What is seismicity?
Seismicity comes from the internal dynamics of the planet Earth. These dynamics imply the implementation of forces at the scale of the globe, and in particular in the earth’s crust. It is these forces that allow the displacement of tectonic plates causing the opening of oceans or the formation of mountain ranges.
These forces propagate in the rock at great distance and can accumulate in the mass of the rock.
When the energy accumulated by the forces in the rock exceeds the threshold accepted by the rock, it is released. This release of energy can be expressed by a deformation of the rock in the mass or by a movement on fault planes. We see a good example of faults on the photo below left, in the quarry of Grand Gandaillat in Clermont-Ferrand.
The movement on the fault causes vibratory waves of different types that propagate in all directions: this is called an earthquake.
The magnitude of an earthquake, often given on the Richter scale, is the measure of the amount of energy released during the earthquake. This scale is particular because it is not linear, that is to say that an increase of 1 in the magnitude of an earthquake is in fact equivalent to a multiplication of the energy released by 30.
For example, the energy released by an earthquake of magnitude 3, which is observed on average 50,000 times per year in the world, corresponds to about 1000 times the energy released by an earthquake of magnitude 1.
Another scale of measurement is the intensity of an earthquake. It measures the consequences of the earthquake on the surface and especially the feeling. The intensity of an earthquake obviously increases with its magnitude, but it also depends on the distance to the source of the earthquake, the topography and the local geology.
To illustrate the importance of geology, let’s take the example of an earthquake that took place in the Grenoble region in 2003. Because of the local geology, it was felt more strongly in the valley of Grenoble composed of loose sedimentary rock than on the surrounding mountains composed of more rigid rocks.
Seismicity in France
Natural seismic activity in metropolitan France is relatively continuous, particularly in the Alps and Pyrenees where the most significant events have been recorded. But it is also well recorded in Brittany and in the Massif Central. The natural seismic risk is considered low to moderate in the Massif Central according to the Environmental Code.
In France, seismic monitoring is carried out by the National Seismic Monitoring Network, also known as ReNaSS, thanks to a network of more than 150 monitoring stations spread throughout the country. Seismicity data is available in real time on their website where earthquakes, quarry blasts and potential explosions are differentiated.
Seismicity induced by geothermal energy
Geothermal energy is part of the human activities that can cause induced seismicity. This seismicity, essentially not felt on the surface, can appear when the pressure in the rock and therefore in the well increases. This pressure depends on the pumping or injection rate in the well, but also on the capacity of the rock to let fluids circulate, also called permeability. If the fluid flow rates applied in the well respect the capacity of the rock to let them circulate, there is no risk of induced seismicity.
During the drilling phase, the only fluid circulation is that of the drilling mud in the well. This is necessary to bring the cuttings to the surface. This circulation is done in a closed circuit in the well and its flow is continuously controlled. The risk of induced seismicity is low.
During the test phases, water is injected into the rock to estimate its permeability. The injection or pumping rates and the pressure in the well are then continuously monitored. The risk of induced seismicity is moderate. Induced microseismicity events may occur, but real-time monitoring allows operations to be shut down if necessary.
During the operation phase, the fluid flow between the plant’s shaft doublet is stable, as is the pressure in the rock. The risk of induced seismicity is low.
For the Geopulse project, all the geoscientific studies carried out since 2015 have made it possible to specify the local parameters influencing the seismic risk. The natural seismicity of the area is low compared to the Fades dam area to the north and the Mont Dore area to the south.
In addition, the permeability in the area targeted by the wells is expected to be high and will therefore be able to promote the circulation of fluids without increasing the pressure in the rock. Note that if the permeability measured in the well is low then there will be no geothermal resource and therefore no continuity of the project.
These parameters imply a reduction of the risk of induced seismicity in the area.
Actions to control the risk
To control the seismic risk related to the project, several actions are underway or will be carried out. First of all, a risk study will be conducted by INERIS, an independent public organization. This study will analyze the link between the magnitude of an earthquake, the potential impact on the surface and the local geological characteristics. The objective of the study is to define vigilance or alert magnitude thresholds to avoid reaching felt earthquakes.
In parallel, a network of seismometers will be installed prior to drilling to carry out continuous and permanent monitoring of local seismicity before, during and after the work. Data from one of the seismometers will be shared with the National Seismic Monitoring Network. These data will allow the monitoring of natural and work-induced seismicity in real time and the stopping of operations if necessary.
Finally, measurements will be taken in the borehole to improve knowledge of the local parameters influencing seismicity. These measurements will also allow the refinement of the pressure thresholds to be respected to avoid the seismicity induced during the fluid circulation operations in the rock.
All the studies already carried out and the operations implemented before and during drilling allow us to say that the seismic risk related to the Saint Pierre Roche deep geothermal project is low to moderate but that it will be controlled.