Drilling work for a planned geothermal power plant in Basel triggered a small earthquake on Friday that caused minor damage to buildings.
The canton Basel City prosecutor has launched an investigation to find if the company behind the Deep Heat Mining project should pay for repairs.
The tremor was felt shortly before 6pm and measured 3.4 on the Richter scale according to the Swiss Seismological Service. Normally, there are three or four earthquakes of this size annually in Switzerland.
Local police and fire departments received hundreds of phone calls from worried citizens, and some buildings suffered from cracks and broken tiles. Nobody was injured.
Basel is one of the areas in Switzerland most prone to seismic activity. In 1356, the city was almost entirely destroyed by a 6.5-magnitude earthquake.
The Deep Heat Mining project plans to recover heat by pumping water deep into the Earth's crust and use it to generate electricity.
Water has been injected into rocks five kilometres below the surface since the beginning of the month. The cantonal authorities had been warned that some minor tremors could take place, according to René Kinderhauser of Geopower Basel, the company behind the project.
Between the time when water was first injected and Friday's tremor, the seismological service registered 36 small tremors, including five with magnitudes of between 2.0 and 3.0.
More were recorded on Saturday, bringing the total to 100. The seismological service said it was unlikely they would cause any structural damage.
Drilling work was stopped on Friday morning after one tremor hit 2.7 on the Richter scale.
The project is now on hold while data from the incident is analysed. There is no word on what the experts' conclusions might be.
The prosecutor's office launched its investigation on Friday evening. The police has already seized computer data.
The judicial authorities will now try to decide whether any laws have been broken and if to prosecute anyone. They will also verify whether Deep Heat Mining can be held responsible for any damage.
The pilot project is the first commercial endeavour worldwide to attempt tapping heat using the so-called hot fractured rock technique.
To extract energy from hot dry rock, water is injected from the surface through boreholes into hot granite underground. The water heats as it flows through cracks in the rock; when it returns to the surface as steam, it is used to generate electricity.
Other boreholes were drilled earlier this year in Basel to measure any potential earthquake activity.
The plan, if the project is successful, is to build a power plant delivering heat to 2,700 homes and electricity to around 10,000.
Geopower estimates that it will cost a total of SFr80 million ($66.3 million) to build the installation, of which SFr56 million has already been spent. Canton Basel City (SFr32 million) and energy suppliers are among the investors.
swissinfo with agencies
The Swiss Deep Heat Mining project aims to set up a geothermal power plant in Basel by 2009.
Until now, environmentally-friendly geothermal power has only been generated in areas where heat is close to the surface, such as in volcanic regions.
The Basel plant should be able to produce 20,000 megawatt-hours of electricity per year, and 80,000 megawatt-hours of heating.
The water injected five kilometres down reaches temperatures of around 200 degrees Celsius.
It would be first commercial power plant of it kind anywhere in the world.
In Switzerland, between 300 and 400 earthquakes are recorded annually.
The last tremor to cause any noticeable damage struck canton Graubünden in 1991.
Basel is one Switzerland's highest risk zones for earthquakes.
The city was almost totally destroyed by a strong earthquake 650 years ago.