damage.
Hydraulic fracturing, often referred to as "fracking," is capable of generating minor to moderate earthquakes in regions that previously had no significant natural seismic activity.
Some intraplate earthquakes (located far from tectonic plate boundaries) are related to the reactivation of ancient fault systems that can be several hundreds of millions of years old.
The accumulated weight of several kilometers of sediment deposited over long periods can be enough to alter local underground stresses and trigger earthquakes even in seemingly stable regions.
The Lisbon earthquake of 1755, one of the deadliest and most destructive in Europe, occurred along a fault located relatively far from the main boundaries of traditional plate tectonics.
Not necessarily. Although these tremors are less frequent, they can be just as destructive, particularly due to the limited preparedness of the affected populations and infrastructures that are less suited to seismic risks.
Yes, certain human activities such as intensive mining, underground fluid injection (hydraulic fracturing), or the creation of large water reservoirs (dams) can induce earthquakes by altering underground stresses.
There are no specific warning signs. However, a gradual increase in local small earthquakes may be an indication of rising local tectonic stress.
Inter-plate earthquakes typically occur at the boundaries between tectonic plates. They are common and well understood. In contrast, intra-plate earthquakes occur within a tectonic plate itself, far from the boundaries, and are often the result of accumulated internal stresses or reactivations of ancient faults.
Unfortunately, there is currently no precise way to predict exactly when earthquakes will occur, whether at plate boundaries or elsewhere. However, ongoing seismic monitoring can help identify areas at increased risk.
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