Up to now, almost all of the ground motion modeling and hazard assessment for induced seismicity in Groningen, the Netherlands, has been focused on the horizontal components of earthquake waves. Including the vertical component in site response studies is hardly being done for low magnitude earthquakes. Since the top part of the soils in the Netherlands is practically always unconsolidated, the elastic waves generated by deeper (~3000m) seated earthquakes will be subject to transformation when arriving in these layers. Recordings over a range of depth levels in a borehole show most of the amplification in the upper 50 meters of the sedimentary cover. We observe not only a strong amplification from shear waves on the horizontal components, but also from longitudinal waves on the vertical component. Furthermore, the seismic velocities show a large lateral heterogeneity. A better understanding of the vertical component of low magnitude earthquakes (to date, max M=3.6 in Groningen) aims to support the design of re-enforcement measures for buildings in areas affected by induced seismicity. This study focuses on longitudinal wave amplification in a sedimentary basin setting.Generally, the vertical component of ground motion is less than the horizontal, but the longitudinal waves are concentrated in a high frequency band which can cause damage to buildings with vertical periods in this range. Furthermore, interference between the longitudinal and shear waves might contribute to extra damage on structures. In Groningen, a dense borehole network is continuously recording seismic activity. From 19 seismic events with a minimum of magnitude two, transfer functions are retrieved between geophones at 50m depth and accelerometers at the surface, for 70 borehole sites. Peak frequencies and amplitudes derived from the transfer functions, do show significant variability across the region. Highest longitudinal wave amplification is measured in the center of the field, which is also the epicenter of most seismic activity. We investigate if the variations in amplification can be linked to the local geology. Additionally, the possibility and consequences of interference between the shear and longitudinal waves are presented.

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.