The Mw 5.4 Pohang earthquake that occurred near a geothermal site in South Korea last year was likely triggered by fluid injection at the geothermal plant, two separate reports conclude. While activity from Enhanced Geothermal System (EGS) sites – which inject large amounts of water underground – has been associated with much smaller earthquake events, the magnitude of the 2017 temblor would make it the largest-known earthquake induced at an EGS site. The Pohang earthquake was the most damaging quake in South Korea since the first seismograph was installed in 1905. After its eruption in November 2017, researchers began to assess whether the event was induced by activity at a nearby EGS location, where many thousands of cubic meters of water had been injected at high pressure starting in 2016. Though induced earthquakes have been well-established in places like Oklahoma, such regions involve oil and gas extraction, not geothermal activity; the latter has not been suspected of inducing quake activity much above Mw 3.4. Here, to illuminate the relationships between injection at the South Korean EGS site and the Pohang quake, Kwang-Hee Kim and colleagues created a local earthquake catalog that allowed them to analyze these relationships closely. Combined with analysis of data on foreshocks and aftershocks surrounding this event, they suggest the quake was probably or almost certainly induced, and by fluid injected directly into a critically stressed subsurface fault zone. Based on the fluid volume, the authors add, injected fluid volumes much smaller than predicted by theory in some circumstances can trigger a relatively large earthquake. In a second paper investigating the likelihood of this quake having been induced, Francesco Grigoli et al. also conclude it probably was, though based on a different data set that included satellite-based information and a range of seismological observations. The 2017 earthquake transferred static stress to larger nearby faults, they add, potentially increasing the seismic hazard in the area more broadly. If the 2017 event was truly induced, as these two studies indicate, the Pohang earthquake would be the largest quake induced by geothermal energy exploitation in history.
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