A magnitude-5.5 earthquake hit the south-eastern city of Pohang on 15 November, injuring at least 70 people, temporarily displacing hundreds, and causing millions of dollars of damage. A team of researchers from the University of Glasgow, ETH-Zurich in Switzerland and GFZ-Potsdam in Germany analysed data from seismographic stations as well as satellite data to locate where the main shock and 46 aftershocks of the quake occurred. They found that the main shock occurred within 1.5km of where plant workers had been pumping thousands of cubic metres of water into the ground, creating or opening fractures in the rocks to enable water to pass between boreholes. The goal was to harness energy from underground heat by injecting water into hot subsurface rocks and then extracting the heated water through a second borehole. Its proximity to an Enhanced Geothermal Systems site, where high pressure hydraulic injection had been performed during the previous two years, raises the possibility that this earthquake was anthropogenic. They also found that the main shock and aftershocks all occurred at depths of between 3 and 7km, which are shallow compared to previous quakes in that area, but similar to the depths at which the water was being injected. South Korean authorities are also exploring a possible link between the two. “It would be a very a remarkable coincidence if this earthquake were to be unrelated to the activity at the site, given that it occurred so close to it,” said Robert Westaway, a senior research fellow at Glasgow university’s school of engineering, and one of the paper’s co-authors. “My own personal view is that it is highly likely there is a connection.”

Westaway said further studies were being carried out to explain the two-month delay between the last round of fracking and the quake. The study may change the way researchers judge how fracking may impact the risk of earthquakes. Previous theories have focused more on the volume of water used, rather than the pressure at which it enters the ground. In the case of the Pohang tremor, workers injected relatively small amount of water – about 10,000 cubic metres (353,000 cubic feet). 'No one ever thought that injecting such a small amount of water could lead to such a large earthquake,' Dr Westaway said. Assessing whether the 2017Mw5.4 Pohang earthquake in South Korea was an induced event, Science (2018). DOI: 10.1126/science.aat2010

Abstract

The Mw 5.5 earthquake that struck South Korea in November 2017 was one of the largest and most damaging events in this country over the last century. The biggest quake occurred in September 2016, when a 5.8 magnitude occurred near the ancient city of Gyeongju, which is close to Pohang. South Korea has relatively little seismic activity, unlike neighboring Japan.
Scientific studies have offered evidence that a geothermal plant may have caused the earthquake in South Korea.