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Unveiling the effects of soil composition on surface motion during seismic events
  • Janneke van Ginkel,
  • Elmer Ruigrok,
  • Rien Herber
Janneke van Ginkel
University of Groningen

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Elmer Ruigrok
Royal Netherlands Meteorological Institute
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Rien Herber
University of Groningen
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The degree of damage on buildings due to earthquakes is strongly dependent on the properties of the subsurface at that specific site. The shallow geology of the Netherlands consists of an heterogeneous soft sediment cover, which has a strong effect on seismic wave propagation characteristics and in particular the amplitude of ground shaking. Where seismic velocities are lower, seismic wave amplitudes are higher. By studying local velocity and amplitude variations from seismic waves, we can obtain constraints on the seismic hazard. Gas extraction in the Groningen field, in the northern part of the Netherlands, is regularly causing shallow (3 km), low magnitude (Mw max= 3.6), induced earthquakes. This region forms an excellent study area due to the presence of a permanent borehole network and detailed subsurface knowledge. The earthquake wavefield consists of shear and compressional waves. Whereas a lot of research has been carried out on the shallow behaviour of the shear waves, this project includes the characterisation of the compressional waves in the shallow subsurface. In this way, ground motions in the vertical direction can be determined in order to support the re-enforcement design for buildings in the areas affected by induced seismicity. The Groningen borehole network is continuously measuring since 2015 and besides earthquakes, it records a wealth of background signals, which is usually called ‘noise’. This noise contains low-energetic elastic waves which also resonate within the sedimentary layers. The local earthquake recordings are used to assess how the shallow unconsolidated subsurface geology influences the amplification of compressional waves, amplification can directly be measured because there are geophones on multiple depth levels. For compressional waves we observe a strong relationship between locations with high amplitudes and the presence of peat in the subsurface. Peat is generating biogas, resulting in a partly gas-saturated soil, hence very low seismic velocities. The noise resonance and earthquake amplification patterns are well-matched. Therefore, the learnings from the densely sampled Groningen region are of interest for other areas in the Netherlands with risk of seismicity since the noise resonance can be used as a first proxy to assess wave amplification.