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South Korea鈥檚 most damaging earthquake for a century may have been man-made. Two investigations both conclude that the quake was caused by injections of water deep underground, as part of a project to harness geothermal energy.

The findings also suggest that seismologists鈥� method for estimating how big an earthquake might be caused by pumping water underground is dangerously flawed.

Several dozen people were hurt and in Pohang by . It was the second most powerful earthquake in South Korea since 1978.

Now two independent studies have found that the quake and its main aftershocks were 2 kilometres or less from a site where water was being injected 4 kilometres underground. The goal was to extract energy from underground heat, by injecting water into deep, hot rocks then drawing the heated water up through a second borehole. During the entire project, which ended last September, engineers pumped down around 12,000 cubic metres of water.

The drilling operations probably caused the quake, both teams conclude based on seismic and satellite data.

鈥淚f the Pohang earthquake proves to be human-caused, it would be the largest known associated with deep geothermal energy, and this would certainly impact future projects,鈥� says team member of the Swiss Seismological Service.

The trouble is, according to our current understanding the water shouldn鈥檛 have caused such a big earthquake.

Since 2014, seismologists have routinely used a formula set out by of the US Geological Survey, which . The formula is applied to geothermal energy projects, and to hydraulic fracturing or 鈥渇racking鈥� to extract oil and gas.

But the formula drastically underestimated the risk to Pohang. 鈥淚t would have needed 1000 times more water to cause a 5.5-magnitude earthquake if the formula was correct,鈥� says team member of the University of Glasgow, UK.

In other words, McGarr鈥檚 formula seems to miss the risk of larger earthquakes. This implies that it 鈥渃an鈥檛 be used to place maximum limits on the size of earthquakes,鈥� says Westaway. 鈥淭hat鈥檚 potentially alarming.鈥�

Water gone awry

However, even magnitude-5 quakes caused by water injection are extremely rare. Westaway says that frackers in the US and Canada routinely inject up to 10,000 cubic metres of water in one go, yet this seldom causes quakes exceeding magnitude-2.

Depth may be a factor. Westaway says frackers only go down 2 or 3km, whereas the Korean geothermal project went down 4km. Deeper injections of water might be more destabilising.

鈥淚鈥檇 support the conclusion that quakes may be triggered by relatively small stress changes if the conditions are right, such as faults critically stressed and orientated favourably,鈥� says Brian Baptie of the . 鈥淏ut the probability of an event with the maximum magnitude would of course remain small.鈥