Yangsan fault and location of the geothermal project near Pohang, South Korea.
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.
Expert commission examins evidence
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.
The Swiss Seismological Service (SED) at ETH Zurich has first informed the public (see
SED news
) in November 2017 about the Pohang earthquake in the context of the release of a best practice report on induced seismicity. At the same time, Geo-Energie Suisse AG had notified the canton of Jura, where currently the only EGS-style deep geothermal project in Switzerland has applied for a construction permit. In consequence, the canton instructed Geo-Energie Suisse AG to evaluate potential implications for the planned Haute-Sorne geothermal project.
Deeper understanding needed
"Such strong earthquakes in connection with deep geothermal projects have not yet occurred worldwide. However, it is known that other technical interventions that led to stress changes in the ground caused similarly large shocks. A deeper understanding of what was happening in Pohang is essential if geothermal energy is to be produced safely and sustainably in the future", says Stefan Wiemer, head of Swiss Seismological Service at ETH Zurich.
ETH Zurich is intensively involved in research in this field. On the one hand through controlled experiments in underground laboratories in Grimsel or soon in the Bedretto Tunnel, on the other hand through the development and validation of adaptive control systems that evaluate all available information in real time and continuously include it in the risk assessment.
