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A magnitude-5.4 earthquake that struck the South Korean city of Pohang on 15 November 2017 was probably triggered by an experimental geothermal power plant injecting water a few kilometres underground, a research team reports. A second independent analysis also finds the plant’s involvement to be plausible.

The pair of studies, published online on 26 April in Science1,2, heighten scrutiny of the potential role of the geothermal plant in the quake, which was South Korea’s second-strongest since observations began in 1978 and the most destructive ever recorded in the country. Eighty-two people were injured and more than 200 homes were seriously damaged.

Earthquakes of similar magnitude in Oklahoma have been linked to the injection of wastewater from hydraulic fracturing (fracking). But the Pohang quake is by far the strongest ever linked to a geothermal power plant — 1,000 times mightier than a magnitude-3.4 earthquake caused by a similar plant in Basel, Switzerland, in 2006.

The findings could shake up the global geothermal industry, the researchers say. “If the Pohang earthquake is really induced, it’s a kind of game-changer in the hydro-geothermal power plant industry,” say Jin-Han Ree, a structural geologist at Korea University in Seoul, and a lead author on one of the studies1.

Geothermal, enhanced

Most conventional geothermal power plants draw heat directly from hot water deep underground, or pump fluid through hot rocks to exchange heat between the ground and the facility. But this requires specific geological conditions.

Enhanced geothermal systems (EGS), such as the one being constructed in southeastern city of Pohang, enable heat extraction at less-ideal locations. In this technique, fluids injected at high pressure into a borehole a few kilometres deep cause the surrounding rock to crack and fissure, which allows the heat-extraction fluid to permeate the rock more easily. Some seismic activity is expected.

In response to foreshocks near the Pohang EGS site, Ree and his colleagues installed eight seismometers near the plant in early November 2017. They suggest that the main earthquake had a depth of just 4.5 kilometres, which was considerably shallower than most earthquakes in South Korea, but consistent with the 4.4-km depth of the plant’s wells.

Data on the geometry of the fault suggest that engineers drilled the plant’s injection well through — or very close to — the active seismic fault, injecting fluid directly into it, say the researchers.

Ree’s team also analysed archival data from a seismic station 10 km away from the plant, which revealed that no earthquakes were recorded at the site﻿ in the five years before drilling was completed. However, 150 microquakes and four quakes larger than magnitude 2.0 happened afterwards — with the vast majority of the seismic activity immediately following periods of fluid injection.

The team concludes that the geothermal plant “probably induced” or “almost certainly induced” the Pohang earthquake.

In the second study2, a separate team of researchers from Switzerland, the United Kingdom and Germany analysed mostly long-distance seismic data and satellite radar data to locate seismic activity on the day of the quake and for two weeks afterwards. The researchers also found a shallow depth for the main shock — 4-4.5 km — and determined that the active fault passes directly beneath the plant.

Francesco Grigoli, a seismologist at Swiss Federal Institute of Technology (ETH) in Zurich and one of the study co-authors, cautioned against drawing firm conclusions without further analysis. But, he said, the results “will certainly impact future projects”.

Unanswered questions

Some researchers are sceptical that the fluid injection triggered the quake. “I do have some doubts,” says geophysicist Ernie Majer of Lawrence Berkeley National Laboratory in California. “Maybe this area was very close to failure and it did not take much to set it off.”

Tae-Kyung Hong, a seismologist at Yonsei University in Seoul, is not convinced that the plant played a part in the quake. He says that the quake’s origin could have been slightly deeper than the latest studies suggest. His unpublished analysis of regional seismic data indicates the quake originated at a depth of 6.2 km, with aftershocks as deep as 10 km, which he argues is not an uncommon depth for earthquakes in South Korea.

The plant’s operator, NexGeo, denied any connection to the earthquake the day after the event, saying fluid injection had ceased nearly two months before the main shock.

Both research teams say that it can take up to months for injected fluids to settle and built up enough pressure to trigger a quake. “Induced earthquakes are still occurring in Basel, even though they shut down the geothermal power plant more than ten years ago,” says Ree.

Ten days after the Pohang quake, the government ordered the plant to suspend operations and opened an investigation into a possible link to the geothermal plant, which is still ongoing.

Some scientists say the latest studies raise questions about whether the Pohang EGS plant operators knew about the fault, or should have known. In 2005, researchers at the Korea Institute of Geoscience and Mineral Resources (KIGAM) in Daejeon — one of NexGeo’s partners on the Pohang plant — published a survey of the region using magnetotelluric soundings, which map the conductivity of rock underground using magnetic readings. It reported a major fault in the area.

But Yoonho Song, a geophysicist at KIGAM and one of the survey’s authors, says data from that kind of surface exploration could not infer whether the fault was active or not. “From any of our research activities, we have never suspected that there is any active fault in that area.” He declined to comment further, citing the pending government probe.

NexGeo did not respond to Nature’s request for comment before publication of this article.

Ole Kaven, a seismologist at the US Geological Survey in Menlo Park, California, says that future EGS projects should take lessons from the Pohang plant to help minimize the possible risks involved. “Every project brings with it the opportunity to learn about additional pitfalls of the process.”