More than 80 earthquakes were recorded in Groningen in 2014. In addition to the impact on people and their homes, the earthquakes also affected dikes, mains networks, bridges, locks and the electricity grid. But how can we predict the impact in order to prevent damage as a result of the shaking and land subsidence? We need to know more about the structure of the subsurface and to study how the ground behaves when there are earthquakes.
In early 2014, the ministry for Economic Affairs asked Deltares to map out how induced earthquakes in the Groningen field might affect critical infrastructure: dikes, the gas transport grid and the electricity grid. In part on the basis of our knowledge and expertise, two guidelines were devised that are already being used in practice for the design of earthquake-resistant structures, buildings and industrial installations.
Copyright foto, Waterschap Noorderzijlvest.
We assessed approximately 70 km of the sea dikes in Groningen and 700 km of secondary dikes to determine their susceptibility to earthquakes caused by gas extraction. These data can be used to answer questions like: which locations should be prioritised for upgrading, and what measures are effective in terms of functionality and risk reduction? We also advised the Noorderzijlvest water authority about how they can strengthen the dikes that they already upgraded in 2014 to cope with high water so that they will also be earthquake-resistant. In this way, a few kilometres of dike that are susceptible to earthquakes have been strengthened by widening them and installing sheet piling. Furthermore, we mapped out the subsurface of all of Groningen on behalf of the NAM. Data showing the location of clay, sand or peat can now be found in a new geological model. This model will allow us to make better predictions in the years to come of how strong earthquakes will be when they reach the surface in Groningen. Including the shallow subsurface and variations in the subsurface makes it possible to predict the impact of earthquakes better and to reduce uncertainty in the risk analysis.
Predicting the consequences of earthquakes is a complex business, but we are getting increasingly better at identifying the areas at risk on the basis of what we know about the subsurface. The next step, limiting or even preventing damage, still represents a major challenge but important steps have been made here, too.