Despite the very large number of deep boreholes drilled worldwide, data on earthquakes occurring in connection with them are rather sparse. One reason for this is that the probability of such quakes is very low. Another is that many deep boreholes have been drilled in uninhabited areas, so potentially noticeable quakes may not have been felt and reported by the public. In many places, such boreholes have not been - and are still not being - seismically monitored. Consequently, it is impossible to reliably record smaller induced earthquakes. In Switzerland, for example, a number of microquakes were recorded when the borehole for the Basel geothermal energy project was cemented. The strongest of these had a magnitude of 0.7, meaning that it released 500 times less energy than a magnitude 2.5 quake. Earthquakes above this magnitude can usually be felt.

The physical processes behind earthquakes triggered in certain circumstances by drilling boreholes are well understood. Deep boreholes sometimes alter local stresses and pore pressures in rock, and in some cases this can reactivate a nearby tectonically pre-stressed fracture, causing an earthquake. However, such stress changes usually only occur in the following two situations: firstly, when drilling into a stratum with high fluid pressures. In this case, under certain conditions, the rock fluid (liquid or gas) can find its way into the borehole, causing overpressure that can usually be reduced in a controlled manner. Alternatively, the borehole is sealed at the corresponding place deep underground. Secondly, when boring into a very liquid-permeable stratum or rock of very low strength. If this happens, some of the drilling fluid or cement may enter the surrounding rock. The drilling fluid is needed to bring the drilling dust to the surface and stabilise the borehole during the driving process. Once a section has been drilled, the borehole is lined with cemented pipe to keep it open in the long term. In most cases, though, stress changes only affect small rock volumes. So the probability of activating quite a large, pre-stressed fracture and thus triggering a fairly large, potentially noticeable earthquake is extremely low.

The Swiss Seismological Service (SED) at ETH Zurich does not normally recommend seismic monitoring in its Guide for Managing Induced Seismicity for deep boreholes (e.g. exploratory drilling). Nonetheless, to record evidence and clarify the distinction between natural and induced seismicity, it may make sense to install an additional monitoring station near a drill site. For this very purpose, to cite just one example, the SED is currently consolidating its network on behalf of Switzerland's National Cooperative for the Disposal of Radioactive Waste (Nagra) with a view to monitoring exploratory drilling in northeastern Switzerland.

The two strongest earthquakes felt over the largest areas by the Swiss population occurred on 17 January and 1 February 2018 in Austria's Kloster Valley (Montafon) near the Swiss border. Both quakes reached a magnitude of 4.1. The most powerful earthquake in Switzerland itself, with a magnitude of 3.2, occurred on 23 August near the twin-peaked mountain Dents de Morcles in the Canton of Valais. The SED received around 400 reports from people who felt this quake, mainly in the Rhone Valley, whose soft subsurface particularly intensified the shaking. Other earthquakes, some of which were also clearly felt, occurred on 15 and 16 May near Châtel-St-Denis in the Canton of Fribourg (with magnitudes of 3.1 and 2.9), on 3 November near Martigny in the Canton of Valais (magnitude 2.9) and on 29 December near Fribourg (magnitude 2.9). Only the quakes in the Kloster Valley ended up causing minor damage, such as cracks in buildings' façades.

In addition, last year saw the occurrence of some remarkable earthquake swarms. A swarm entails numerous quakes occurring over a fairly long period, without there being any clear sequence of foreshocks, mainshock and aftershocks. The most notable swarm involved a series of earthquakes northeast of St. Léonard, near Sion in the Canton of Valais. This sequence was related to a fault that has repeatedly been associated with phases of heightened seismic activity since 2014. It is believed to be part of the Rhone-Simplon fault, which appears to be broken into separate segments in this area. Another earthquake sequence worth mentioning occurred in the border area between Italy, France and Switzerland, in the east of the Mont Blanc Massif. Last year, the SED pinpointed the locations of close to 100 earthquakes with magnitudes between 0 and 2.2 in that area.

In general, in 2018, as in previous years, most earthquake activity occurred in the Valais region, the Canton of Graubünden and the areas along the Alpine front. Despite this concentration of seismic activity, historically it has been shown that no parts of Switzerland are earthquake-free regions. Based on the long-term average, a serious earthquake with a magnitude of 6 or more occurs in the earthquake country Switzerland every 50 to 150 years.

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Currently, the physical and chemical parameters governing leakage through faults, and the effects of rock deformation and chemical interactions leading to induced seismicity, are not fully understood. There is further limited knowledge on Swiss-specific conditions, making it difficult to judge to what extent underground CO₂ storage could be an option in this country. This is why scientists from the Swiss Seismological Service at ETH Zurich and the SCCER-SoE are conducting an experiment, in collaboration with the Department of Mechanical and Process Engineering and the Institute of Geophysics at ETH Zurich as well as Swisstopo and EPFL. The experiment is taking place at the Mont Terri rock laboratory and part of the ELEGANCY project funded by the European Commission and the Swiss Federal Office of Energy.

Scientists will investigate how CO₂ migrates withing a rock with faults, under what circumstances induced seismicity may occur, and how such a storage should be monitored best. Therefore, they will inject small amounts of CO₂-enriched saltwater into a borehole that cuts through a small fault zone. To study how the fault zone reacts to the CO₂ injected, they will observe the stability of the rock and analyse the coupling between fault slip, pore pressure, and fluid migration. Active and passive seismic sensors will monitor the variations of seismic velocities around the injection and register possible micro-earthquakes with magnitudes below zero.

In contrast to a full-scale CO₂ storage project, this experiment only investigates the relevant processes with small amounts of CO₂-enriched saltwater. Nonetheless, its findings will contribute to a better understanding of the relevant processes influencing the migration of CO₂ in faults. Thereby, the experiment will also contribute to an enhanced site characterization. Worldwide, about twenty CO₂ storage projects are already in operation, each sequestering up to three million tons of CO₂ per year, and numerous plants are in the planning. In Switzerland, there is currently no CO₂ storage project planned. 

Learn more about the ELEGANCY project:

www.sintef.no/elegancy/

www.sccer-soe.ch/research/pilots-demos/elegancy/