What is a seismic risk analysis

Seismic risks in geothermal projects

(27.1.2010) In the course of a geothermal project, earth tremors occurred in Basel - according to the Richter scale with magnitudes of up to 3.4. The project was then interrupted because there was a risk that a major earthquake could be triggered. As a reminder: Basel is located in an active earthquake area and was badly damaged by an earthquake in the Middle Ages. The interruption of the project created time for a seismological report.

The report is currently only known in a short version, the publication of the overall report should take a few more months. Conclusions are therefore difficult to evaluate at the moment.

So far, based on extensive calculations, the report has come to the conclusion that there is no risk that geothermal energy could trigger a large natural earthquake. Because the effects on existing geological faults are far too small. However, these considerations should be viewed as very specific to the Basel location. They could hardly be transferred to other locations. Basel is an isolated case in many aspects.

The events directly generated (induced) by the individual project are also predictable and thus controllable. In addition to the methods of risk analysis that have been used up to now, the authors see a relationship to the size of the reservoir used in the subsurface. The authors saw this relationship when working through a large number of cases in which liquids were pressed into the subsoil. Since the size of the reservoir can be specified and adjusted by the operators of a plant, the seismological risk can also be controlled and geothermal plants are still possible.

During a stimulation in Basel, water was injected at high pressure (300 bar at the wellhead) to generate an artificial heat exchanger. This situation cannot be compared with the operating status of a geothermal system, for which significantly lower pressures are used for economic reasons. It is also a fact that a geothermal system has been in operation in Riehen near Basel for years without seismicity.

The fact that the report will probably also mean the end of the geothermal project in Basel does not diminish its basic statements. In Basel, not only was the underground heat exchanger installed in an unfavorable way, but the “vulnerability” of the surroundings due to inner-city development and industry was extraordinarily high. Accordingly, the authors of the report also calculate considerable and certainly atypical maximum damage sums.

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