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> 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. </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> SEOUL, South Korea (AP) — South Korea’s weather agency said a magnitude 3.2 earthquake was detected in North Korea on Saturday close to where the country recently conducted a nuclear test, but it assessed the quake as natural.

The quake was detected in an area around Kilju, in northeastern North Korea, just 6 kilometers (3.7 miles) northwest of where the North conducted its sixth and most powerful nuclear test on Sept. 3, according to an official from Seoul’s Korea Meteorological Administration.

The area isn’t where natural earthquakes normally occur. A South Korean expert said the quake could have been caused by geological stress created from the recent nuclear explosion. Other possible causes include landslides or the collapsing of test structures such as tunnels, said Hong Tae-kyung, a professor at the department of Earth System Sciences at Yonsei University.

“It could be a natural earthquake that really was man-made as the nuclear test would have transferred a lot of stress,” he said. “The quake is small enough to suspect that it could have been caused by a tunnel collapse, and satellite data shows there have been many landslides in the area since the nuclear test.”

China’s official Xinhua News Agency said the country’s seismic service detected a magnitude 3.4 quake in North Korea and saw the likely cause as an explosion. But the South Korean weather agency official said the analysis of seismic waves and the lack of sound waves clearly showed that the quake wasn’t caused by an artificial explosion. She spoke on condition of anonymity, citing office rules.

Another Korea Meteorological Administration official, who also didn’t want to be named, said the agency saw the possibility of the earthquake being caused by a tunnel collapse as low.

The U.S. Geological Survey said that it detected a magnitude 3.5 quake in the area of previous North Korean nuclear tests, but that it was unable to confirm whether the event was natural.

North Korea’s weakest nuclear test, its first one, conducted in 2006, generated a magnitude 4.3 quake. The USGS measured this month’s nuclear test at magnitude 6.3. The latest test was followed by a second magnitude 4.1 quake that experts said could have been caused by landslides or a tunnel collapsing after the explosion.

Analysts examining satellite images of North Korea’s mountainous test site after the latest nuclear test said they spotted landslides and surface disturbances that were more numerous and widespread than what was seen from any of the five previous tests.

North Korea has been maintaining a torrid pace in nuclear and weapons tests as it accelerates its pursuit of nuclear weapons that could viably target the United States and its allies in Asia.

North Korea said its recent nuclear test was a detonation of a thermonuclear weapon built for its developmental intercontinental ballistic missiles. In two July flight tests, those missiles showed potential capability to reach deep into the U.S. mainland when perfected.

WHAT OTHERS ARE CLICKING ON:
• MAN HITS WOMAN IN THE FACE ON BART IN THE EAST BAY </s> China's earthquake administration said on Saturday it had detected a magnitude 3.4 earthquake in North Korea that was a "suspected explosion", raising fears the isolated state had conducted another nuclear bomb test weeks after its last one.

BEIJING/SEOUL: A small earthquake near North Korea's nuclear test site on Saturday was probably not manmade, the nuclear proliferation watchdog and a South Korean official said, easing fears Pyongyang had exploded another nuclear bomb just weeks after its last one.

Chinese earthquake officials said the magnitude 3.4 quake detected at 0829 GMT was a "suspected explosion" but both the CTBTO, which monitors nuclear tests, and a South Korean meteorological agency official said they believed it was a natural quake.

"A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake," said the South Korean official, who asked for anonymity. "In this case we saw none. So as of now, we are categorising this as a natural earthquake."

The earthquake, which South Korea put at magnitude 3.0, was detected in Kilju county in North Hamgyong Province, where North Korea's known Punggyeri nuclear site is located, the official said.

All of North Korea's previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on Sept 3 registered as a 6.3 magnitude quake.

A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the Sept 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was an advanced hydrogen bomb.

The head of the nuclear test monitoring agency CTBTO said on Saturday that analysts were "looking at unusual seismic activity of a much smaller magnitude" than the Sept 3 test in North Korea.

"Two #Seismic Events! 0829UTC & much smaller @ 0443UTC unlikely Man-made! Similar to "collapse" event 8.5 mins after DPRK6! Analysis ongoing," CTBTO Executive Secretary Lassina Zerbo said in a Twitter post, referring to Sept 3 test.

Russia’s emergency ministry says background radiation in nearby Vladivostok was within the natural range.

The U.S. Geological Survey said it could not conclusively confirm whether the quake, which it measured at magnitude 3.5, was manmade or natural.

"The depth is poorly constrained and has been held to 5 km by the seismologist," USGS said. "The Air Force Technical Applications Center (AFTAC) is the sole organisation in the U.S. federal government whose mission is to detect and report technical data from foreign nuclear explosions."

There was no immediate reaction from China's Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.

Tensions have continued to rise around the Korean peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of U.N. sanctions.

North Korea's Foreign Minister Ri Yong Ho, currently in New York for a United Nations meeting, warned on Thursday that Kim could consider a hydrogen bomb test of an unprecedented scale over the Pacific.

Ri is due to address the United Nations later on Saturday.

