St. James Church in Lorca, Spain after an earthquake on May 11, 2011.
One of the things people often wonder about earthquakes is whether human activity can play a role in their occurrence. Sometimes that comes from a desire to assign blame, but often it’s related to a bigger question: could we actively trigger small earthquakes to prevent the big, damaging ones from occurring? While that lofty piece of geoengineering may not be feasible (or even possible), it is true that humans can sometimes trigger earthquakes.
Earthquakes are fundamentally controlled by two factors. The first is the movement of rock, such as tectonic plates. This constant, gradual movement is the source of conflict in an active fault zone—one region of rock is being forced past another. If the two blocks simply slid smoothly by each other along the fault surface, this would be a pretty peaceful process. But this is where the second factor comes along—the friction between the blocks. The stress builds up until it’s great enough to overcome that friction, at which point seismic energy is released violently as the blocks catch up on decades' (or centuries') worth of motion in just a few seconds.
For the most part, the accumulating stress that creates this situation is much too large for human activities to make a difference. We can, however, affect the friction that locks up the fault. Hydraulic fracturing, where fluids are pumped into the ground at extremely high pressures to crack rocks that release natural gas and oil, has been shown to do just that in certain situations. Increasing the fluid pressure inside the fault partially de-stabilizes the friction-lock, lowering the stress threshold necessary to trigger an earthquake just enough for one to occur.
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