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by Mark Zastrow
26 April
2018
from
Nature Website
Credit: YONHAP/EPA-EFE/REX/Shutterstock
Researchers say the 2017
disaster
is a potential game-changer
for the geothermal energy
industry.
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 kilometers 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 Science,1,2
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 studies.1
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 kilometers 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, Jin-Han 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 kilometers, 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 analyzed 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 micro-quakes
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 study,2 a separate team of researchers from
Switzerland, the United Kingdom and Germany analyzed 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 skeptical 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."
References
-
Kim, K.-H. et al.
Science -
Assessing whether the 2017 Mw 5.4
Pohang earthquake in South Korea was an induced event
- (2018).
-
Grogoli, F. et
al. Science -
The November 2017 Mw 5.5 Pohang
earthquake: A possible case of induced seismicity in South
Korea - (2018).
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