A recently published overview study investigating the current challenges associated with monitoring and handling induced seismicity showed that real-time monitoring of seismic activities in the subsurface is still not standard practice in many places. This makes it difficult to intervene promptly and take preventive measures. Furthermore, there are no standardised requirements or international norms regarding such monitoring, which especially creates difficulties for projects carried out in border areas.

The geothermal project in Basel launched in 2006 sought to create an artificial fracture system in rock 4,000-5,000 m underground to use as a geothermal reservoir for power generation. To this end, cold water was injected into the substrate under high pressure. In the course of this process, a large number of microearthquakes occurred, some of them noticeable, and one earthquake with a magnitude of 3.4 magnitude (M_L), which caused minor damage to buildings. As a result, work on the project was interrupted and then stopped altogether in 2009 after a comprehensive risk analysis. The borehole was opened in December 2006 after increased seismicity, and closed again in April 2011.

Earthquake activity in the vicinity of the borehole has been monitored by a seismic network since the start of the project. The indications from the amassed data are that seismic activity in the stimulated area has more or less continually decreased since the project ended in 2006. Roughly a year after the closure of the borehole in April 2011, seismic activity in the immediate vicinity of the borehole markedly increased again. This increase has been particularly noticeable since the second half of 2016 and typically consisted of swarms of microearthquakes, with phases of increased activity over a number of weeks being followed by quieter periods. So far, none of these earthquakes were felt by the public.

Apart from the seismic activity, in recent months the spatial distribution of the quakes has also shifted. The latest earthquakes occurred at the southern and northern edges of the previously affected area, suggesting that the artificially induced fractures are spreading. In addition, measurements show that the hydraulic pressure in the reservoir (pore pressure) has steadily increased since the borehole was closed. A detailed analysis of the seismic data and modelling of the relationship between earthquakes and increasing pore pressure have shown that even modest pressure increases in the reservoir can significantly increase seismicity.

Analyses performed by the Swiss Seismological Service (SED) show that a noticeable minor earthquake with a magnitude of 2 may well occur within the next 12 months unless steps are taken to lower the pressure. The probability of this happening is between 55% and 85%. The current probability of an earthquake as strong as the one that occurred in 2006, which had a magnitude of 3.4, is around 5%. Based on its modelling and the decline in seismicity observed between 2007 and 2011, the SED expects that opening the borehole for the next one or two years should lower the average seismicity rate by between 50% and 90% percent.

Over the past decade, the SED has advised and supported project operators and in particular cantonal authorities (e.g. Basel-Stadt, Jura, Vaud, Thurgau and the city of St. Gallen) on deep geothermics. This work focussed on seismological aspects of the environmental impact assessment (EIA), seismic monitoring and the review of operational and seismic safety concepts.

Learn more

Background report on induced earthquakes within area covered by the geothermal project in Basel (in German)

Press release by the Canton of Basel Stadt on the opening of the borehole (in German)

SED information on the geothermal project in Basel

Earthquakes and geothermal energy – a brief explanation of key correlations

The shaking from this event was felt by the majority of people across Central Switzerland. The Swiss Seismological Service also received more than 5000 earthquake reports from a region of about 200 km diameter, including Ticino and the cantons of Bern, Aargau, Basel, Zurich and Grisons up to Chur. The high public interest was evident also in the up to 500’000 requests per minute on the SED websites. This high demand led to the SED's web pages partially not being accessible in the first 40 minutes after the event, and after that for some time with delays.

The epicenter is located in the Helvetic nappes. The preliminary focal depth of 5 km indicates a source at the border of the sedimentary cover and the crystalline basement. The moment tensor solution for this earthquake indicates it had a moment magnitude of M_W 4.1 with a strike-slip mechanism, with either a NNW-SSE or WSW-ENE striking fault plane, consistent with other events in this region. These mechanisms indicate northwest-southeast oriented compression of the crust in this region of the Helvetic domain. Last night’s earthquake occurred close to the magnitude 4.0 (M_L) Urnerboden earthquake of 5 May 2003. The depth and the focal mechanism of that event are very similar to yesterday’s earthquake and it seems likely that both events are associated with the same fault system.

Generally speaking, tectonic stresses in the Alps are the result of the collision between the European and the African lithospheric plates. However, due to the complex tectonic structure and history of the Alpine collision zone, significant along-strike variations in the tectonic regimes are observed.

The highest acceleration measured by seismic instruments were reported from Linthal (GL) and reached 85 cm/s². On average, earthquakes of this size happen approximately once every 5 years in Switzerland. The last earthquake with a similarly large magnitude was the Vallorcine (F) (4.9 M_L) event close to the Swiss border near Martigny (VS) on 8 September, 2005. It was strongly felt in the Valais.

On Wednesday 7 March, the SED installed two additional stations at Urnerboden and Bisisthal. These stations will allow characterizing the aftershock sequence and thus the properties of the activated fault in more detail.