One major challenge for past and future deep geothermal power projects is how to produce a working heat exchanger with sufficient rock permeability without triggering damaging or felt earthquakes as a result. The problems associated with this challenge are varied and complex, such as how to distinguish between natural (tectonic) and man-made (induced) earthquakes. In purely physical terms, there is no difference between these types of quake. A distinction, albeit one that rarely leads to a conclusive classification, can only be drawn between them based on the earthquake's location, time of occurrence, focal mechanism and connection with any human activities.
Just how difficult such a classification can be is currently apparent in South Korea. On 15 November 2017, a shallow earthquake with a magnitude of 5.4 occurred near Pohang, a few kilometres away from a geothermal plant currently under construction. Since 2016, the subsurface has been repeatedly stimulated in two boreholes up to 4.3 km deep, with a view to subsequently forcing water through the rock to heat it up. The latest such stimulations took place in August and September 2017 and did not trigger any major earthquakes. Within the framework of Project DESTRESS, the Korean operators and authorities, flanked by international experts, are currently investigating a possible connection between the quake and the human interventions that have occurred.
Such a line of enquiry and the successful strategies (“good practices”) derived from it are dealt with in a scientific report commissioned by the Swiss Seismological Service (SED) at ETH Zurich. It has been accomplished as part of GEOBEST-CH, a project supported by EnergieSchweiz, which examines good practices for dealing with potential induced seismicity. The report is intended to provide useful tips, in particular for operators of geothermal power plants and the licensing authorities, on how to gauge and limit the risks of man-made earthquakes.
It starts off with an introduction to natural earthquake activity in Switzerland, some background information on induced seismicity and a summary of the lessons learnt during the failed geothermal projects in St. Gallen and Basel. Building on this, it then contemplates guidelines for the seismic monitoring of geothermal projects and presents recommendations for regulators and field operators on how to estimate seismic activity and the risks and hazards associated with it. The objective is to develop measures that take account of the need to protect people, the environment, goods and buildings as well as considering local geological conditions and plant design.
The Geothermal Risk of Induced seismicity Diagnosis grid (GRID, Trutnevyte & Wiemer, 2017) mentioned in the report is the recommended tool for drawing up such a comprehensive situational analysis for a specific deep geothermal energy project. The resulting recommendations provide for meaningful measures at all phases of the project, including hazard and risk assessments, seismic monitoring, traffic-light systems, the interpretation of reflection seismic data and the involvement of stakeholders and authorities (see also Measures to Control Induced Seismicity). Detailed information on the individual aspects of the recommendations can be found in the linked pages or in the individual chapters of the report. These fundamental data help regulators to impose restrictions and field operators to develop measures aimed at reducing the risk to an acceptable level by reasonable, feasible means.
German, French and Italian versions of the summary of the English report, including its key findings and recommendations, will be published in February 2018.