U.S. President Donald Trump called the North Korean leader a "madman" on Friday, a day after Kim dubbed him a "mentally deranged U.S. dotard" who would face the "highest level of hard-line countermeasure in history".

North Korea's nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons programme.

North Korea has launched dozens of missiles this year, several of them flying over Japan, as it accelerates a weapons programme aimed at enabling it to target the United States with a nuclear-tipped missile.

While China has been angered by North Korea's repeated nuclear and missile tests and has signed up for the increasingly tough U.N. sanctions, it has also stressed the need to resume dialogue and for all sides to take steps to reduce tensions.

In a series of meetings this week at the United Nations, Chinese Foreign Minister Wang Yi has reiterated to various foreign counterparts that apart from sanctions, the resolutions also call for dialogue to resume and that this needs to happen.

Earlier on Saturday, China said it will limit exports of refined petroleum products from Oct. 1 and ban exports of condensates and liquefied natural gas immediately to comply with the latest U.N. sanctions. It will also ban imports of textiles from North Korea.

The United States and South Korea are technically still at war with North Korea because the 1950-53 Korean conflict ended with a truce and not a peace treaty.

The North accuses the United States, which has 28,500 troops in South Korea, of planning to invade and regularly threatens to destroy it and its Asian allies. </s> China says North Korean quake ‘suspected explosion’, South Korea says likely natural

A meeting of DPRK of the central committee is held as they vow a sacred war against the US during an anti-US rally, in this undated photo released by North Korea’s Korean Central News Agency (KCNA) in Pyongyang September 22, 2017. — Reuters picBEIJING, Sept 23 — China’s earthquake administration said today it had detected a magnitude 3.4 earthquake in North Korea that was a “suspected explosion”, raising fears the isolated state had conducted another nuclear bomb test weeks after its last one.

An official at South Korea’s meteorological agency said they were analysing the tremor, which they put at magnitude 3.0, but the initial view was that it was a natural quake.

“We use several methods to tell whether earthquakes are natural or manmade,” said the official, who asked for anonymity. “A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake. In this case we saw none. So as of now we are categorising this as a natural earthquake.”

The earthquake was detected in Kilju county in North Hamgyong Province, where North Korea’s known Punggyeri nuclear site is located, the official said.

The Chinese administration said in a statement on its website that the quake, which occurred around 0830 GMT, was recorded a depth of zero kilometres.

All of North Korea’s previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on September 3 registered as a 6.3 magnitude quake.

A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the September 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was a hydrogen bomb.

The head of the nuclear test monitoring agency CTBTO said today that analysts were “looking at unusual seismic activity of a much smaller magnitude” in North Korea, and that it would have more details to come.

There was no immediate reaction from China’s Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.

Tensions have continued to rise around the Korean peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of UN sanctions.

North Korea’s Foreign Minister Ri Yong Ho, currently in New York for a United Nations meeting, warned on Thursday that Kim could consider a hydrogen bomb test of an unprecedented scale over the Pacific.

US President Donald Trump called the North Korean leader a “madman” on Friday, a day after Kim dubbed him a “mentally deranged U.S. dotard” who would face the “highest level of hard-line countermeasure in history”.

North Korea’s nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons programme. — Reuters </s> China’s earthquake administration said on Saturday it had detected a magnitude 3.4 earthquake in North Korea that was a “suspected explosion”, raising fears the isolated state had conducted another nuclear bomb test weeks after its last one.

An official at South Korea’s meteorological agency said they were analyzing the tremor, which they put at magnitude 3.0, but the initial view was that it was a natural quake.

“We use several methods to tell whether earthquakes are natural or manmade,” said the official, who asked for anonymity. “A key method is to look at the seismic waves or seismic acoustic waves and the latter can be detected in the case of a manmade earthquake. In this case we saw none. So as of now we are categorising this as a natural earthquake.”

The earthquake was detected in Kilju county in North Hamgyong Province, where North Korea’s known Punggyeri nuclear site is located, the official said.

The Chinese administration said in a statement on its website that the quake, which occurred around 0830 GMT, was recorded a depth of zero kilometres.

All of North Korea’s previous six nuclear tests registered as earthquakes of magnitude 4.3 or above. The last test on Sept 3 registered as a 6.3 magnitude quake.

A secondary tremor detected after that test could have been caused by the collapse of a tunnel at the mountainous site, experts said at the time. Satellite photos of the area after the Sept 3 quake showed numerous landslides apparently caused by the massive blast, which North Korea said was a hydrogen bomb.

The head of the nuclear test monitoring agency CTBTO said on Saturday that analysts were “looking at unusual seismic activity of a much smaller magnitude” in North Korea, and that it would have more details to come.

There was no immediate reaction from China’s Foreign Ministry, but the news was widely reported by Chinese state media outlets and on social media.

Tensions have continued to rise around the Korean peninsula since Pyongyang carried out its sixth nuclear test, prompting a new round of U.N. sanctions.

