The 5.4 magnitude earthquake that hit the South Korean city of Pohang on November 15, 2017 – one of the strongest to strike the region since instrumental monitoring of seismic activity began there in 1903 – may have been triggered by activities at a nearby geothermal power generation site, according to new studies.
If further investigation bears out these findings, it would be “the largest and most damaging earthquake ever to have been associated with enhanced geothermal systems, making it a potential ‘game changer’ for the geothermal industry worldwide”, says a report by lead author Francesco Grigoli, from the Swiss Federal Institute of Technology, in Zurich.
Although there were no fatalities, the Pohang quake injured dozens of people and left about 1500 homeless in the seaport and industrial centre, which is about 270 kilometres south of the South Korean capital, Seoul.{%recommended 1320%}
The key point of interest in the Grigoli report, published in the journal Science, is the linking of the earthquake to a site containing an Enhanced Geothermal System (EGS) plant. EGS produces steam-generated electricity, by pumping large amounts of cold water under high pressure deep underground, creating new fractures and enhancing existing ones. The process is similar to that of hydraulic fracturing, or fracking.
The report says the Korean peninsula is generally considered stable, “with low to moderate intraplate seismic activity”. But it notes that historically there has been a wide variance of earthquakes in number and magnitude. Activity was much higher between the fifteenth and eighteenth centuries.
Given this, the report says, the increase represented by recent earthquakes “is not completely inconsistent with the historically varying rates of seismicity. This leaves open the possibility that the occurrence of earthquakes close to the EGS site is a coincidence.”
But it concludes that “it is plausible that the occurrence of the Pohang earthquake was influenced by the nearby stimulation activities”.
The report says natural and induced earthquakes are indistinguishable using their waveform characteristics: “Distinguishing induced earthquakes typically relies on building a convincing chain of evidence.”
It says the hypothesis that the Pohang earthquake sequence was caused by human activity is supported by correlations between the main shock, its aftershocks, and the EGS injections.
Although induced earthquakes are common in regions involved in oil and gas extraction, geothermal activity has not previously been implicated in inducing quake activity stronger than magnitude 3.4.
In a second investigation of the Pohang quake, South Korean geologist Kwang-Hee Kim and colleagues created a local earthquake catalogue that allowed them to analyse relationships between EGS injections and seismic activity.
Combined with analysis of data on foreshocks and aftershocks surrounding the Pohang event, they suggest the quake was almost certainly induced by fluid injected directly into a critically stressed subsurface fault zone.
Based on the fluid volume, the researchers add, injected fluid volumes much smaller than predicted by theory in some circumstances can trigger a relatively large earthquake.
The Grigoli report further concludes that the Pohang earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area more broadly.
