A magnitude 5.5 earthquake shook the industrial city of Pohang in South Korea on 15 November 2017, injuring almost 100 people and damaging thousands of buildings at a cost of millions of US dollars. Six months on, two academic papers have suggested that fracking was probably the cause of this earthquake.
A local geothermal energy project had been injecting highly pressurised water into two, four kilometre-deep boreholes for almost two years. This process, known as hydraulic fracturing or fracking, creates or enhances fractures in rock to harness the heat stored there.
Once the network of fractures connected the two boreholes, the plan was to pump water into one, circulate it through the granite rock, absorbing heat, then extract it from the other borehole and use the heat to generate electricity. Afterwards, the cooled water would be reinjected to begin the process over again.
The proposed cause-and-effect connections now identified make the Pohang earthquake by far the largest recorded for which fracking is the likely cause. The previous record holder linked to geothermal development '“ of magnitude 3.4 '“ occurred a decade ago in the Swiss city of Basel.
Ten days after the Pohang earthquake, the South Korean government suspended the geothermal plant's operations and ordered an investigation into a possible link, which is still ongoing.
The area of Pohang where the 5.5 magnitude earthquake occurred on November 15, 2017.GoogleEarth, Author provided
Seismographs, satellites and faults
The first of the papers reports a collaboration between researchers from Zurich, Switzerland, Potsdam, Germany, and myself. We used public domain data from seismographs (instruments for recording ground motion caused by earthquakes) and remote-sensing satellites to determine the location and position of the geological fault that slipped in the earthquake.
Both types of data indicate the rupture of a fault running southwest to northeast and dipping steeply to the northwest. Recorded by satellites, the rock above this fault moved upward, lifting the Earth's surface by four centimetres. This analysis indicates that the fault slipped over a depth range of three to six kilometres, encompassing the depth of the injection and passing within hundreds of metres of the boreholes. This points strongly to a connection between the high-pressure fluid injection and the earthquake.
The second paper, by Korean colleagues, reports the locations of the many aftershocks of the Pohang earthquake, more accurately defining its fault plane. As shown in the cross-section below, their study indicates the fault passing between the bases of the two boreholes where water was injected, cutting across one borehole at a depth of around three to four kilometres. The results from the two papers are consistent with each other, despite the different types of data used, providing strong confirmation.
Preliminary surveys for the geothermal project over a decade ago identified a fault (its position at a depth of four kilometres is shown in orange on the image below) with essentially the same position and alignment as that which slipped in the November 2017 earthquake (its position, also at a depth of four kilometres, is shown in red for comparison). Taking into account the uncertainties in each of the analyses, this comparison indicates that these surveys revealed the fault that is now known to have slipped in the November 2017 earthquake.
Further surveys taken before drilling started led the developers to revise their plans to focus on a WNW-ESE fracture in the granite beneath the site. The boreholes, shown in the cross-section above, were designed for flow in this direction.
During drilling, fluid (or drilling 'mud') leaked from one borehole at roughly the depth where the fault cuts across the well bore, with fault gouge '“ crushed rock debris produced by the movement of the rocks on either side of a fault '“ indicating a fault there before any injection had taken place.
The geothermal project site, north of Pohang, marked in green.Google Earth
Testing the cause-and-effect connection
More than 20 years ago, a set of criteria was devised for assessing whether earthquakes are caused by high pressure fluid injection. The Pohang earthquake meets most of these criteria. Notably, it occurred within hundreds of metres of the injection, and at the same depth. Also, during much of the injection, the pressure was high enough for standard calculations to predict the slipping of the fault, if water at this pressure reached this fault.
However, the two-month interval between the end of injection in September 2017 and the earthquake in November provides a potential argument against any cause-and-effect connection. A possible explanation for this delay is that once water entered the fault it began to dissolve the granite, gradually weakening the fault so it eventually failed and slipped. It seems entirely plausible at this stage to believe that the earthquake was caused by the injection, and to examine the implications.
One reason this seismicity is significant is the disproportion between the size of the main shock and the volume of water injected. A theory has been developed for determining the 'worst case scenario' earthquake feasible for a given volume of fluid injection. The overall volume injected at Pohang was roughly 12,000 cubic metres, whereas this theory requires around 1,000 times more volume to cause an earthquake as large as magnitude 5.5. This suggests that the theory needs improving, possibly to incorporate the injection pressure as well as the injected volume.
The drilling rig at the Pohang geothermal project site. Work here is now suspended.Rob Westaway, Author provided
In the meantime, designs for future geothermal fracking projects might require independent expert assessment, as is already the case for projects involving fracking for shale gas in the UK. At Pohang, this could have highlighted the potential implications of a fault cutting across the site, leading to recommendations such as limiting the injection pressure, which could have lessened the force of the earthquake.
Elsewhere in the world, successful deep geothermal projects have been designed to incorporate circulation of water along faults, requiring high pressure injection to create or enhance fractures. Pohang illustrates the need for accurate information on the geometry of faults on which project designs can be based.
Still, the disproportion between the small volume of water injected at Pohang and the size of the November 2017 earthquake may give geothermal fracking developers worldwide pause for thought. It may well be the game changer for their industry.
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