• A report commissioned by the Bas-Rhin prefecture points to the responsibility of the operator Fonroche in the multiple earthquakes which shook the Strasbourg agglomeration at the end of 2020.

  • The tremors are thus “to be associated with the operations […] carried out on the [geothermal] wells from October to December 2020”, write the experts in the conclusions of this report.

  • For them, the “persistent seismicity” in November 2020 “should have alerted” Fonroche (since renamed GéoRhin).

New episode in the geothermal series.

After the multiple earthquakes felt at the end of 2020 in the Strasbourg conurbation, a report on Thursday pointed to the responsibility of the operator Fonroche.

"A bundle of consistent clues to the existence of a link" between the company's activity and these earthquakes.

The tremors of magnitude 2.1 to 3.6 recorded at the time are thus "to be associated with the operations [...] carried out on the [geothermal] wells from October to December 2020", write the experts in the conclusions of this report commissioned in February 2021 by the Bas-Rhin prefecture and published Thursday on its site.

The study was presented on Wednesday during a monitoring committee of the geothermal site of Reichstett-Vendenheim, north of the Strasbourg conurbation, which has been shut down since the end of 2020 following a series of earthquakes that have occurred since 2019 and which had caused a stir in Strasbourg and its region.

“Among the causes of this seismicity, the large volumes of fluid injected since September 2020, into a reservoir (…) of reduced size” which “resulted in the pressurization of the bottom of the well” and “induced a destabilization of faults and fractures pre-existing," write the scientists.

"Operations on the wells followed one another in a rapid manner compared to the relaxation time of the reservoir", they note again.

For them, the “persistent seismicity” in November 2020 “should have alerted” Fonroche (since renamed GéoRhin) “to the occurrence of an instability phenomenon […] and should have called into question the continuation of the injections, without waiting » a new earthquake, at the beginning of December.

"Poor assessment of the risk of seismicity"

"In view of the great depths reached, the existence of major fault zones, significant tectonic loading" and "the large volumes of fluid injected", the Vendenheim site "presented a level of seismic hazard that could be considered as high”.

This should have “encouraged more precautions in the conduct of operations”, reprimand the experts, for whom “the lack of solid data […] involved […] a poor assessment of the risk of seismicity”.

This does not mean "that all geothermal projects in such conditions are a priori not viable but that they require [...] more precautions in the conduct of operations", they still consider, judging "essential to continue to monitor the (Vendenheim) reservoir in the months and years to come”.

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