Interview: Samuel Schlaefli

Mr. Deichmann, a magnitude 3.4 earthquake was felt in Basel on 8 December 2006, accompanied by a loud bang. What are your memories of that day?

Several quakes occurred that day. At the time of the 3.4 magnitude quake I was speaking by phone to a geophysicist with the Geothermal Explorers operating company in Pratteln to report to him the tremor values we had measured for a previous quake. The reaction of the whole team on site to the noticeable quake was one of enormous surprise, but nonetheless they began the prearranged precautions straight away. The first journalists and anxious townspeople called us only a few minutes after that phone conversation. I was immediately aware that the population had insufficient information about the risk of an earthquake.

What caused this quake?

The principle of the Hot Fractured Rock process used in Basel is that cold water is forced into rock strata at a depth of about five kilometres. There it is heated and the resulting steam is conveyed through a second borehole to the surface where it is used for commercial energy generation. To make this possible the rock must be sufficiently water-permeable, which by nature it is normally not. To achieve this, rather large quantities of water under high pressure are forced into the rock through an initial borehole. The higher the water pressure in the strata, the lower the strength of the rock. The micro-quakes caused in Basel as a result, more than ten thousand of them, led to rock fractures and thus to the required permeability in the deep rock strata. The noticeable quake was a consequence of these procedures.

What magnitudes are to be expected with induced earthquakes of this kind?

Micro-quakes are an integral constituent of the Hot Fractured Rock process. Very few of them are perceptible by the local residents. In Basel the population noticed about a dozen quakes between early December and early February, four of them very distinctly. The strongest events in other similar projects have reached magnitudes between 2.9 and 3.7.

The earthquakes recorded by the SED during the stimulation phase of the geothermal energy project in Basel. Top left-hand corner: the quake of 8 December 2006 with a magnitude of 3.4. (Diagram: SED)

Have comparable earthquakes occurred in Switzerland recently?

Yes, on average eight quakes per year with magnitudes of 3 or greater occurred in Switzerland and the surrounding areas over the last 30 years. However, up to now we do not know of any damage caused by quakes with magnitudes of 3 to 3.5. The damage reported in Basel – mainly plaster cracks – may be due to the relatively shallow depth of the focus and to the fact that these quakes took place below an exceptionally densely built-up area of the town. At this magnitude, however, damage to the load-bearing elements of buildings is impossible.

How large is the risk of human activity triggering a major quake in Basel?

That is precisely the million dollar question which no one can answer for the actual case in Basel at present. However, we know that humans can cause major earthquakes. For instance they have happened during excavations in mines or when earth-filling dams, where again the water pressure on the deep rock strata was increased. In Colorado there was an example of a waste disposal operation where forcing liquid wastes into deep rock strata for many years caused a quake with a magnitude of more than 5.

What about the petroleum industry, where the method used to empty oil reservoirs is the same as that in Basel?

A relatively large number of empirical values are available in this area, and it is claimed that so far there is no known quake damage linked to it. Here again, however, the situation is different because in petroleum extraction the water is introduced into sedimentary rocks rather than into crystalline rock as in Basel.

To return to the million dollar question: what would be needed to allow it to be answered?

It will remain impossible to answer the question definitively even after a thorough risk analysis of the kind being discussed at present. In my opinion the aim must be to enable the bandwidth of the possible risk to be quantified, i.e. to establish a best and worst case scenario with their corresponding probabilities.

Nicholas Deichmann of the SED: “It is for the public, not the experts, to decide whether or not it wants to take a risk in order to benefit from new forms of energy.” (Photo: SED)

What data would such a risk analysis need to contain?

Certainly we must attempt to map the fractures in the rock strata as accurately as possible. In addition we need to build computer models with which the flow of water in the rock can be simulated. During the project in Basel it has been possible to collect important data for such models. The seismological behaviour of the rock during the long-term cooling resulting from the operation of a completed geothermal energy plant is also of interest. Naturally one must also calculate the damage scenarios in the event of further quakes to enable an economic balancing of interests to be carried out as well.

Who must make the final decision on this balance?

The experts must try to interpret the accumulated data correctly and thus attempt to assess the situation as objectively as possible. However, the public must take the final decision regarding the continuation of the geothermal energy project. After the publication of a risk analysis, it must decide whether or not it is willing to take the calculated risk so as to benefit from new forms of energy.

What role will the Swiss Seismological Service play in this risk analysis?

The project is currently on hold and the Great Council will have to reach a decision this summer on the conduct of such a study. Our main hope is that we will be allowed to have a say in drafting the specifications for the content of a risk study. The more experts that collaborate on such a report and the more intensive the scientific discussion about it, the better will be the chances of a soundly based forecast. Moreover the management of the study should be in the hands of an independent organisation that is untainted by the previous political happenings.

When could a new geothermal energy project in Basel be tackled again?

I estimate in two years at the earliest.

Concerns were voiced in public that those responsible in the Geopower Basel AG Company and the Swiss Seismological Service had backed out and that possible aftershocks were not being recorded at all. Is that true?

Seismic activity monitoring by the Swiss Seismology Service and Geopower Basel AG is continuing without any changes. From the very outset we stipulated that the measuring equipment installed by Geopower in six boreholes under Basel will remain in operation even if the project is abandoned. These instruments enable accurate observation of the changes in situ, and they continue to yield valuable scientific data.

What is your personal prediction: has geothermal energy any chance of continuing in Switzerland?

The basis to make such an assessment is lacking. The public’s fears must be taken seriously and the best possible quantification of the risks must be made. However it would be a shame to reject the geothermal energy project at present. The opportunity and the responsibility now on offer to check out the geothermal energy option in the form of a detailed risk study must be grasped. In this sense Basel has a knock-on effect extending far beyond Switzerland.