All three earthquakes have been caused by the high pressure water injections necessary to fracture the deep-seated rock in Kleinhuenigen, Basel where the drilling for the "Deep Heat Mining" project from Geopower Basel AG is taking place. The epicenter was near the borehole. The project uses the Hot-Dry-Rock (HDR) geothermal energy extraction method.

The drilling was stopped after the first earthquake and the drilling rig is being dismantled. Geopower Basel AG submitted a report to the Swiss authorities on the incident yesterday. The company has also published a press release on their website (in German). The company's executive board has decided to re-evaluate their original plans, but no take-up of drilling is expected in the year 2007.

The magnitude of the earthquakes has come as a surprise to the projects investors as well as to the population of Basel. Geopower Basel AG had expected smaller earthquakes, but the population was not aware of these risks. The region of Basel is prone to earthquakes; the last large one was on October 18, 1356 when a few hundred citizens died (mainly from unattended fires following the quake).

Why does fracturing rock cause earthquakes? Fracturing (or "fraccing" in industry colloquial terms) involves creating and enlarging small fractures in the rock from about 0.3 millimeter in size to about 1mm in diameter by using high pressure water injection. The fraccing is done at the bottom of the borehole; in the Basel project this is at 5000 meters depth. Although the fractures are relatively small, millions of tons of rock are being moved in the process. This creates stress and can result in the accumulated pressure suddenly being released in the form of an earthquake. The Soultz-sous-Forêts geothermal drillings in France have also caused a series of 93 earthquakes in the summer of the year 2000 ranging from 1.0 to 2.9 in magnitude on the Richter scale.