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.

The story of the May 11, 2011 earthquake near Lorca, Spain is a different one. This magnitude 5.1 quake occurred at a shallower depth than usual (less than 4 kilometers), leading to surprisingly strong shaking at the surface that caused extensive damage in the city and nine deaths. A study published in Nature Geoscience shows this earthquake was probably related to another geologic phenomenon in the area—unsustainable use of groundwater.

Records show the water table around Lorca has dropped a remarkable 250 meters over the past 50 years. Just as in California’s San Joaquin Valley and the scenic city of Venice, depleting large volumes of groundwater actually causes the land surface to sink. This is because the sediments compact without the water pressure that helps hold spaces between grains open. As a result, the land surface around Lorca is dropping in elevation by as much as 16 centimeters per year.

But that’s only half of the story. The removal of all that water also represents a large removal of weight that was pressing down on the rocks below. And just as a massive ice sheet depresses the land surface—and when it melts away, the surface rebounds—the loss of water causes the rocks to pop up a bit.

To see how all this related to the May 11 earthquake, the researchers turned to satellite data and computer models. The satellites recorded subtle changes in elevation near the fault that occurred during the earthquake. Together with seismometers, this helps scientists work out exactly where the fault slipped, and by how much.

Armed with this information, they used computer models to calculate the effects of the groundwater depletion. The model showed where the depletion altered the stresses along the fault—in exactly the location where the earthquake occurred. That could be a coincidence, but the researchers offer good reasons to think it really is connected.

The removal of the mass of groundwater, and the resulting rebound in the crust, acts to reduce some of the force clamping the fault closed. Like the example of hydraulic fracturing, this lowers the amount of friction that needs to be overcome for an earthquake to occur.

The rebound also created some motion parallel to the fault that probably contributed. To picture this, imagine a pad of paper with one edge placed against a wall. If you make the pad bulge upwards in the middle, the left and right sides will slide along the wall as they are pulled toward the middle.

Critically, all this may also explain why the earthquake was so shallow—and therefore, damaging. This was exactly the depth where the effects of the groundwater depletion were greatest.

It’s not as though an earthquake wouldn’t happen in this location without the groundwater depletion. The stresses that build up and create the conditions where an earthquake can occur have very little to do with human activities. However, it appears that this earthquake occurred where and when it did because of the groundwater depletion.

And that’s what makes this story so important to seismologists. When evaluating the seismic risks in an area, it could be critical to understand how human activities are interacting with the natural processes at work. That’s not an easy thing to do, but it may be necessary. When it comes to preparing for earthquakes, the very last thing you want is a surprise.

Nature Geoscience, 2012. DOI: 10.1038/NGEO1610 (About DOIs).