2017 Pohang earthquake triggered by human activity

Water pumped into geothermal wells cause microquakes along same fault as Pohang quakeA conclusion has been reached in the debate about whether the 5.4-magnitude earthquake that struck the South Korean city of Pohang in 2017 was a natural phenomenon or man-made through geothermal power generation. The answer came in a report about the correlation between the Pohang earthquake and geothermal power generation released by a government research team on Mar. 20. “The process of excavating geothermal wells and pumping water in and out during research for geothermal power generation in Pohang triggered microearthquakes along the fault. As time passed, this ultimately led to the Pohang earthquake,” the researchers concluded.
The Pohang earthquake on Nov. 15, 2017, took a human toll, leaving one dead and 117 injured, and also caused 85 billion won (US$75.58 million) in property damage, with some 2,700 buildings affected. After some suggested that an experimental geothermal research facility in Pohang’s Heunghae Township might have been the cause of the earthquake, the government put together a team of foreign and domestic experts in March 2018. The team has been investigating and analyzing the Pohang earthquake since then.

Relation between induced microearthquake and triggered microearthquake (space)

The decisive evidence that led the government research team to conclude that geothermal generation activity was the cause of the Pohang earthquake was the fact that the plane of the microearthquakes caused by the geothermal activity coincided with the fault plane along which scientists believe the Pohang earthquake was triggered. “What the team of researchers put the most effort into was determining the location of the seismic focus. If the Pohang earthquake were more than a few kilometers away from the geothermal wells used in power generation, our conclusion might have been different,” said Lee Gang-geun, a professor at Seoul National University and the head of the research team. The research team conducted precious analysis of the location of the 98 earthquakes (among a total of 520 that occurred around Pohang after Jan. 1, 2009) whose epicenter was within 5km of the geothermal generation facility and whose seismic focus was no deeper than 10km underground. Through this analysis, the researchers showed that the microearthquakes triggered when water was pumped into the second geothermal well (PX-2) exhibited a distribution approximating a plane, which was nearly identical to the strike and slope of the fault plane solution for the main earthquake at Pohang. Beginning in December 2010, the Pohang geothermal generation project drilled two geothermal wells and pumped 12,800 cubic meters of water into the wells and 7,000 cubic meters out of them on five occasions between January 2016 and Sept. 28, 2017, as part of repair work. Dozens of microearthquakes occurred during this period, the largest of which was a 3.2-magnitude earthquake on Apr. 15, 2017, which struck after water was pumped into the wells for the third time. Pohang earthquake was “triggered” earthquake that exceeded bounds of “induced” quake Shemin Ge, a professor at the University of Colorado and the head of the research team’s foreign contingent, gave the following explanation about why the Pohang earthquake was described as having been triggered, and not induced: “We defined an induced earthquake as one that occurs within the spatial bounds of the rock because of changes in pressure and stress and a triggered earthquake as one whose original cause is man-made activity, but whose magnitude greatly exceeds the spatial bounds affected by that activity.” The professor said that the term “triggered” had been used to distinguish the Pohang earthquake from a natural earthquake, though it had exceeded the bounds of an induced earthquake. The research team said the evidence that the earthquake had been triggered by man-made activity had been found even after its occurrence. While the first geothermal well (PX-1) allows photos to be taken as far as 4,100m underground, the second geothermal well (PX-2) is blocked around 3,800m. That’s consistent with the depth of the fault plane of the Pohang earthquake, if it were extended outward. The implication is that the exploratory well was ruptured by the earthquake. A sudden drop in the water level in the well and the change in the chemical composition of the groundwater there also supports the conclusion that the Pohang earthquake was triggered, the research team said. “In our calculations of the stress that the 2011 Japan earthquake and the 2016 Gyeongju earthquake would have exerted on the fault in the Pohang earthquake, we found that the stress accumulated thusly would not have been sufficient to move the fault,” said Lee Gang-geun. “Since this has demonstrated the danger of altering a fault that’s already in a state of critical stress, we need a different approach to our method of risk management for geothermal power generation.” By Lee Keun-young, senior staff writer Please direct comments or questions to [english@hani.co.kr]