Separate studies offer evidence of geothermal plant causing Pohang earthquake

April 27, 2018

by Bob Yirka , Phys.org

Pohang, South Korea, Geothermal Power Plant with surface deformation and seismogram recorded after the November 2017 Mw 5.5 earthquake. Credit: Rob Westaway

Two teams working independently have found evidence implicating a geothermal plant in South Korea as a cause of a major earthquake in the city of Pohang last year. One of the teams, made up of researchers from several universities in Korea, used data from onsite seismic monitors to reach their conclusions. The other team was made up of researchers from across Europe and used satellite data to make their assessments. Both teams have published their findings in the journal Science.

Last year, on November 15, a 5.5-magnitude earthquake struck the South Korean city of Pohang, causing structural damage and numerous injurious to citizens. The quake was a surprise to much of the world because the area was not sitting on a known fault line and no earthquake had ever been recorded there. It was not a surprise to the Korean team of researchers, however, who had already observed numerous smaller earthquakes in the area that they suspected were caused by a geothermal plant—the size of the quake, on the other hand, was a major surprise. Because of the smaller quakes, the researchers had installed an array of seismic monitors around the plant. Just five days later, the big quake came. Data from the sensors indicated the earthquake was centered directly below the geothermal plant, strongly suggesting its activities were involved.

Geothermal plants such as the one proposed in Pohang (it has since been shut down) involve drilling two holes into the ground and then forcing water down them, causing the ground between them to crack. Once it does, water can be pumped down one hole and extracted from the other hole. The water is heated as it passes through the cracks underground. In most cases, such plants work without problem, but in this case, it appears the water served as a lubricant for an unknown fault.

Yangsan fault and location of the geothermal project near Pohang, South Korea. Credit: ETH Zurich

The second team used data from regional seismic monitors and also from satellites to study ground movement. They noted the earlier, smaller earthquakes and the larger one were all centered around the geothermal plant—they also noted that all of the quakes were unusually shallow. Taken together, the evidence suggests that the geothermal plant activities were the cause of the earthquakes.

More information: 1. F. Grigoli et al. The November 2017Mw5.5 Pohang earthquake: A possible case of induced seismicity in South Korea, 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. 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. We have combined seismological and geodetic analyses to characterize the mainshock and its largest aftershocks, constrain the geometry of this seismic sequence and shed light on its casual factors. According to our analysis it seems plausible that the occurrence of this earthquake was influenced by these industrial activities. Finally we found that the earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area.

2. Kwang-Hee Kim et al. Assessing whether the 2017Mw5.4 Pohang earthquake in South Korea was an induced event, Science (2018). DOI: 10.1126/science.aat6081

Abstract

The Mw 5.4 Pohang earthquake, the most damaging event in South Korea since instrumental seismic observation began in 1905, occurred beneath the Pohang geothermal power plant in 2017. Geological and geophysical data suggest that the Pohang earthquake was induced by fluid from an enhanced geothermal system (EGS) site, which was injected directly into a near-critically-stressed subsurface fault zone. The magnitude of the mainshock makes it the largest known induced earthquake at an EGS site.

Press release

Journal information: Science

© 2018 Phys.org</s>One of South Korea’s largest earthquakes on record may have been caused by hydraulic fracturing – or fracking – according to a study published on Friday in the journal Science.



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. In the aftermath, residents and researchers have questioned whether the quake could be connected to a geothermal plant – the country’s first – less than 2km (about 1 mile) away.

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 last of those injections was about two months before the quake.



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.

The researchers have been careful to say only that a connection is plausible. Korean authorities are also conducting their own investigation.

“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.

Earthquakes caused by hydraulic fracturing have prompted concerns around the world. Two earthquakes in 2011 led to the UK imposing a fracking moratorium. In Oklahoma, meanwhile, experts have reported a millennium’s worth of quakes in two years because of fracking there.

For decades, the Korean peninsula has seen almost no seismic activity. On 12 September 2016, another magnitude-5 quake struck just south of Pohang in Gyeongju. Since then the country has seen hundreds of smaller earthquakes.

The study’s conclusions could also change how fracking is studied in relation to 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 quake, a relatively small amount of water – about 10,000 cubic metres – was injected.

“No one ever thought that injecting such a small amount of water could lead to such a large earthquake,” Westaway said.</s>Humans may be to blame for the second most damaging earthquake in South Korea's history.

The quake hit the south-eastern city of Pohang on 15 November, injuring nearly 100 people, temporarily leaving 1,500 homeless, and causing millions in damage.

Two investigations have now concluded the 5.5-magnitude tremor was caused by an experimental fracking plant injecting water deep underground.

The 'alarming' results could change the way we assess the risks of fracking.

A team of researchers from the University of Glasgow, ETH-Zurich in Switzerland and GFZ-Potsdam in Germany analysed data from seismographic stations.

They also studied satellite data to pinpoint where the main shock and 46 aftershocks of the quake occurred.

The main shock hit within 0.9 miles (1.5km) of where workers at a geothermal plant had been injecting thousands of cubic metres of water into the ground.

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.

The last of these injections was around two months before the earthquake.

Researchers also discovered the main shock and aftershocks hit at unusual depths of between 1.9 and 4.3 miles (3-7 km) beneath the Earth's surface.

The tremors were surprisingly shallow compared to precious tremors in the region but hit at a similar depth to the geothermal plant's fracking injections.

The European researchers said their finding does not prove the fracking caused the quake, but does outline a connection that merits further investigation.

South Korean authorities are also exploring a possible link between the two.

Study coauthor Dr Robert Westaway, a research at Glasgow University, told the Gaurdian: '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.'

'My own personal view is that it is highly likely there is a connection.'

The study may change the way researchers judge how fracking may impact the risk of earthquakes.

Previously, risk assesments have focussed on the volume of water used rather than the pressure it is pumped at.

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.

The 5.4 magnitude earthquake was South Korea's second-strongest in decades.

More than 1,000 houses and dozens of other buildings and cars were damaged or destroyed, and cracks and other damage were found in military facilities, bridges, port facilities and water supply facilities.

Media images showed crumbled walls piled on damaged cars, broken windows and cracks in buildings.

It was the second-strongest quake in South Korea since the country officially began monitoring them in 1978.

The biggest quake occurred in September 2016, when a 5.8 magnitude occurred near the ancient city of Gyeongju, which is close to Pohang.

That quake also caused injuries but no deaths.

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.