Wave propagation modelling of induced earthquakes at the Groningen gas production site

Gas extraction from the Groningen natural gas field, situated in the Netherlands, frequently induces earthquakes in the reservoir that cause damage to buildings, and pose a safety hazard and a nuisance to the local population. Due to the dependence of the national heating infrastructure on Groningen gas, the short-term mitigation measures are mostly limited to a combination of spatiotemporal redistribution of gas production and strengthening measures for buildings. All options become more effective with a better understanding of both source processes and seismic wave propagation. Detailed wave propagation simulations improve both the inference of source processes from observed ground motions and the forecast of ground motions as input for hazard studies and seismic network design. The velocity structure at the Groningen site is relatively complex, including both deep high-velocity and shallow low-velocity deposits showing significant thickness variations over relatively small spatial extents. We performed a detailed 3-D wave propagation modelling study for an induced earthquake in the Groningen natural gas field using the spectral-element method. We considered an earthquake that nucleated along a normal fault with local magnitude of ML = 3. We created a dense mesh with element size varying from 12 to 96 m, such that with a dominant source frequency of 7 Hz, waveforms with a frequency content up to 10 Hz could be accurately modelled. The velocity/ density model is constructed using a 3-D geological model of the area, including both deep high-velocity salt deposits overlying the source region and shallow low-velocity sediments present in a deep and narrow tunnel valley. The results show that the 3-D density/velocity model in the Groningen area clearly plays a large role in the wave propagation and resulting surface ground motions, and causes significant lateral variations in site response. The highvelocity salt deposits have a dispersive effect on the radiated wavefield, reducing the seismic energy reaching the surface near the epicentre. In turn, the presence of low-velocity tunnel valley deposits can locally cause a significant increase in peak ground acceleration. Here we study induced seismicity on a local scale and use SPECFEM3D to conduct full waveform simulations and show how local velocity variations can affect seismic records.