A recently published paper in Science, written with contributions from DESTRESS partners, explores the potential link between magnitude 5.5 (Mw) earthquake in South Korea and the nearby geothermal project. If proven to be anthropogenic, it would be the largest earthquake known to have been associated with the exploitation of deep geothermal energy.

Seismograms of induced earthquakes are generally indistinguishable from those of natural earthquakes. Therefore, investigations focus on a range of indicators, taking into account the location of earthquakes, their depths, and the history of stimulation activities conducted underground. By analyzing publically available continuous waveform and geodetic data from satellites, the paper contributes to the understanding of the nature of the event.

The study shows that the mainshock and its largest aftershocks occurred within 2 km distance or less of the geothermal site. Furthermore, they are located within 1.5 km distance of an event, which occurred in April 2017 during one of the underground stimulation operations. The locations are confirmed by a Korean study, which has been simultaneously published in Science. In general, the closer an earthquake sequence is located to a geothermal site and to previous related seismic activity, as well as the sooner it happens after underground operations, the greater the chance of being related to it. The mainshock and the 46 aftershocks detected between 15 and 30 November 2017 all occurred at depths of 3 to 7 kilometres. Such depths are unusually shallow when compared to previously registered natural events in the area. The analysis of satellite data illustrates that the mainshock displaced the surface of the earth permanently by up to 4 cm, indicating that the activated, and previously unknown fault, is a steeply dipping and very shallow thrust fault that passes directly beneath the bottom hole section.

These indications combined lead to the conclusion that a connection between the magnitude 5.5 earthquake in South Korea and the nearby geothermal project is plausible. However, the mainshock occurred about two months after the last stimulation activities. So far, there is no quantitative model available that relates the injection activities conducted to the occurrence of this event. The Korean government has appointed an independent expert commission to examine all pieces of evidence and to evaluate if the event was triggered or induced by the nearby stimulation activities. To our understanding, the commission will (re-)analyze all available data and models, including microseismicity data, injection volumes, reservoir pressures, and detailed hydrological and geological data, which are essential to understand the relation between the injection operations and the earthquake sequence.

DESTRESS emphasizes the tentativeness of the results and therefore will continue to support analyses respective to the seismic events near Pohang. In contrast to the Science paper, DESTRESS differentiates between induced and triggered events. Triggered earthquakes are also defined as anthrophonic, but release tensions on a press-stressed fault and therefore might provoke bigger events. Numerous members of the DESTESS community categorize the Pohang earthquake as a triggered and exclude that the event was induced. As a next step, DESTRESS will organize a workshop involving all DESTRESS partners working on the Pohang sequence in order to bundle competences. The results of the workshop will be shared with the independent expert commission.