North Korea’s Foreign Minister Ri Yong Ho, currently in New York for a United Nations meeting, warned on Thursday that Kim could consider a hydrogen bomb test of an unprecedented scale over the Pacific.

U.S. President Donald Trump called the North Korean leader a “madman” on Friday, a day after Kim dubbed him a “mentally deranged U.S. dotard” who would face the “highest level of hard-line countermeasure in history”.

North Korea’s nuclear tests to date have all been underground, and experts say an atmospheric test, which would be the first since one by China in 1980, would be proof of the success of its weapons programme. </s> BEIJING — Research by Chinese geologists suggests that the mountain above North Korea's main nuclear test site has likely collapsed, rendering it unsafe for further testing and requiring that it be monitored for any leaking radiation.

The findings by the scientists at the University of Science and Technology of China may shed new light on North Korean President Kim Jong Un's announcement that his country was ceasing its testing program ahead of planned summit meetings with South Korean President Moon Jae-in and President Donald Trump.

The results also support some of the findings of an earlier study by another group of Chinese researchers that was published last month by the journal Geophysical Research Letters.

Nuclear explosions release enormous amounts of heat and energy, and the North's largest test in September was believed early on to have rendered the site in northeastern North Korea unstable.

Chinese authorities have said they've detected no radiation risk from samples collected along the border. Calls to those departments were not immediately answered on Thursday.

The data in the latest Chinese study was collected following the most powerful of North Korea's six nuclear device tests on Sept. 3, which is believed to have triggered four earthquakes over the following weeks. The yield of the bomb was estimated at more than 100 kilotons of TNT, at least 10 times stronger than anything the North had tested previously. (The bomb the United States dropped on Hiroshima in 1945 had a yield of about 15 kilotons.)

The University of Science and Technology of China paper, authored by Tian Dongdong, Yao Jiawen and Wen Lianxing, said the first of those earthquakes, which occurred 8 ½ minutes after the explosion, was "an onsite collapse toward the nuclear test center," while those that followed were an "earthquake swarm" in similar locations.

"In view of the research finding that the North Korea nuclear test site at Mantapsan has collapsed, it is necessary to continue to monitor any leakage of radioactive materials that may have been caused by the collapse," the authors said in a summary dated Monday and viewed Wednesday on the university's website.

The study is peer-reviewed and has been accepted for publication by the journal Geophysical Research Letters.

The Chinese study makes sense and is based on well understood research, said Rowena Lohman, a seismologist at Cornell University who wasn't part of the work. She said she believes there's an international effort that monitors these tests for radiation.

A study published last month by the journal, authored by a team led by Liu Junqing at the earthquake bureau in Jilin province along the border with North Korea, found similar results of the Sept. 3 explosion. It described the aftershock that followed seconds later as most likely a "rapid destruction of an explosion-generated cracked rock chimney due to cavity collapse."

North Korea's nuclear tests are of special concern to Beijing, since the test site near the town of Kilju is less than 60 miles from the border with China.

North Korean nuclear tests have caused seismic events in Chinese border towns and cities, forcing evacuations of schools and offices, sparking fears of wind-born radiation and leading to a backlash among some Chinese against their country's unpredictable traditional ally.

Ties between the sides have been deteriorating for years, although Kim made a long-anticipated visit to Beijing last month after China's implementation of United Nations economic sanctions reduced trade between them by as much as 90 percent.

The quakes that followed the September test were not man-made and didn't appear to cause any damage in the area. The region isn't one where earthquakes naturally occur and no quakes were detected after the five smaller nuclear tests North Korea has conducted since 2006.

Kune Yull Suh, a professor of nuclear engineering at Seoul National University, warned last year that further tests could threaten to cause a volcanic eruption at Mount Paektu, which is about 60 miles away.

On Saturday, North Korea announced it will close its nuclear testing facility and suspend nuclear and intercontinental ballistic missile tests — a move welcomed by Trump as "big progress" — and which comes ahead of the planned summit between him and Kim.

However, the North stopped short of suggesting it will give up its nuclear weapons or scale back its production of missiles and their related components. </s> An earthquake measuring 5.6 on the Richter scale was detected around North Korea’s nuclear test site on Sunday, the Seoul military said indicating that Pyongyang has conducted its sixth nuclear test.

The artificial quake wave was detected around the Punggye-ri site in the North Hamgyeong province, according to the Joint Chiefs of Staff.

The military said it was analysing whether the temblor came from a nuke test, reports Yonhap News Agency.

Seoul’s weather agency said that the seismic wave was recorded at 12.36 p.m.

The USGS said the epicentre was recorded 24 km east-northeast of Sungjibaegam and zero km deep.

South Korean President Moon Jae-in has convened an emergency National Security Council (NSC) meeting following the earthquake and a report by North Korean state-media that Kim Jong-un inspected a hydrogen bomb being loaded into a new intercontinental ballistic missile (ICBM) after it claimed to have made a “more developed nuke”.

Since 2006, North Korea has carried out five nuclear tests, including two last year.
Scientific studies have offered evidence that a geothermal plant may have caused the earthquake in South Korea.